Physics P4

Physics revision P4

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  • To start anything moving, a force is required- eg. two boats on a lake without oars would move apart if one pushed away from the other. Both boats would move in opposite directions.
  • This is because forces always arise from an interaction between two objects.
  • They therefore work in pairs, and the two forces are equal in size and opposite in direction.
  • As well as this, the two forces act on different objects.
  • Anything can exert a force if its involved in an interaction- if someone wearing roller skates pushes against a wall, they immediately start to move backwards. The wall therefore exerts a force on them.
  • Where two interacting objects touch each other, the forces are known as contact forces. However, there are also some forces that act at a distance. These are caused by gravity and magnetism. However, all forces arise from interactions.
  • For example, if an apple falls from a tree, the force of gravity pulls it down towards the centre of the earth. However, gravity is an interaction between two objects, so the apple exerts an equal and opposite force on the Earth. However, this does not have much of an effect, as the Earth is too large.
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  • Normally when objects explode, the pieces trave out in all directions.  However, if the object is deseigned so that the pieces go in one single direction, it is a rocket.
  • Burning hot gases are pushed from the base of a rocket, thus making the object move in the opposite direction.
  • Jet engines use the same idea as rockets- air is drawn in and then pushed out at the back.  As a result, the engine moves forward.
  • When a car starts moving, there must be an interaction between the wheel and the ground.  The wheel causes a backward force on the ground surface.  The other force of the interaction pair is the forward force on the car.  If the surface is good, the force exerted by the wheel on the ground will be high.  This means that the car will move forward, as the forces are equal.  However, if the surface is bad, the wheel will not grip, and the car will not move forward.
  • When a person walks, they push back on the ground with each foot in turn.  The ground then pushes them forward.  However if a surface is slippery, it is much harder to push against, and walking becomes a lot more difficult.
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  • Friction is an unusual force, as it adjusts in size in response to the situation- up to  a limit.  This limit depends of the objects and the surfaces involved.
  • When two surfaces are put together, the bumps on one can fit into the hollows of the other (as no surface is completely smooth).  When one object has to slide over another, it must ride over the the bumps.  As surfaces are rough, they can only touch at a few points (when a bump on one meets a bump on the other).  So there are very few points of contact.  The pressure at these points is therefore very large (large enough to 'cold weld' them together).  When one surface slides over another, the tiny welds must be broken.  This requires a force.
  • If a workman pushes a boz with a force of 25N, to try and slide it along, it does not move.  This is because the friction exerted by the floor on the box is 25N.  The workman's push is balanced.  If he pushes with 50N, the box still does not move, as the friction has increased to 50N, thus balancing his push.  However, if the workman pushes with 70N, the box moves, as 70N is bigger than the maximum possible friction force for the box and the floor surface.
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Reaction of surfaces

  • If a ball is dropped, it falls to the floor due to the force of gravity.  However, if it is places on a table so that it does not roll around or move, it does not move.  This is because the table exerts an upward force on the ball that balances the force of gravity.
  • In the same way, if an object such as a bag is places on a foam surface, it will sink down until it reaches the point where the push on the foam exactly balances the downward pull of gravity.
  • When a hard surface such as a table is squashed (like the foam surface), it is not visible to the naked eye.  However, it does happen.  We call this upward force exerted by the table on the object the Reaction of a surface.
  • The table cannot always exert an upward force on an object to balance the downward force on it.  If the force exerted on the table goes beyond a certain limit, the table will break.  However, up to this point, it exerts an upward force that matches the downward force exerted on it.
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  • To calculate average speed, you must do speed= distance/time
  • However, very often the speed changes on a journey, and so the average speed is not particularly useful.
  • Instantanious speed can be measured by taking the average speed over very short time intervals.
  • Very often, graphs are used to describe a journey.  A distance-time graph shows how far a moving objects is from its starting pointat every instant during it's journey.
  • A speed-time graph shows the speed of the moving object at every interval at every instant during it's journey.
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