Physics GCSE Revision

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P4 Explaining Motion

-Speed= Distance / Time   eg. A cat skulks 20 metres in 40 seconds. Find: a) its speed b) how long it takes to skulk 75m.       answer: using the formula: a) s=d/t= 20/40 = 0.5m/s  b) t=d/s= 75/0.5= 150s = 2 mins30s  

-Speed itself is just a number, Velocity has direction too!!!  (velocity can be +tive or -tive)    eg2 hamsters strap themselves to rockets and set off to the north and south pole to escape global warming. hamster 1 has a velocity of 200 m/s due north, whereas hamster 2 has avelocity of 230 m/s due south. another way of saying this is that hamster 1's velocity is +200 m/s and hamster 2's is -230 m/s.



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P4 Explaining Motion

The green arrow shows the force on the weights as the weightlifter pushes upwards.

The red arrow shows the downwards force on the weightlifter’s arm muscles.

These two forces are an interaction pair. They are equal in size, and opposite in direction.

You only have an interaction pair if the forces are caused by the interaction. In this case, the compression in the weightlifter’s muscles is caused by the weight pushing down, and the upwards force on the weight is caused by the weightlifter’s muscles.

 Stick figure holding up weight (

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P4 Explaining Motion

Sometimes a force is produced as a response to another force – these are not the same as interaction pairs.

A book on a table has a downwards force (its weight) due to gravity.

This downwards force, pushing on the table, produces an upwards force called reaction.

Diagram showing reaction forces - the downwards force is weight caused by gravity and this produces an upwards force called a reaction force (

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P4 Explaining Motion

Another common force is friction.

When two surfaces slide past each other, the interaction between them produces a force of friction.

In this diagram, the book is moving to the right across the table as shown by the red arrow.

Diagram showing interaction pairs: The book is moving to the right and this produces friction on the table in the same direction as the book is moving (to the right) and friction on the book in the opposite way to the direction the book is moving (to the left). The two friction forces are an interaction pair. (

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P4 Explaining Motion

Sometimes several forces act on the same object. Look at this diagram of a moving car:


There are several forces acting on the car, shown by the arrows.

  • Gravity pulls down on the car.
  • The reaction force from the road pushes up on the wheels.
  • The driving force from the engine pushes the car along.
  • There is friction between the road and the tyres.
  • Air resistance acts on the front of the car.
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P4 Explaining Motion

Resultant force

The resultant force is the sum of all the different forces acting on the car.

You have to take account of the directions – the reaction forces on the wheels (blue arrows) add up to the same as the weight (green arrow), so these cancel out.

The driving force from the engine (yellow arrow) is in the opposite direction to the counter forces of friction (red arrows) and air resistance (purple arrow).

When the car is increasing its speed then all these forces add to give a single resultant force forwards.

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P4 Explaining Motion

  • If the car is already moving, it will carry on at a steady speed in a straight line.
  • If the car is not moving, it will stay still.


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