Weight and the effects of forces

Effects and balance of forces

Several forces can act at the same time

On a given object, it is common for several forces to act simultaneously. For example, a book resting on a table is subject to:

its weight P, which pulls it downward (a force at a distance exerted by the Earth),
the force exerted by the table, which pushes it upward (a contact force, called the support's reaction).

When forces cancel each other out

If an object is stationary (at rest) or moves in a straight line at constant speed, we say that the forces acting on it cancel each other out. This means their overall effect is zero.

For two forces to cancel each other out, they must have:

the same direction,
opposite senses,
the same value (magnitude).

Example of the book on the table

Force	Direction	Sense	Value
Weight P	vertical	downward	5 N
Table's reaction R	vertical	upward	5 N

Since P and R have the same direction, opposite senses and the same value, they cancel each other out: the book remains stationary.

When forces do not cancel each other out

If the forces do not cancel each other out, the object's motion changes: it can start, accelerate, slow down or change direction. For example, when you release a ball in the air, only its weight acts on it (nothing balances it): it falls and accelerates toward the ground.

Classic pitfall to avoid