4.2 How forces act
 Forces are pushes or pulls that try to change the shape or
position of an object.
 Forces always act in pairs that work in opposite directions.
 If you are holding an object in the air, you are exerting an upward
force on it, but the object is also exerting a downward force.
 Force diagram show the direction and size of forces on an
object. Each force is shown as a force arrow. When the forces are
equal, the force arrows are the same length.
 The downward force on an object is caused by gravity pulling
it down.