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P4 Explaining P4 Explaining MotionMotion

M Barker

Shirebrook Academy

OCR 21OCR 21stst Century Century

20/04/23P4.1 How can we describe motion?

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Distance, Speed Distance, Speed and Timeand Time

Speed = distance (in metres)

time (in seconds)

D

TS

1) Freddie walks 200 metres in 40 seconds. What is his speed?

2) Hayley covers 2km in 1,000 seconds. What is her speed?

3) How long would it take Lauren to run 100 metres if she runs at 10m/s?

4) Jake travels at 50m/s for 20s. How far does he go?

5) Izzy drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?

5m/s

2m/s

10s

1000m

500s

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Distance, Speed Distance, Speed and Timeand Time

Speed = distance (in metres)

time (in seconds)

D

TS

1) Sarah walks 2000m in 50 minutes. What is her speed in m/s?

2) Jack tries to walk the same distance at a speed of 5m/s. How long does he take?

3) James drives at 60mph (about 100km/h) for 3 hours. How far has he gone?

4) The speed of sound in air is 330m/s. Molly shouts at a mountain and hears the echo 3 seconds later. How far away is the mountain? (Careful!)

0.67m/s

400s

300km

495m

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Distance vs DisplacementDistance vs Displacement“Distance” is how far you have gone, “displacement” is how far you are from a point and can be positive or negative:

Start

1 metre-1 metre

Distance =

Displacement =

Distance =

Displacement =

Distance =

Displacement =

Distance =

Displacement =

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Distance-time graphsDistance-time graphs

40

30

20

10

0 20 40 60 80 100

4) Diagonal line

downwards =

3) Steeper diagonal line =1) Diagonal line =

2) Horizontal line =

Distance

(metres)

Time/s

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40

30

20

10

0 20 40 60 80 100

1) What is the speed during the first 20 seconds?

2) How far is the object from the start after 60 seconds?

3) What is the speed during the last 40 seconds?

4) When was the object travelling the fastest?

Distance

(metres)

Time/s

0.5m/s

40m

1m/s

40-60s

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40

30

20

10

0 20 40 60 80 100

1) What was the velocity in the first 20 seconds?

2) What was the velocity between 20 and 40 seconds?

3) When was this person travelling the fastest?

4) What was the average speed for the first 40 seconds?

Distance

(metres)

Time/s

1.5m/s

0.5m/s

80-100s

1m/s

20/04/23Distance-time graph for changing speeds

40

30

20

10

0 20 40 60 80 100

Distance

(metres)

Time/s

Object is speeding up

here Object is slowing

down here

20/04/2320/04/23Speed 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

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Speed vs. VelocitySpeed vs. Velocity

1) Is this car travelling at constant speed?

2) Is this car travelling at constant velocity?

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Speed vs. VelocitySpeed vs. Velocity“Speed” means “how fast you are going”, “velocity” means “how far you are going in a certain direction”. If the following journeys take 1 second then work out:

Start

1 metre-1 metre

Speed =

Velocity =

Speed =

Velocity =

Speed =

Velocity =

Speed =

Velocity =

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AccelerationAcceleration V-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?

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

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

2m/s2

120m/s

5s

2000m/s2

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AccelerationAcceleration V-U

TA

Acceleration = change in velocity (in m/s)

(in m/s2) time taken (in s)1) Will accelerates from standstill to 50m/s in 25 seconds.

What is his acceleration?

2) Pierre accelerates at 5m/s2 for 5 seconds. He started at 10m/s. What is his new speed?

3) Elliott is in trouble with the police. He is driving up the A29 and sees a police car and brakes from 50m/s to a standstill. His deceleration was 10m/s2. How long did he brake for?

4) Another boy racer brakes at the same deceleration but only for 3 seconds. What speed did he slow down to?

2m/s2

35m/s

5s

20m/s

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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 =

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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?

10 20 30 40 50

Velocity

m/s

T/s

40m/s

2m/s2

3m/s2

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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 40 to 50s?

10 20 30 40 50

Velocity

m/s

T/s

10m/s

4m/s2

6m/s2

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20

10

0

-10

-20 20 40 60 80 100

Velocity

(metres)

Time/s

1) When did the object have zero acceleration?

2) What is the average acceleration from 0 to 40s?

3) What was the acceleration from 40 to 60s?

4) When did the object change direction?

A higher tier question…

20 - 40s

0.5m/s2

2m/s2

50s

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P4.2 What are Forces?

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Introduction to ForcesIntroduction to ForcesA force is a “push” or a “pull”. Some common examples:

Weight (mg) – pulls things towards the centre of the Earth

Air resistance/drag – a contact force that acts against

anything moving through air or liquid

Upthrust – keeps things afloat

Friction – a contact force that acts against anything

moving

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Paired forcesPaired forces

The Earth pulls Newton down with a gravitational force of 700N.

Newton pulls the Earth up with a gravitational force of 700N.

what on what

direction

type

size

Action and reaction are equal and opposite!!

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Free body force diagramsConsider a man on a sloping table:

Notice that the reaction force always pushes up on the object and the friction force always acts up the slope!

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Free body force diagrams1) Draw a free body force diagram for a ladder against a wall.

2) A car pulls a caravan along the M25. Draw a free body force diagram for the caravan.

4) Draw a free body force diagram for a 2-wheel drive (engine at the front) car driving up the M1 as well.

3) Draw a free body force diagram for a 4-wheel drive car driving up the M1.

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Action and Reaction in RocketsConsider a rocket leaving the Earth’s atmosphere:

A chemical reaction happens inside the rocket that ______ out exhaust gases. There is an equal and ______ reaction to this force – this _______ acts on the rocket and pushes it _______. The same process happens in ____ engines.

Words – jet, pushes, reaction, upwards, opposite

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P4.3 Forces and Movement

20/04/2320/04/23Balanced 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.

20/04/2320/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

What would happen if we took the road away?

Weight

Reaction

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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”).

20/04/2320/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

20/04/2320/04/23Balanced and unbalanced Balanced and unbalanced forcesforces

1) This animal is either ________ or moving with _______ _____…

4) This animal is also either _______ or moving with ________ ______..

2) This animal is getting ________…

3) This animal is getting _______….

Words - Stationary, faster, slower or constant speed?

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SummarySummaryIf an object is stationary and has NO resultant force on it the object will…

If an object is stationary and a resultant force acts on it the object will…

If an object is already moving and NO resultant force acts on it the object will…

If an object is already moving and a resultant force acts on it the object will…

Complete these sentences…

…continue to stay stationary …accelerate in the direction of the resultant force

…continue to move at the same speed and the same direction

…accelerate in the direction of the resultant force

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Resultant ForceResultant ForceCalculate the resultant force of the following:

500N 100N 700N 600N

700N 700N

200N

800N 800N

100N

50N

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MomentumMomentumAny object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula:

Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s)

P

VM

What is the momentum of the following?

1) A 1kg football travelling at 10m/s

2) A 1000kg Ford Capri travelling at 30m/s

3) A 20g pen being thrown across the room at 5m/s

4) A 70kg bungi-jumper falling at 40m/s

10kgm/s

30,000kgm/s

0.1kgm/s

2800kgm/s

20/04/2320/04/23Force and change in Force and change in momentummomentum

To calculate the change the momentum of an object you need to know two things – how much force acts and how long it acts for:

mv-mu

TF

Force = Change in momentum Time(in N)

(in kgm/s)

(in s)

For example, Ronaldo takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate:

1) The change in momentum of the ball (its impulse),

2) The speed the ball moves away with

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Example questionsExample questions1) Paddy likes playing golf. He strikes a golf ball with a

force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed.

2) Courtney thinks it’s funny to hit tennis balls at Kit. She strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed.

3) Tom takes a dropkick by kicking a 0.4kg rugby ball away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball.

4) Jenny strikes a 200g golf ball away at 50m/s. If she applied a force of 50N calculate how long her club was in contact with the ball for.

16Kgm/s, 80m/s

4.5Kgm/s, 18m/s

40N

0.2s

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Safety featuresLet’s use this equation to explain how airbags work:

mv

TF

Basically:

1) The change in momentum is the same with or without an airbag

2) But having an airbag increases the time of the collision

3) Therefore the force is reduced

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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, balance,

accelerates

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Terminal VelocityTerminal Velocity

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 ________ _______.

Words – slowing down, decrease, increases, terminal

velocity, weight

20/04/23P4.4 Describing Motion with Energy Changes

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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

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Example questionsExample questions

1) Bex drives her car at a speed of 30m/s. If the combined mass of her and the car is 1000kg what is her kinetic energy?

2) Emma rides her bike at a speed of 10m/s. If the combined mass of Emma and her bike is 80kg what is her kinetic energy?

3) Rob is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running?

4) Josh is walking to town. If he has a kinetic energy of 150J and he’s walking at a pace of 2m/s what is his mass?

450,000J

4000J

5m/s

75kg

20/04/23Gravitational Potential Energy

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

GPE = Weight x Change in height

(Joules) (newtons) (metres)

GPE

Hmg

(Remember - W=mg)

20/04/23Some example questions…How much gravitational potential energy have the

following objects gained?:

1. A brick that weighs 10N 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 50cm by a friend.

How much GPE have the following objects lost?:

1. A 2N football dropping out of the air after being kicked up 30m,

2. A 0.5N egg falling 10m out of a bird nest,

3. A 1,000kg car falling off its 200cm ramp.

100J

20KJ

60J

5J

20KJ

350J

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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

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

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

2. Courtney lifts a laptop 2m into the air with a force of 10N. How much work does she do? What type of energy did the book gain?

3. Tom does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? What type of energy did the wheelbarrow gain?

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

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

25J

20J, GPE

4m, KE

50N

2MJ

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Stopping a car…Stopping a car…What happens inside the car when it stops?

In order to stop this car the brakes must “do work”. This work is used to reduce the

kinetic energy of the vehicle.Amount of energy transferred

= work done

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An example question…An example question…

15m/s = 11.25m stopping distance

30m/s = 45m stopping distance (4 times greater)

This car can apply a maximum braking force of 10,000N. If the car’s mass is 1000Kg how far is its stopping distance when it is travelling at a speed of 15m/s (roughly 30mph) and 30m/s

(roughly 60mph)?

20/04/2320/04/23A Practical Example of Doing Work

Consider a rocket re-entering the Earth’s atmosphere:

The rocket would initially have a very high _______ energy. This energy would then _____ due to friction caused by collisions with _______ in the atmosphere. These collisions would cause the rocket to ____ up (_____ is “being done” on the rocket). To help deal with this, rockets have special materials that are designed to lose heat quickly.Words – work, kinetic,

particles, heat, decrease

20/04/2320/04/23Energy Changes in Roller Energy Changes in Roller CoastersCoasters1) 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

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Dropping objectsIf I drop this ball 1m how fast will it be

going when it hits the floor?

Use GPE at top = Kinetic energy at bottom

mgh = ½mv2

gh = ½v2

10 x 1 = ½ x v2

v2 = 20

v = 4.5m/s

1m

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Some Questions…Some Questions…1) Energy is always conserved, so what happens to the

electrical energy eventually?

2) If the height of the drop was 100m and assuming there was a 100% conversion from gravitational to kinetic energy, how fast was the car moving at the bottom of the ramp?