get it right - 1e1 science · © 2007 marshall cavendish international (singapore) pte ltd© 2008...

© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd 85Acids and Alkalis

Some reactions of dilute sulphuric acid are given below.

a) Identify salt M and gas N.

b) How can gas N be identifi ed?

Recall the properties of acids. Refer to the Science Matters Textbook Volume A, section 8.1 on Acids.

Get It Right

+

Solid magnesium

+ Gas Q

Salt M Gas N

+Salt P Salt X + Substance Y

Sodium hydroxide solution

Solid copper carbonate

c) Identify salt P, gas Q and substance R.

d) How can gas Q be identifi ed?

Recall the properties of acids. Refer to the Science Matters Textbook Volume A, section 8.1 on Acids.

Substance R

Dilute sulphuricacid

Salt M is magnesium sulphate. Gas N is hydrogen gas.

A lighted splint is extinguished with a ‘pop’ sound.

Salt P is copper sulphate. Q is carbon dioxide. R is water.

It is passed through the limewater. If limewater turns milky, gas Q is carbon dioxide.

LSSWB_08.indd 85LSSWB_08.indd 85 8/2/07 6:11:32 PM8/2/07 6:11:32 PM

© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte LtdAcids and Alkalis86

Refer to the Science Matters Textbook Volume A, section 8.3 on Neutralisation — Mixing Acids and Alkalis and section 8.4 on Indicators.

f) Name the process that produces salt X and substance Y.

g) Both dilute hydrochloric acid and sodium hydroxide solution are colourless. How would you identify that salt X has been formed?

h) Identify salt X and substance Y.

e) State the nature of sodium hydroxide solution. How would you know this?

Refer to the Science Matters Textbook Volume A, section 8.2 on Alkalis.

Sodium hydroxide is alkaline. It turns red litmus paper blue.

Neutralisation

Salt X is sodium sulphate. Substance Y is water.

By adding Universal Indicator to either hydrochloric acid or sodium hydroxide solution before mixing

them. When the Universal Indicator turns green, it means salt X has been formed.


77Acids and Alkalis© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

AcidAcids and Alkalis s and Alkalis

Worksheet 8.1 For Science Matters Textbook Volume A, sections:

8.1 Acids 8.3 Neutralisation — Mixing Acids and Alkalis

8.2 Alkalis

Without referring to the textbook, try to answer these questions. If you cannot answer a question, look up the relevant section in your textbook.

1. Which of the following is true of acid?

A It has a pH of more than 7. B It produces a gas with an alkali. C It produces effervescence with baking powder. D It turns Universal Indicator blue. ( )

2. When sulphuric acid is added to calcium carbonate, the gas produced .

A extinguishes a lighted splint with a ‘pop’ sound B relights a glowing splint C turns limewater chalky D turns litmus paper green ( )

3. Which of the following is an example of neutralisation in everyday life?

A Adding bleach to whiten clothes B Adding common salt to meat C Rubbing ointment to skin D Taking antacids for gastric pains ( )

4. Acid rain causes lakes to be polluted. Which substance is used to neutralise the acids in the water?

A Calcium B Calcium nitrate C Calcium oxide D Calcium sulphate ( )

Secure Your Basics

ChapterChapter 8

C

C

C

D

Name: Class: Date:


Acids and Alkalis78 © 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

5. Complete the following word equations.

a) + Iron sulphate + Hydrogen

b) + Hydrochloric acid Zinc chloride + + Water

c) Calcium hydroxide + Nitric acid +

d) + Sulphuric acid + Water

6. Give a use for each of the following substances.

a) Sulphuric acid:

b) Nitric acid:

c) Potassium hydroxide:

Taste

Turn litmus

paper .

React with some

and metal .

Complete the graphic organiser that follows the question below.

What are the main differences between acids and alkalis?

Taste

Turn litmus

paper .

with metals and metal carbonates.

Map It Out

Acids Alkalis

Iron Sulphuric acid

Zinc carbonate Carbon dioxide

Calcium nitrate Water

In car batteries / For making fertilisers

For making fertilisers / explosives

Ingredient in paint remover

sour

red

blue

carbonates

metals

bitter

blue

red

Do not react

Aqueous ammonia Ammonium sulphate



Attempt the following questions within the time allocated.

1. Given two substances, an acid and an alkali, you cannot identify them by adding to these substances.

A a piece of magnesium B an indicator C calcium carbonate D hydrochloric acid ( )

2. The diagram shows what happens when red and blue litmus paper are dipped into solution X.

• 25 min •Challenge Yourself

Which of the following substances could possibly be solution X?

A Distilled water B Lime juice C Limewater D Vinegar ( )

3. Which of the following word equations does not show neutralisation between an acid and an alkali?

A Aqueous ammonia + Nitric acid Ammonium nitrate + Water B Carbonic acid + Calcium hydroxide Calcium carbonate + Water C Sodium hydroxide + Sulphuric acid Sodium sulphate + Water D Zinc + Sulphuric acid Zinc sulphate + Hydrogen ( )

Red litmus paper turns blue.

Blue litmus paper remains blue.

solution X

D

C

D

Name: Class: Date:


Acids and Alkalis80 © 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

a) What variables need to be controlled for a fair experiment?

b) Suggest why the shells must be completely immersed into the acid.

c) Suggest the function of the gas syringe in the experiment.

d) Write down the word equation for the reaction that takes place.

e) Explain how you can determine which type of shell contains the most amount of calcium carbonate from the results.

f) What assumption is made in the experiment?

gas syringe

30 cm3 ofhydrochloric acid

shell

4. Marie is conducting an experiment to fi nd out the type of shell that contains the greatest amount of calcium carbonate. She intends to add 30 cm3 of hydrochloric acid to three types of shells (chicken eggshell, quail eggshell and oyster shell) and measure the volume of carbon dioxide produced.

To ensure that the reaction is complete.

To measure the volume of carbon dioxide produced.

No other substances in the shells react with hydrochloric acid to produce carbon dioxide.

The shell which produces the greatest volume of carbon dioxide contains the greatest amount of

calcium carbonate.

Calcium carbonate + Hydrochloric acid Calcium chloride + Carbon dioxide + Water

Same mass of the different types of shells used e.g. 1 g each.

Same (concentration of) hydrochloric acid used.



Worksheet 8.2 For Science Matters Textbook Volume A, section:

8.4 Indicators


1. Which of the following is not an indicator?

A Datalogger B Methyl orange C Phenolphthalein D Red and blue litmus ( )

Study the table below and answer questions 2 – 3.

Secure Your Basics

2. Which substance will turn blue litmus paper red?

A S B S and T C S and U D T and U ( )

3. When equal volumes of T and U are mixed, the pH is likely to become .

A 5 B 7 C 8 D 14 ( )

4. Where is it most unlikely for the measurement of pH to be done?

A In car engines B In farm soil C In fi sh farms D In swimming pools ( )

5. Label the pH scale to show the appropriate positions of the substances listed below.

Substance S T U

pH 3 8 6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

distilled water toothpaste vinegarpotassium hydroxide hair conditioner acid in car battery

A

B

C

A

Acid in car battery

Vinegar ToothpasteHair conditioner

Distilledwater

Potassiumhydroxide

Name: Class: Date:



6. A big piece of zinc is added to a beaker of hydrochloric acid containing a few drops of Universal Indicator. A reaction takes place and a small piece of zinc remains at the end of the reaction.

a) What is the colour of the Universal Indicator in the acid?

b) What changes to the colour of Universal Indicator are observed?

c) What happens to the piece of zinc? What other observations can be seen?

d) What substances are produced in this reaction? Write a word equation for the reaction.

e) A gaseous product is obtained in this reaction. Describe a test for this substance.

7. State whether each of the following statements is true or false.

a) Fresh lemon juice turns litmus red. b) Common salt solution turns litmus paper blue.

c) Indicators can help us know when alkalis have been neutralised

by acids.

d) Methyl orange and Universal Indicator turn red in acids.

hydrochloric acid + Universal Indicator

zinc

True

False

True

True

Red

Red Orange Yellow Green

The piece of zinc becomes smaller in size and sinks to the bottom of the beaker. Effervescence is

also observed.

Zinc chloride and hydrogen gas are produced in this reaction.

Zinc + Hydrochloric acid Zinc chloride + Hydrogen

Insert a lighted splint into the gas, and observe if the splint extinguishes with a ‘pop’ sound. If it

does, then the product is hydrogen gas.


© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd 83Acids and Alkalis


1. Which is likely to happen when a substance is added to a solution with pH 10?

Substance added Result A Hydrochloric acid Carbon dioxide gas produced B Iron powder Hydrogen gas produced C Methyl orange Turns red D Phenolphthalein Turns pink ( )

2. What would be expected when sodium hydroxide solution (in excess) is added gradually to a conical fl ask of dilute nitric acid containing Universal Indicator?

A The colour of the indicator changes from blue to red. B The colour of the indicator changes from red to green. C The pH increases from 3 to 7. D The pH increases from 3 to 14.

( )

3. When aqueous solutions of P and Q are mixed, neutralisation occurs and a solution with a pH of 7 is obtained. What could the pH values of P and Q be?

pH of P pH of Q

A 2 5 B 3 7 C 3 12 D 7 12 ( )

4. The graph below shows the pH change of a colourless alkali when a colourless acid is gradually added to it.

0 10 20 30 40

4

8

12

Volume of acid (cm3)

2

6

10

14

5 15 25 35

pH value

• 20 min •Challenge Yourself

sodium hydroxidesolution

dilute nitricacid + Universal Indicator

D

D

C



Bottle Physical appearance

1 White powder

2 Colourless solution

3 Colourless solution

a) If a few drops of Universal Indicator was added to the alkali before the addition of acid, describe the change in colour of the Universal Indicator throughout the reaction.

b) How do you know neutralisation had occurred?

c) What is the volume of the acid required to neutralise the alkali?

5. Labels have fallen off three bottles. The labels read “Hydrochloric acid”, “Zinc carbonate” and “Sodium hydroxide”. Use the information given and describe how you can conduct tests to confi rm the identity of the chemicals in the respective bottles. Write down observations of the tests described. You can use chemicals commonly found in the science laboratory.

Add sulphuric acid to a sample of the white powder in bottle 1. Effervescence and a colourless gas

(carbon dioxide) is obtained. Pass the gas into limewater. The limewater turns milky. Hence, bottle 1

contains salt; zinc carbonate is the only salt among the three.

Add a few drops of Universal Indicator or dip litmus paper into the samples from bottles 2 and 3.

If the indicator turns blue, the substance is sodium hydroxide as it is alkaline. If the indicator turns red,

the substance is hydrochloric acid as it is acidic.

The Universal Indicator turned from violet to dark blue, then appeared green, became yellow and

fi nally turned red.

The Universal Indicator turned green.

25 cm3


© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

Here is a picture of a man being sandwiched between two beds of nails. Another man is standing on the upper part of the bed.

a) What is the force that holds man A against the nails?

b) Will man A get injured? Explain your answer.

c) Which bed of nails should have higher density of nails, the one at the top or the one at the bottom? Explain.

Recall that a force can be distributed evenly onto many objects, i.e. many objects can share the load to support a force. Refer to the Science Matters Textbook, section 10.3 on What is Pressure? to understand how a force is related to pressure.

Get It Right

man A

man B

Gravitational force

He will not get injured. The weight of man A is distributed equally over the many nails. Hence, a very

small force acts on each nail. A small pressure is exerted onto the skin by each nail; this pressure is not

large enough to make a hole through the skin.

The one at the bottom should have higher density of nails. This is because it supports more weights (two

persons’ weights, man A’s and man B’s). Hence, more nails are required to distribute the force evenly

into small force on each nail.

Force and Pressure104


Force and PressureForce and Pressure


10.1 What are Forces? 10.2 Forces Around Us

Secure Your Basics


1. Which of the following statements about mass and weight is true?

A An object has weight but not mass.B Mass is a force, weight is not.C Mass is the weight of an object.D Weight is the force exerted by gravity on an object. ( )

2. Which word would you not associate with a force?

A Predict B Pull C Push D Twist ( )

3. Which of the following activities does not involve forces?

A B

C D

( )

4. Which of these statements is true?

A Catapults when pulled back and released will only then push.B Gravity is only a pushing force.C Magnetism is only a pulling force.D Springs can only pull at what is attached to them. ( )

ChapterChapter 1010

D

A

B

A

95Force and Pressure

Class: Name: Date:

101© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd


10.3 What is Pressure?

Secure Your Basics


1. You recently layered the fl oor of your home with expensive wooden parquet fl ooring. In order to preserve the fl oor, identify which of the following footwear would cause the most damage to the fl oor.

A Japanese fl ip-fl ops B Padded bedroom slippersC Rubber-soled shoes D Stiletto heels ( )

2. A cube with sides of 2 m is placed on the ground. It has a mass of 20 kg. What is the pressure exerted on the ground by this cube?

A 0.5 Pa B 5 Pa C 50 Pa D 5,000 Pa ( )

3. a) After weighing in at 60 kg, Isaac measures the area of his foot. Each foot is roughly a rectangle of 10 cm × 25 cm. He then stands on both his feet. Calculate the pressure on the ground under his feet due to his weight.

b) If Isaac now stands on one foot, what would be the pressure on the ground under his foot?

4. A girl weighs 45 kg, and stands with her full weight on one of her stiletto heels. The base of the heel has an area of 1.0 cm2. What is the pressure she is exerting on the fl oor in pascals?

D

C

Pressure = Force———Area

Force = 60 kg × 10 = 600 N

Area of feet = 2 × (0.1 × 0.25) = 0.05 m2

Pressure = 600 N————

0.05 m2 = 12,000 Pa

Force = 45 kg × 10 = 450 N

Area = 0.0001 m2

Pressure = 450 N————

0.0001 m2 = 4,500,000 Pa

If the area in contact is halved, the pressure is doubled.

Pressure = 2 × 12,000 Pa = 24,000 Pa

101

Name: Class: Date:

Force and Pressure


5. What is the main force that causes your shoes to wear out?

A Friction B PolishingC Pressure D Weight ( )

6. Observe the following illustration.

Explain how the three types of forces — gravitational force, frictional force and magnetic force — act in the situations illustrated above.a) Effect(s) of gravitational force

b) Effect(s) of frictional force

c) Effect(s) of magnetic force

A

Gravitational force pulls all objects downwards, keeping them in contact with the ground. The boy,

the fridge and all its contents are not fl oating in the air.

Frictional force allows the hand to grip the milk bottle. It also allows the boy to stand without

slipping.

Magnetic force allows the fridge magnets to stick onto the fridge door. The door of the refrigerator

is kept closed by a magnetic lock.



7. The diagrams below show the basic principles of how a beam balance and a spring balance work.

Beam balance

a) How does a beam balance measure the mass of object M?

b) Why does a beam balance measure mass and not weight?

c) In the diagrams below, sketch arrows to represent the size and direction of the forces acting on the objects due to their weights. [Hint: The greater the force, the longer the arrow.]i) On the moon ii) On the earth

M m

a b c

m Spring balance

M m

a b c

M m

a b c

very light rod

It measures the mass of object M by placing a counterweight, m, whose mass is known to balance

the rod. If the arms ab and bc are equal, and the the rod is very light, the mass of object M is equal

to that of the counterweight, m.

Beam balance has two objects hanging onto it at its two ends, M and m. Since both M and m are on

the same place (for example on earth), gravitational forces that act on M and m (i.e. the weights) are

the same. The weights of M and m still balance the rod because they have the same mass. Therefore,

a beam balance measures mass and not weight.



A B C

Based on your answer in (e), state if each of the instruments shown above measures mass or weight.

d) How does a spring balance measure the weight of object m?

e) By looking at the diagram of the spring balance and the beam balance, what is the difference between the instruments for measuring mass and the instruments for measuring weight?

f) Observe the diagrams below.

0

1

2

3

45

6

7

8

9

standard weights

counterweight

A measures mass. B measures weight. C measures mass.

By quantifying the extension of the spring that is being pulled by the weight of m.

Instruments for measuring mass have counterweights while instruments for measuring weight

have springs.




What are the effects of forces?

Map It Out

Effects

Forces

Change in

Change in size

Move a object

a moving object

Speed up or the motion of of an object

Change in of a moving object

shape

stationary

Stop

slow down

direction


100 © 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

Challenge Yourself • 10 min •


1. Which of the following is true about a spring balance?A The mass of the spring will always try to balance the mass of the object hanging

from it.B The more the pull of gravity, the greater is the stretch of the spring.C The density of the object hanging on the spring balance is greater than the

density of the spring balance.D The spring’s weight will balance the weight of the object hanging from it. ( )

2. The diagrams show the weights of a 500-g mass on the earth and on the moon.

5 N

500 g

0.8 N

a) Indicate which diagram shows the weighing on the earth and which shows the weighing on the moon. Draw arrows to show the size and direction of the weight. [Hint: The greater the force, the longer the arrow.]

b) An object with a mass of 1.75 kg is weighed on the earth and on the moon. i) What is the weight of the object on earth?

ii) What is the weight of the object on the moon?

c) What causes this difference in weight?

g 005

B

Difference in gravitational force acting on the object when it is on the earth compared to when it

is on the moon.

The weight of a 1 kg mass on earth = 1 kg———

0.5 kg × 5 N = 10 N

Therefore, the weight of a 1.75 kg object on earth = 1.75 × 10 N = 17.5 N

The weight of a 1 kg mass on the moon = 1 kg———

0.5 kg × 0.8 N = 1.6 N

Therefore, the weight of a 1.75 kg object on the moon = 1.75 × 1.6 N = 1.8 N

On the moonOn the earth

100 Force and Pressure



To carry heavy groceries, would you use a shoppping bag with narrow handles or the one with wide handles? Why?

Map It Out

Assuming the weight of the groceries in each bag is the same, from the

relationship Pressure = ForceArea

, we can deduce that as the area of the

handle , the pressure .

The area of the handle is

.

Pressure exerted on the

fi ngers is .

The area of the handle is

.

Pressure exerted on the

fi ngers is .

It may your fi ngers. It is more .

Therefore, I would use a shopping bag with handles to carry the groceries.

Carrying heavy grocerieswith a shopping bag that has …

Differences

Narrow handles Wide handles

increases decreases

small

large

large

small

hurt convenient

wide



2. A spacesuit provides air and keeps the air pressure at a comfortable level to the astronaut. Without the spacesuit, the astronaut’s body could burst in the vacuum of space. Why do you think a person going to the depths of the sea requires a rigid diving suit instead of a fl exible one?

3. You went to an overseas trip to a cold country. It was winter then and the weather was snowy. On the way to the hotel, you had to walk through a path covered with snow a metre deep. You sank into the snow at every step of the way.a) Explain why you kept sinking into the snow as you walked.

b) You are given the following items: the grills from the previous evening’s barbecue, some ping-pong bats, newspapers and a pair of fl ip-fl op sandals.


1. A 4 kg brick has a dimension of 20 cm × 30 cm × 10 cm. Calculate the maximum pressure that can be exerted by the brick.

Aluminium grills Ping-pong bats Newspapers Flip-fl op sandals

Suggest what you can do to avoid sinking into the snow the next time you go to countries with snowy weather. Use any of the items given. Explain your reasoning in fi nding the solution to this problem.


In the depths of the ocean, the water pressure is far greater than the air pressure humans are used to. To

prevent the high water pressure from crushing the diver, a rigid diving suit is needed. This suit keeps the

air inside at a pressure lower than the water pressure outside the suit. A fl exible suit would collapse under

the water pressure, crushing the diver within.

The pressure of weight on the snow is high because of the small surface area of the soles of the

shoes in contact with the snow.

Wear any of the available items under your shoes such that the surface area in contact with the

ground is increased. Anything that increases the surface area you stand on will help decrease the

pressure of your weight on the snow.

Pressure = Force———Area

Force = 4 kg × 10 = 40 N

Smallest area = 0.1 m × 0.2 m = 0.02 m2. Since the smallest area will exert the greatest pressure,

Pressure exerted = 40 N————

0.02 m2 = 2,000 Pa


The picture below shows a simple watermill. A watermill is an example of application of unbalanced moment.

a) Explain how a watermill works.

Analyse how a watermill work in relation to application of moment of force.

b) Sketch how a watermill works. Label where a force is applied and where the pivot (fulcrum) of the water mill.

c) Assuming that the force of the water hitting the paddles remains steady, what will happen if the radius of the watermill is increased? Explain.

Recall the formula of moment of force. Refer to the Science Matters Textbook Volume A, section 11.2 on Moment of a Force.

Get It Right

rotation of watermill

fl ow of water

The force exerted by the water to the blade creates a

turning effect (moment) on the watermill. The watermill

turns in the direction of water fl ow.

The moment of force (the turning effect) will be higher as moment is proportional to the perpendicular

distance between the fulcrum and effort. Increasing the radius means increasing the perpendicular

distance.

fulcrum

force fl ow of water

Force and Work116 © 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

Force and WorkForce and Work


11.1 What is Work Done?

Secure Your Basics


1. Work has the same unit as energy because .

A forces are needed to create energy and forces do workB it is diffi cult to think of a new name for a new unitC while doing work, heat is generated and heat is energy D work measures the energy changing from one form to another ( )

2. Which of the following pictures shows no work done?A C

B D

( )

3. The most work done against gravity on a 5 kg bag is when it is .

A carried a distance of 10 m from the gate of the house to the bus stop B carried at the bus stop for ten minutes C carried up the bus by climbing three steps, each 28 cm high D carried while standing in the bus as it moves 120 m ( )

ChapterChapter 1111

D

D

C

105Force and Work© 2007 Marshall Cavendish International (Singapore) Pte Ltd© 2008 Marshall Cavendish International (Singapore) Pte Ltd

Class: Name: Date:

4. Which activity will use up the highest amount of energy?

A Climbing a hill 500 m highB Climbing down a 30 m ladderC Standing on a boat moving 2.5 km out to seaD Walking up a fl ight of stairs 4 m high ( )

5. When an item of weight W goes up against gravity, the energy required to move it up by a height H is given by .

A H + W, which is the total energy for supporting and moving itB HW, which is the work done on the weight against gravity

C H—W

, which is the specifi c height achieved by the worker

D H—W

, which is the pressure on the hand that does the work ( )

6. Marie, who has a mass of 40 kg, climbs up to a look-out point 25 m above the road. All the work done against gravity was converted into heat. How much heat energy was produced?

A 10 kJ B 100 kJC 1,000 kJ D 10,000 kJ ( )

7. A force equals to the weight of a backpack is used to carry it upstairs. The work done is given by the formula:

A Work = Height of stairs + Weight B Work = Height of stairs / WeightC Work = Weight × Height of stairsD Work = Weight / Height of stairs ( )

8. A pebble of mass 120 g leaves a catapult with kinetic energy of 18 J. It rises vertically to a height of .

A 1.5 m B 6.7 mC 15 m D 150 m ( )

9. Whenever energy changes from one form to another, .

10. The family car broke down. Isaac gets out to push the car with all his might. The maximum force he can exert is 800 N. After trying hard, without success, he gives up. Isaac thinks he has lost a lot of energy as he is hot and sweaty. How much work has he done? How did you arrive at this answer?

E

E

E

E

work is done

A

B

A

C

C

No work was done. The car did not move.



11. What is the force needed to do 2.4 kJ of work in pulling a cart over a distance of 0.5 km?

12. Calculate the work done:a) to draw 8 kg of water, in a 2 kg bucket, from a well 5 m deep.

b) when a force of 500 N tries to pull a car parked along a road without budging it a little.

c) when a force of 4 kN hammers a nail 1.2 cm into a wall.

13. A soggy wet towel weighing 1.2 kg falls from the bamboo pole on a fl at balcony to the pavement 42 m below. (1 kg of mass weighs 10 N.)

a) What is the weight of the towel?

b) What is the work done by the towel’s weight to pull it down to the ground?

c) What is the work done by the owner of the towel to bring the towel up to the balcony again? Explain your answer.

E

E

E

Work done = Force × Distance

2,400 J = Force × 500 m

Force = 2,400 J———500 m

= 48 N


= 100 N × 5 m = 500 J


= 500 N × 0 m = 0 J


= 4,000 N × 0.012 m = 48 J

Weight = 1.2 kg × 10 = 12 N


= 12 N × 42 m = 504 J


= 12 N × 42 m = 504 J

The work done to bring the towel up to the balcony is equal to the work done when the towel falls.

The force and distance are unchanged.



14. The initial kinetic energy of a 5 kg stone thrown upwards is 325 J.

a) What is the weight of the stone?

b) How much energy has the stone at its highest point? Explain your answer.

c) How high will the stone go? Explain your answer.

15. Marie, who has a mass of 40 kg, walked around in the supermarket. She moved a total distance of 55 m. What is the total work done in moving herself against gravity? Explain your answer.


What are the three conditions for work to be done?

Map It Out

Work done

The movement of the object is in the

of the force.

The object .A acts on the object.

E

E

Weight = 5 kg × 10 = 50 N

325 J. Energy is conserved. Hence, it has the same amount of energy as the kinetic energy at the

start. Energy changes from one form (K.E.) to another (P.E.).

The work done against gravity is stored as potential energy. As the stone rises, kinetic energy

decreases until all the stone’s energy becomes potential energy at the maximum height of the throw.

P. E. at the peak of throw = K. E. at the start of throw

Weight × Maximum height = 325 J

Maximum height = 325 J——50 N

= 6.5 m

Marie’s weight has a downward direction. However she travelled a horizontal distance, which is not in

the direction of the weight. Hence, there is no work done.

moves

direction

force





1. A rock in outer space moving in a straight line with a constant speed. Which of the following is true the about the rock?

A It burns up like a meteorite and does work. B It does no work because there is no force acting on it. C It has energy, which is mass multiplied by distance moved. D It has kinetic energy, and hence, work is done. ( )

2. The diagram shows 7 steps.

15 cm

20 cm

Geno, having a mass of 50 kg, runs up the steps to the top. What is the potential energy gained by Geno?A 525 J B 1,225 JC 5,250 J D 12,250 J ( )

3. When a motorcycle accelerates up a hill, the work done by the motorcycle causes it to .

A lose both kinetic and potential energyB gain both kinetic and potential energyC lose kinetic energy but gain potential energyD gain kinetic energy but lose potential energy ( )

4. Marie, whose mass is 49 kg, pushed the wall of a building with a force of 40 N. What is the amount of work Marie did?

A 530 N B 490 NC 400 N D 0 N ( )

B

A

B

D


5. A cart weighing 2.5 kN is lugged up a hill by a rope having a force F. The distance moved by the cart is as shown in the diagram.

12 m

16 m

F

20 m

a) What is the work that needs to be done against gravity to lift the cart up the hill?

b) What then, is the potential energy the cart would have gained at the top of the hill? Explain your answer.

c) The work done by force F provides the 30 kJ required to bring the cart to the top of the hill. Find the force F.

30 kJ. The work done is stored as potential energy.


= Weight of cart × Vertical height

= 2,500 N × 12 m = 30,000 J

Weight done = Force × Distance

Force = Work done—————Distance

= 30,000 J—————

20 m = 1,500 N

An exercise involving conversion from gravitational

potential energy to elastic potential energy is available

in the Inquiry Worksheets (Teacher’s Resources). This

exercise is on the Safety of a Bungee Cord.

Note to Teacher:



11.2 Moment of a Force 11.3 Levers

Secure Your Basics


1. The moment of a force about a point does not depend on .

A the location of the line of action of the forceB the magnitude of the force C the perpendicular distance of the line of action from the fulcrumD the time during which the force acts ( )

2. Which of the following actions does not make use of the turning effect of a force?

A Dabbing on paint with a paint brushB Picking up an ice cube with ice tongsC Tightening a nut with a spannerD Turning on the tap to wash your hands ( )

3. The scale pan weighs 40 g. The uniform metre rule is supported at its centre. What is the mass of the object X in the scale pan if the standard weight given is 500 g?

A 1,085 g B 1,125 gC 1,960 g D 2,000 g ( )

4. Slotted weights of 50 g each are hung on a metre rule as shown. The weight hanger has the same weight as each slotted weight shown. Which diagram shows a balanced metre rule?

A B

C D

( )

X500 g

90 cm40 cm

E

D

A

C

D


Class: Name: Date:


5. Which of the following is not true for a lever?

A It moves about a stationary point called the pivot.B The force a person uses to operate the lever is called the effort.C The force that has to be overcome is called the load.D The load and effort must always be equal. ( )

6. In this diagram, at the position of the knife shown

A The fulcrum is at the pointed tip of the knife. B The fulcrum is the point where the knife touches the board. C The fulcrum is where the object being cut touches the board.D The fulcrum is on the right of the load. ( )

7. The cutter shown is used to cut a woody stem.

The smallest effort can be used if .

A the load is moved to the left to the extreme tip of the cutter and the effort moved close to the fulcrum.

B the load and effort are both moved closer to the fulcrum. C the load and effort are both moved away from the fulcrum.D the load is moved closer to the fulcrum and the effort moved away from the fulcrum to the right. ( )

8. A pair of tongs is used to pick an ice cube as shown below.

At which position should effort be applied such that the least force is required? ( )

A B C D

L

L F

E

E

D

B

D

D


9. L, the load force, is 30 cm away from the fulcrum, F, of this wheelbarrow. The effort is applied at the handle, which is 120 cm away from F. What is the effort, E, required to lift a 20 kg load of soil?

10. The diagram below shows a see-saw type of lever that has a length 8 L. On the left of the fulcrum is a body P with 6 units of mass. Find the mass of the body M in terms of these units if the see-saw is balanced. [You do not need to know what units they are.]

11. A light metre rule is pivoted at the zero end. A spring balance supports the other end. What is the reading of the spring balance if a weight of 50 N is hung from the80 cm mark?

12. AC is a plank of wood 3 metres long. It is supported at the centre and bolted at a point B, one metre from A. The end C overhangs a swimming pool. What force is acting at the bolt when a diver weighing 50 kg stands at C?

LF

E

30 cm

120 cm

M6 units

P?

A B P C

E = 80 cm———100 cm

× 50 N = 40 N

F × 0.5 m = 500 N × 1.5 m

F = 500 N × 1.5 m———0.5 m

= 1,500 N

200 N × 30 cm = E × 120 cm

E = 200 N × 30 cm————120 cm

= 50 N

Clockwise moment = Anticlockwise moment

M × 3 L = 6 × 2 L

M = 6 × 2 L——3 L

= 4 units

500 N

80 cm

L50 N

E

F

1 m




For questions 1 and 2, you may have to add up all clockwise moments and add up all anticlockwise moments to see if they are equal.

1. A metre rule has 50-cent coins laid onto it. Which diagram shows an unbalanced metre rule?

A B

C D

( )

2. Weights of a standard unit are hung on a metre rule as shown. All weights and hangers have the same mass. Which diagram shows a balanced metre rule?

A B

C D

( )

3. Jack and Jill sit 52 cm and 72 cm respectively from the fulcrum of a see-saw. The fulcrum is at the centre of the see-saw. Jill weighs 46 kg.

a) What does Jack weigh?

b) What is the force at the fulcrum supporting the see-saw if the see-saw weighs 50 kg?

D

Jack weighs 72—52

× 46 = 63.7 kg

Total mass = 63.7 kg + 50 kg + 46 kg = 159.7 kg

Force on pivot = 159.7 kg × 10 = 1,597 N

Jack Jill46 kg

F

52 cm 72 cm

D


5. A bucket of water can be very heavy. By using the wheel and axle shown, the force required to draw water from the well can be reduced.

If the diameter of the wheel is 60 cm and the axle’s diameter is 20 cm, how much force would you need to pull up a bucket of water weighing 600 N?

F

axle

wheel

R

W

r

F

4. A light rod 80 cm long is supported at both ends by two spring balances P and Q. A weight of 40 N is hung 10 cm from P along the rod. What does each balance read?

P QQ reads (40 ×

10—80

) N = 5 N

P reads (40 – 5) N = 35 N

Let the required force be F.

Taking moments about the centre of the wheel and axle, notice that the perpendicular distances are equal to the radius of both wheel and axle.

Clockwise moment = Anticlockwise moment F × R = W × r F × 30 cm = 600 N × 10 cm

F = 600 × 10 cm———30 cm

= 200 N

I need to exert only 200 N to pull up the bucket of water.

40 N

10 cm



get it right - 1e1 science · © 2007 marshall cavendish international (singapore) pte ltd© 2008...

Documents