Contact and Non-contact Forces
Some forces, like friction, act when two objects are touching. Others, like gravity and the electrostatic force, act at a distance.
Weight vs Mass
The force on an object due to gravity is called its weight. The size depends on its mass (which is the same anywhere in the universe) and how strong the gravitational field strength is at that point.
Friction and Drag
When two objects that touch move past each other there is a force that opposes their motion - we call this friction, drag or air resistance.
The Normal Contact Force
Even stationary objects, like your chair or the ground beneath your feet, exert an upwards force which we call the normal contact force (sometimes called the reaction force).
Identifying Forces: Example A
Now it's time to put your new knowledge into practise with a couple of simple examples.
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