Interactions and Forces
Forces happen because of an interaction between two objects. They happen in pairs.
- the two forces in an interaction pair are always the same size.
- the two forces in an interaction pair act in opposite directions.
- the two forces in an interaction pair act on a different object.
This is the first law of motion:
If the resultant force acting on an object is zero, its momentum will stay the same.
This means that a stationary object will stay where it is. It will not start moving if there is no resultant force. A moving object will carry on at the same speed. In both cases the momentum does not change because the resultant force is zero.