sce 3105 - pract 2

PRACTICAL 1

TOPIC: FORCE AND MOTION

AIM/ OBJECTIVE:

1) To identify the different types of force in different contexts.

2) To measure the acceleration of a freely falling object.

HYPOTHESIS:

There are forces acts on any objects in any situation or contexts. There can

be different types of force acting on the objects in their different contexts

where a situation context is not only involving just one type of force but also

can be more than one type.

The acceleration of freely falling objects on this earth will give the same value

of acceleration due to the gravity that is 9.8 ms-2, where the acceleration is

independent of mass.

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Activity 1: Identifying Different Types Of Force In Different Contexts .

STATEMENT OF PROBLEM:

What kind of different types of force identified in different context?

THEORETICAL KNOWLEDGE:

1. Force is a vector quantity. Force has both magnitude and direction. There are

six types of forces. They are friction, normal, spring, tension, thrust and

weight.

Force Definition

Friction, Ff The contact force that acts to oppose sliding

motion between surfaces.

Normal, FN The contact force exerted by a surface on an

object.

Spring, Fsp A restoring force, that is, the push or pull a spring

exerts on an object.

Tension, Ff The pull exerted by a string, rope, or cable when

attached to a body and pulled taut.

Thrust, Fthrust A general term for the forces that move objects

such as rockets, planes, cars and people.

Weight, Fg A long- range force due to gravitational attraction

between two objects, generally Earth and an

object.

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2. When a body is in equilibrium, the resultant force on it is zero. For example,

the Figure 1 below shows the force acting on the book and a box. The forces

that act on them are balanced. The forces cancel each other so that the

resultant force is zero. So the book and the box are said to be in equilibrium.

Figure 1

INSTRUMENTS AND MATERIALS:

Materials Instruments

A wooden block No

A ping-pong ball

A stretched wire on a wooden block

A beaker

A book

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VARIABLES:

Manipulated: Contexts of situation

Responds: Types of force identified

Kept constant: Acceleration due to gravity (9.8 m s-2)

PROCEDURE:

1. A wooden block is placed on the floor. The wooden block is pushed and the

observations are recorded.

2. A ping-pong ball is thrown up into the air. The observations are recorded.

3. A piece of wire is tied tightly between 2 nails hammered into a piece of wood.

Then, the wire is touched. The observations are recorded

4. A ping pong ball is placed into a beaker of water. The ball is tried to be

pushed into the water. The observations are recorded.

5. A book is placed on the palm of the hand. The observations are recorded.

6. The inferences are stated for each of these observations.

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RESULTS:

Procedure Observation Inferences

Procedure 1 The wooden block is moving after

being pushing and then stop.

Presence of normal force (FN),

weight force (Fg), friction force

(Ff) and thrust force (Fthrust).

Procedure 2 The ping-pong ball is moving up into

the air and then falling down.

Presence of weight force (Fg),

thrust force (Fthrust) and friction

force (Ff).

Procedure 3 The wire is stretched or moved

slightly from its initial position and

then turn back to the initial position.

Presence of tension force (FT)

Procedure 4 The ping-pong ball resists from the

water or cannot sink into the water.

Presence of weight force (Fg) and

thrust force (Fthrust).

Procedure 5 The book stays static or immobile on

the palm of hand.

Presence of weight force (Fg) and

thrust force (Fthrust).

Table 1

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DISCUSSION:

1. Activity 1 shows me that there are forces acting on any objects in any

situations. There can be many types of forces acting on an object depend on

its situation context. There is also situation that just involving a force action.

The forces can either make the objects moves or stay static (equilibrium

state).

2. I notice that an object is being acted by forces to be in its situation. For

example, Procedure 1 that makes the wooden block move after being

pushing and then stop is because of the presence of normal force (FN), weight

force (Fg), friction force (Ff) and thrust force (Fthrust) which acting on it. Here,

the wooden block is moving because of the thrust force and then slows down

because of the friction force.

3. The ping-pong ball is moving up into the air and then falling down because of

the presence of weight force (Fg), thrust force (Fthrust) and friction force (Ff).

The ping-pong ball is moving up into the air by the thrust force, and then

falling down because of the Earth’s gravity is pulling it downwards with a

weight force. At the same time, the air resistance that acts again gravity

causes a friction force which pulls the ball downwards.

4. The wire is stretched or moved slightly from its initial position and then turn

back to the initial position due to the presence of tension force (FT) along the

wire.

5. The ping-pong ball resists from the water or cannot sink into the water

because of the presence of weight force (Fg) in the ball itself and thrust force

(Fthrust) in the medium of water.

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6. For the last procedure, the book stays static or immobile on the palm of hand.

Why? It is because of the presence of weight force (Fg) and thrust force

(Fthrust). The book is said to be in equilibrium. The forces of weight and thrust

that act on them are balanced. The forces cancel each other so that the

resultant force is zero.

Activity 2: Measuring The Acceleration Of A Freely Falling Object

STATEMENT OF PROBLEM:

How to measure the acceleration of a freely falling object

THEORETICAL KNOWLEDGE:

1. Gravitational field is a region around the earth in which an object

experiences a force towards the centre of the earth. This force is the

gravitational attraction between the object and the earth. The gravitational

field strength at the surface of the earth is 9.8 N kg-1. This means that an

object of mass 1 kg will experience a gravitational force of 9.8 N.

2. Measurement with more accurate instruments shows that the acceleration

due to the gravity at the surface is also 9.8 m s-2. The acceleration due to

gravity does not depend on the mass of the falling object. All objects falling

freely fall with the same acceleration.

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3. In Diagram 1 below, all the masses are falling freely (gravity is the only force

acting). From F = ma, it follows that all the masses have the same downward

acceleration, g. This is the acceleration of free fall.

Diagram 1

4. Acceleration tells us how fast the speed of something in changing.

Acceleration can be shown very clearly on a speed- time graph. We calculate

the acceleration by working out how much the speed changes in one second.

5. We can say the units as ‘metres per second, per second’. This is because the

acceleration tells us how much the speed has changed (in metre per second)

in each second.

Acceleration = Change in speed

Time taken

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INSTRUMENTS AND MATERIALS:

Materials Instruments

A ticker-timer No

Ticker tape

Clamp

Power pack

Connecting wires

Masses of 300 g, 450 g and 600 g

Cellotape

VARIABLES:

Manipulated: Masses of the objects

Responds: Accelerations of the freely falling objects

Kept constant: Frequency of ticker timer (50 Hz)

PROCEDURE:

1. A chair is placed on top of a table. A ticker-time is clamped onto the upper

side of the chair as shown in Picture 1.

2. A piece of ticker tape is fixed through the ticker-time and a 300g mass is

cellotaped to the tape.

3. The ticker time is switched on and the mass is left to be falled freely

downwards.

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4. The tape is cut into 2 dots strips and they are arranged to form a graph.

5. Step 3 is repeated by using masses of 450 g and 600 g.

6. For each case, a graph is build from their strips and then their accelerations

are determined.

Picture 1

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RESULTS:

Mass (g) Acceleration (ms-1)

300 7.361 ms-1

450 7.963 ms-1

600 8.242 ms-1

Table 2

A) Calculating acceleration for mass 300 g:

a) Time covered for the 3- tick strip:

= 3 x 0.02

= 0.06 s

b) The average initial velocity over 3- tick strip, u:

u = 2.0

0.06s

= 33.33 cms-1

c) The average final velocity over 3- tick strip, v:

v = 19.2

0.06s

= 320 cms-1

d) Time taken for the change in velocity, t:

t = T7 - T1

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= 0.42 – 0.06

= 0.36 s

e) Acceleration of the freely falling 300g load, a300:

a = v – u

t

a = 320 - 33.33

0.36 s

= 796.31 cms-2

B) Calculating acceleration for mass 450 g:


= 3 x 0.02

= 0.06 s


u = 2.1

0.06 s

= 35 cms-1


v = 18.0

0.06 s

= 300 cms-1


t = T7 - T1

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= 0.42 – 0.06

= 0.36 s


a = v – u

t

a = 300-35

0.36 s

= 736.1 cms-2

C) Calculating acceleration for mass 600 g:


= 3 x 0.02

= 0.06 s


u = 6.6

0.06 s

= 110 cms-1


v = 24.4

0.06 s

= 406.67cms-1


t = T7 – T1

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= 0.42 – 0.06

= 0.36 s


a = v – u

t

a = 406.67-110

0.36 s

= 824.08 cms-2

► Average of acceleration value:

= 796.31cms-2 + 736.1cms-2 + 824.08cms-2

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= 785.5 cms-2

= 7.86 ms-2

DISCUSSION:

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1. In activity 2, I calculate the value of acceleration for each mass as recorded

by using the below formulas in Table 1. The time for a tick is 0.02 s is due to

the main voltage in Malaysia that is an alternating current (AC) with a

frequency of 50 Hz. This provides a very reliable means of measuring time

because the interval between each cycle is exactly 1 s (i.e 0.02 s).

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Find 1st strip 7th strip

The displacement of the 10-

tick strip, x

x1 X7

The time covered for the 3- tick

strip

t1 = 0.06 s T7 = 0.06 s

Average velocity over the 3- tick

strip, u

u = x1

t1

v = x7

t7

Change in velocity between the

two 3- tick strips, v - u

Final velocity – Initial velocity

= v - u

Time taken for the change in

velocity, t

t = Time taken for the six strips – Time taken for

the first strip

Acceleration, a a = Change in velocity (v – u)

Time taken (t)

Table 1

2. Then, I get the average of the acceleration values of freely falling objects. The

average is 7.86 ms-2. This value is not accurate enough compared to the

actual value of gravity acceleration that is 9.8 ms-2 because of some small

errors made while doing the experiment.

3. An object is falling freely when it is falling under the force of gravity only.

However, the air resistance can affect the fall either being smoothly in freely

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falling or not. The objects of masses are considered to fall freely because the

air resistance is small compared to the pull of gravity and therefore is

negligible. So, the experiment is considered successful although the

acceleration value is not so accurate because of the air resistance is

everywhere on this Earth except in vacuum.

4. Another error while doing the experiment is the angle between the ticker- time

and the mass which is going to fall freely is not in 90° degree. Besides, there

is friction between the ticker- timer and the ticker tape also. These all affect

the last result because the masses are not freely falling smoothly.

5. During the experiment, the time of the mass fall is not synchronizing with the

time to switch on the power pack. This is also one of the small reasons of why

the acceleration values of different masses are not so accurate. Anyhow, the

experiment is still considered successful.

PRECAUTIONS:

1. Make sure the ticker timer is in good condition before connecting it to the

power supply.

2. We must be careful of the environment during the experiment. The fan should

be switched off because it can interrupt the falling of the object either may be

freely or not.

3. We must take at least three strips to get a more accurate acceleration in

average.

4. The ticker-time must be clamped onto the upper side of the chair wisely so

that it results accurately.

5. The ticker tape should be fixed through the ticker-time thoroughly because it

will influence the smoothness of freely falling downwards of the objects.

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QUESTIONS AND ANSWERS:

1. Are the values of acceleration of the different masses obtained equal to g

(acceleration due to gravity on earth)? Explain.

The value of acceleration of the different masses obtained is not so equal to g

(acceleration due to gravity on earth). It is because of some errors done in the

experiment. The air resistance affects the freely falling of the masses from being

smoothly. However, the experiment is considered successful although the

acceleration value is not so accurate because of the air resistance is everywhere

on this Earth except in vacuum. Then, the angle between the ticker- time and the

mass which is going to fall freely is not in 90° degree. Besides, there is friction

between the ticker- timer and the ticker tape also. These all affect the last result

of not being so equal enough because the masses are not freely falling smoothly.

Lastly, the time of the mass fall is not synchronizing with the time to switch on the

power pack producing the acceleration values of different masses are not so

accurate.

2. What conclusion can be made about the force acting on a freely falling

object?

The force acting on a freely falling object is a weight force or gravity due to Earth.

Object is exerted by air resistance, which opposes its motion The acceleration of

freely falling objects on this earth gives the same value of acceleration due to

gravity that is 9.8 ms-2, where the acceleration is independent of mass. The small

air resistance interrupts the acceleration a bit but still shows their equality

because air resistance is everywhere except in vacuum.

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3. If this experiment was done on the moon, will the value of acceleration of

the different masses obtained be equal to g? Explain.

No. If this experiment was done on the moon, the value of acceleration of the

different masses obtained will not be equal to g. It is because the value of gravity

on the moon is 1/6 to the value of gravity on the earth. If the experiment is done

in vacuum on Earth, yes, they will be equality in acceleration although different in

mass because the objects will fall freely as same as the Earth gravity without any

disturbance or air resistance. It is due to the gravity acceleration that is the same

everywhere on the Earth. Yes, being at the moon is just being in vacuum but its

gravity is not the same as the Earth.

Mastery Of Knowledge And Skills Questions:

(a) What have you learnt from the practical?

This experiment has taught me the different types of forces exist in different

contexts of situation. The objects involved in any situations either in moving or

static are because of the forces action. Besides, the types of force involved can

be more than one. I learn how to make such correct observations in identifying

the forces. I then can explain scientifically about the reasons towards the

observations.

I also learn how to handle the instruments and materials involved in the

experiment correctly and thoroughly. I have learned how to measure the

acceleration of the objects especially free fall objects using the formulas. Lastly, I

know the relation between the forces and the free fall object. I know why their

accelerations are the same although different in mass that is due to the gravity

acceleration.

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(b) Which part of the primary science curriculum teaches this particular topic?

The part is in Investigation Force and Energy theme, Year 5.

(c) Discuss how you can use the above practical in your teaching and

learning?

First of all, I will teach the student about the concept related to this experiment in

the class before doing experiment in the laboratory. I will make sure that they are

understood first before doing the experiment. How to teach them are many ways.

I can do demonstration or relate the situation with their experiences or daily life.

Then, when turning to the hands on application for meaningful learning, I will

make them do the experiment. I will show them how to do first. I will show the

skills required. I will give a clear briefing to the students regarding the experiment

while demonstrate it first. I will show how to use the apparatus well in order to get

an accurate result and record the result. Then, I will let them do themselves in

group. Lastly, I will be with them in the calculation process for the last result.

(c) What was the role of the teacher in the above practical?

Teacher acts as a facilitator, guider and demonstrator in the above experiment.

Teacher guides students how to do the experiment and being together with the

students along the experiment in order to facilitate them. Indeed, teacher always

demonstrate how to do first before let the students do themselves. Teacher

should encourage the students in their application of scientific skills such as

solving problem, measuring and analyzing. Here, the support from the teacher is

important for the students to elaborate their answers. Yes, they will learn the best

from the hands on activity in the experiment.

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sce 3105 - pract 2

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