P4 revision

Forces and motion

Interaction pairs

Forces are equal in magnitude(size)

Forces act in oppostie directions

Each force acts on a different body.

Question

A boy kicks a ball in the park. Discuss the situation.

Include a point about interaction pairsName the other force.

(2marks)

Moving objects

In this situation we still have our interaction pair. Having an interaction pair does not mean that you will not get a resultant force.

It does mean that you get friction, there are three types of friction :

Friction:

Friction between solid surfaces which are gripping. Eg. Earths crust.

Friction between solid surfaces which are sliding past each other. Eg. Pieces of a car engine.

Friction or drag from from fluids(liquids or gases)

Question

The overall force on a rocket at takeoff is an upwards force. Explain this using the idea of interaction pairs and resultant forces.(3marks)

Speed

Units

In physics, units are really important. Although you are used to using mph in everyday life in physics we use m/s or metres per second. Distance is measured in metres and time in seconds.

Look for tricky questions where you are given distance in km or time in minutes or hours. You might need to convert. Look at the units of the answer for guidence.

Distance- Time graphs

A

B

C

D

Speed – Time graphs

Velocity- Time graphs

Velocity (m/s)

Time (s)

A

B

C

DE

Speed

Average speed: This is the distance travelled in a given time.

Instantaneous speed: This is the average speed but over a very small time. (used in spee cameras)

Velocity: Speed in a given direction. Eg. 5m/s north. Velocity is a vector as it has a length an a dircetion. Speed is scalar.

Techographs

Question...

Give two reasons why haulage businesses fit their lorrys with tachographs. (2 marks)

Momentum

P= momentum (kgm/s)

m= mass (kg)

v = velocity (m/s)

Again, you will need to be careful with your units here. You might be given something in g, but we use kg!

Conservation of momentum

The momentum before and interaction and after an interaction is alwas the same. So if it is zero before say, a car accident, it must be zero afterwards. This is achieved by a change of mass or velocity.

Example

Mass = 30kg Mass = 20kg

We can take this a before. They are not moving. Therfore their velocity is zero, and so is their momentum.

mxv + mXv = p

(30)(0) + (20)(0) = 0kgm/s

If they then push away from each other so they move in opposite directions. The child in the pink coat has a velocity of 2m/s what is the velocity of the child in the red coat?

After

mXv + mXv = P

(30)(2) + (20)v = 0 kgm/s

60 = -20v As the momentum v = -3m/s is conserved.

Change in momentum

If the resultant force on an object is not zero, its momentum changes in the direction of the force.

Change in momentum = Force X Time

Again be careful with units. Force should be in newtons and time in seconds.

Car safety

If someone’s momentum changes very quickly, the forces on the body will be very large and more likely to cause injury.

We want the change in momentum to take place over the largest time possible so the force is minimal.

In a car we have many safety features to try and do this.

Safety features

Crumple ZonesAirbagsSeat beltsBike and motorcycle helmets

Question

A force of 500N acts on a car of mass 1500kg for half a minute. What is the speed of te car after this time?

Clue you need to use both equations.

Work Done

Work done = Force applied x Distance moved

Question: If a force of 1500N is applied to a wall however the wall cannot be moved, what is the work done?

Work done is measured in joules (J)

Work done

Work done = Energy Transferred

Energy is also measured in joules and this is a really important equation. It will be on the cover of your exam paper but it is easly forgotten and often examined without being directly asked. You might be asked for an assumption you made or why the work done is not equal to the energy transferred.

Kinetic Energy

KE= ½ m X v2

Energy is always conserved (it cannot be created of destroyed) but it is often lost as heat or sound. (if friction is causing a loss this is heat too) So in reality the kinetic energy is a little less than the energy transferred.

Gravitational Potential Energy

GPE = m x g x h

g is the gravitational field strength and at GCSE is rounded to 10N/kg.

GPE to KE

Falling objects convert GPE to kinetic energy.

Kinetic energy = Potential energy

Gained Lost

This is a commonly asked question

Example

A rolercoaster carriage has a weight of 5000N (a mass of 500kg) and is 20m above its lowest point. Assuming the roller coaster is stationary at its greatest height, what is the speed of the carriage when it reaches its lowest point?

(These questions are often asked in stages)

Steps

1. Write down what your are given to see what equations to use.

Mass = 500kgWeight (mass x g) = 5000NH = 20mPossible eqns: KE= ½ mv2

GPE = WhGPE lost = KE gained.

Step 2

So we can calculate GPE and then because this is equal to KE gained, the KE.

GPE = Wh(5000)(20) = 100,000JTherefore KE gained = 100,000J

Step 3

Ke= 100,000JM= 500kgV= ?We need to rearrange our equationKE = ½ mv2

2KE = mv2

2KE = v2 √ (2KE/m) = v m

Answer

√ (2KE/m) = v

√((2 x 100,000)/500) = v

v = 20m/s

These are the hardest questions you get asked.

Questions

A 600kg(6000N) rollercoaster carriage is travelling at 40m/s. What is the maximum height it could climb if all its kinetic energy is transferred to gravitational potential energy?

(this is the opposite)

A trolley is stationary at the top of a hill. The trolley weighs 200N(mass 20kg) and the hill is 50m high. Assuming all of its GPE is converted into KE, how fast will the trolley be going when it reaches the bottom of the hill?