# P4 Explaining Motion Revision Notes

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P4 Explaining Motion

Forces and Friction

A force is a pull or push, but they never come on their own.

Remember that for every action there is a reaction.

Forces occur when two objects interact.

When an object exerts a force on another object, it always experiences a force in return, and this is called an interaction pair.

Interaction pairs are always:

• equal in size.
• opposite in direction.
• occur on two different objects.

This also applies to objects on a surface.

For example a man standing on a floor exerts a force downwards (due to his weight), and the floor exerts an equal force opposite in direction, i.e upwards and this is known as a reaction (reaction of a surface) force (remember the force is equal in size. If the upwards force was greater he would be flying!)

[ weight= mass x gravity ]

[ (N) (kg) (N/kg ]

Moving objects normally experience friction.

Friction occurs when two objects more relative to one another, and therefore both experience a force in the direction that opposes their movement.

Three types of friction are as follows:

• friction between solid surfaces which are gripping (static friction).
• friction between solid surfaces which are sliding past each other.
• restistance or "drag" from fluids (liquids or gases) (air resistance/water resistance)

Forces and Motion

Remember that arrows show the size and direction of a force.

The length of the arrow shows the size.

The direction of the arrow shows the direction fo the of the force (relative to the picture).

Resultant Force

The resultant force is the overall force acting on an object.

If there is a resultant force, than the object either accelerate or deaccelerate in that direction.

If there is no resultant force, then the object remains at constant speed (or may not move-depending on wherether it was moving in the first place!).

If forces are acting in the same direction - add them together.

If forces are acting in opposite directions - subtract them

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