19/04/23

OCR Additional ScienceOCR Additional Science

AGAC

P3 Forces for TransportP3 Forces for Transport

19/04/23Speed vs. VelocitySpeed vs. Velocity

Speed is simply how fast you are travelling…

Velocity is “speed in a given direction”…

This car is travelling at a speed of 20m/s

This car is travelling at a velocity of 20m/s east

19/04/23

Distance, Speed Distance, Speed and Timeand Time

Speed = distance (in m)

time (in s)

d

tS

Dave walks 20 metres in 4 seconds. What is his speed?

Laura covers 12m in 3 seconds. What is her speed?

How long would it take to run 100 metres if you run at 10m/s

Steve travels at 50m/s for 20s. How far does he go?

Susan drives her car at 40m/s. How long does it take her to drive 200m?5

m/s4 m/s

10s

1000m

5

s

(in m/s)

19/04/23

Distance-time graphsDistance-time graphs

40

30

20

10

0 2 4 6 8 10

4) Diagonal line

downwards =

3) Steeper diagonal line =1) Diagonal line =

2) Horizontal line =

Distance

(metres)

Time/s

Constant speed

Not moving

Faster constant speed

Constant speed…back to start

19/04/23

40

30

20

10

0 2 4 6 8 10

What is the speed during the first 2 seconds?

How far is the object from the start after 6 seconds?

What is the speed during the last 4 seconds?

When was the object travelling the fastest?

Distance

(metres)

Time/s

10/2= 5m/s

40m40/4= 10m/s

4 to 6 seconds

ds x t

19/04/23

Distance-time graph for non-uniform motionDistance-time graph for non-uniform motion

40

30

20

10

0 20 40 60 80 100

Distance

(metres)

Time/s

Object is accelerating up to here

Object is now

decelerating

19/04/23

AccelerationAccelerationv-u

ta

Acceleration = change in velocity (in m/s)

(in m/s2) time taken (in s)

1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration?

2) A ball is dropped and accelerates downwards at a rate of 10m/s2 for 12 seconds. How much will the ball’s velocity

increase by?

1) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take?

2) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?

10/5= 2m/s2

10x12=120m/s

10/2= 5s

4000/2= 2000m/s2

19/04/23

Velocity-time graphsVelocity-time graphs

80

60

40

20

0 10 20 30 40 50

Velocity

m/s

T/s

1) Upwards line =

2) Horizontal line =

3) Upwards line =

4) Downward line =constant acceleration

constant velocity constant acceleration

constant deceleration

19/04/23

80

60

40

20

0

1) How fast was the object going after 10 seconds?

2) What is the acceleration from 20 to 30 seconds?

3) What was the deceleration from 30 to 50s?

4) How far did the object travel altogether?

10 20 30 40 50

Velocity

m/s

T/s

40m/s

2m/s2

-3m/s2

½ x 10x40 =

½ x 10 x 20=

10 x 40=

½ x 20 x 60=10 x 40=

200m + 400m + 400m +100m +600m = 1700m

clue: it’s the areaunder the graph!

va x t

19/04/23

Speed-time graph for non-uniform motionSpeed-time graph for non-uniform motion

40

30

20

10

0 20 40 60 80 100

Distance

(metres)

Time/s

Object’s acceleration is increasing

Object’s acceleration is decreasing

19/04/23

What is a force?What is a force?A force is a “push” or a “pull”. Some common examples:

________ – pulls things

downwards

___ ________ (drag) – acts against anything moving through air

______ – keeps things afloat_____ – acts against anything moving

Words – upthrust Air resistance friction weight

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

Consider a camel standing on a road. What forces are acting on it?

Weight

Reaction

These two forces would be equal – we say that they are

BALANCED. The camel doesn’t move anywhere.

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

What would happen if we took the road away?

Weight

Reaction

19/04/23

What would happen if we took the road away?

The camel’s weight is no longer balanced by anything, so the

camel falls downwards…

Balanced and unbalanced Balanced and unbalanced forcesforces

19/04/23

Air ResistanceAir ResistanceAir resistance is a force that opposes motion through air. The quicker you travel, the bigger the air resistance:

The same applies to a body falling through a liquid (called “drag” or “upthrust”).

The bigger the arrow, the bigger the force

19/04/23

Examples of Air ResistanceExamples of Air Resistance

19/04/23Balanced and unbalanced Balanced and unbalanced forces 1forces 1

In each of thesesituations the

car is not movingnot movingbefore each force is

applied car accelerates

car accelerates

car does not move

car accelerates

19/04/23Balanced and unbalanced Balanced and unbalanced forces 2forces 2

Stage 1

Stage 2…continuing

from stage 1 (moving)

Car starting offcar accelerates

car still accelerates but not as much as in

Stage 1Next slide for stage 3

19/04/23Balanced and unbalanced Balanced and unbalanced forces 2forces 2

Stage 3…continuing from Stage 2 (moving)

Stage 4…continuing from

Stage 3 (moving)

Car moves at asteady speed

Car decelerates

Now see the next slideTo see how this can be

Put into a graph …..

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

Velocity

Time

Stage 1 Stage 2 Stage 3 Stage 4

accelerating

acce

lera

ting

decelerating

constantspeed

19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

1) This animal is either ________ or moving with

_____ _____…

4) This animal is…

2) This animal is getting _________…

3) This animal is getting _______….

stationaryconstant

speed

faster

slowerstationary

19/04/23

Force and accelerationForce and acceleration

If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers:

Force (in N) = Mass (in kg) x Acceleration (in m/s2)

F

AM

19/04/23

Force, mass and accelerationForce, mass and acceleration

1) A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate?

2) A force of 3000N acts on a car to make it accelerate by 1.5m/s2.

What mass has the car?

3) A car accelerates at a rate of 5m/s2. If it has a mass of 500kg how much driving force is the

engine applying?

4) A force of 10N is applied by a boy while lifting a 20kg mass.

How much does it accelerate by?

F

AM

1000 =500

2m/s2

3000= 1.5

2000Kg

500 x 5 = 2500N

10 =20

0.5 m/s2

19/04/23

Stopping a car…Stopping a car…

Braking distance

Too much alcohol

Thinking distance

Tiredness

Too many drugs

Wet roads

Driving too fast

Tyres/brakes worn out

Icy roads

Poor visibility

Stopping distance Thinking distance Braking distance= +

The following factors INCREASE distances

19/04/23

Car Safety FeaturesCar Safety Features

Air bags Rigid cage Seat belts

Paddle controls

Crumple zone

19/04/23Work Done Work Done

A B C

1

2

3

4Each Worker needs to do the same amount

of work in lifting these boxes on to the

shelves

A B C

19/04/23Work Done Work Done

AA A

A B C

1

2

3

4

A

Worker A

lifts 4 boxes on to the 1st shelf

A B C

19/04/23Work Done Work Done

AA A

A B C

1

2

3

4

BB

A

Worker B

lifts 2 boxes on to the 2nd shelf

A B C

19/04/23Work Done Work Done

AA A

A B C

1

2

3

4C

BB

A

Worker Clifts 1 box on to the 4th

shelf

A B C

19/04/23Work Done Work Done

AA A

A B C

1

2

3

4C

BB

A

No. of boxes x height4 x 1 = 4

No. of boxes x height2 x 2 = 4

No. of boxes x height1 x 4 = 4

A B C

“box” = weight or force

“height” = distance moved

19/04/23

Work doneWork done

When any object is moved around work will need to be done on it to get it to move (obviously).

We can work out the amount of work done in moving an object using the formula:

Work done = Force x distance moved

in J in N in m

W

DF

19/04/23

Example questionsExample questions1. Bori pushes a book 5m along the table with a force of

5N. He gets tired and decides to call it a day. How much work did he do?

2. Alicia lifts a laptop 2m into the air with a force of 10N. How much work does she do?

3. Martin does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it?

4. Chris cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use?

5. Carl drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do?

25J

20J

4m

50N

2 000 000J

19/04/23

Work and PowerWork and PowerThe POWER RATING of an appliance is simply how much work it does (i.e. how much energy it transfers) every second.

In other words, 1 Watt = 1 Joule per second

W

TP

W = Work done (in joules)

P = Power (in watts)

T = Time (in seconds)

19/04/23

Some example questionsSome example questions1) What is the power rating of a light bulb that transfers

120 joules of energy in 2 seconds?

2) What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?

3) Isobel runs up the stairs in 5 seconds. If she transfers 1,000,000J of energy in this time what is his power rating?

60W

2000W

200 000W

19/04/23

Kinetic energyKinetic energyAny object that moves will have kinetic energy.

The amount of kinetic energy an object has can be found using the formula:

Kinetic energy = ½ x mass x velocity squared

in J in kg in m/s

KE = ½ mv2

19/04/23

1) Emily drives her car at a speed of 2m/s. If the combined mass of her and the car is 1000kg what is

her kinetic energy?

2) Max rides his bike at a speed of 10m/s. If the combined mass of Max and his bike is 80kg what is his

kinetic energy?

Example questions Example questions KE KE =1/2mv=1/2mv22

KE =1/2 mv2

KE = 0.5 x 80 x (10)2

KE = 2000 J

KE =1/2 mv2

KE = 0.5 x 1000 x (2)2

KE = 4000 J

19/04/23

Terminal VelocityTerminal VelocityConsider a skydiver:

1) At the start of his jump the air resistance is _______ so he

_______ downwards.

2) As his speed increases his air resistance will _______

3) Eventually the air resistance will be big enough to _______ the

skydiver’s weight. At this point the forces are balanced so his

speed becomes ________ - this is called TERMINAL VELOCITY

Words – increase, small, constant, equal, accelerates

smallaccelerates

increase

equal

constant

19/04/23Terminal Terminal VelocityVelocity

Consider a skydiver:

4) When he opens his parachute the air resistance suddenly

________, causing him to start _____ ____.

5) Because he is slowing down his air resistance will _______ again until it balances his _________.

The skydiver has now reached a new, lower ________ _______.

slowing down

decrease

terminal velocity

weight

Words:

increases

terminal velocity, weight,decrease, increases, slowing down

19/04/23Velocity-time graph for terminal Velocity-time graph for terminal velocity…velocity…

Velocity

Time

Speed increases…

Terminal velocity reached…

Parachute opens – diver slows down

New, lower terminal velocity reached

Diver hits the ground

On the Moon

No air resistance !

19/04/23Gravitational Potential Gravitational Potential EnergyEnergy

To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation…

GPE = Mass x Acceleration of free-fall x Change in height

(Joules) (newtons) (=10N/kg) (metres)

19/04/23Gravitational Potential Gravitational Potential EnergyEnergy

To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would

use the simple equation…

GPE = Mass x gravitational field strength x Change in height

(Joules) (kg) (N/kg) (metres)

GPE

hgm

On Earth g = 10N/kg. This means that every

kilogram on Earth experiences a

downwards weight of 10N

Now let us consideran earlier

problem….

19/04/23Work Done Work Done

AA A

A B C

1

2

3

4C

BB

A4x10x1 = 40J

2x10x2 = 40J

1x10x4=40JIn this example the work done

produces an increase in potentialenergy

Each box has a mass of 1Kg and the shelves are

1m apart

GPE= mxgxh

GPE= mxgxh

GPE= mxgxh

Each worker has transferred 40J into GPE for the boxes

19/04/23

Example questionsExample questions

How much gravitational potential energy have the following objects gained?:

1. A brick that has a mass of 1kg and is lifted to the top of a house (10m),

2. A 1,000kg car lifted by a ramp up to a height of 2m,

3. A 70kg person lifted up 50m by a ski lift.

How much GPE have the following objects lost?:

4. A 0.2kg football dropping out of the air after being kicked up 30m,

5. A 50g egg falling from a nest 10m above the ground

6. A 1,000kg car falling off its 2m ramp.

1. GPE = 1x10x10= 100J

2. GPE=1000x10x2= 20 000J

3. GPE=70x10x50= 35000J

4. 60J 5. 5J 6. 20 000J

GPE

mxgxh

19/04/23

Roller CoastersRoller Coasters1) Electrical energy is transferred into gravitational potential energy

2) Gravitational potential energy is transferred into kinetic energy

3) Kinetic energy is transferred back into gravitational potential energy