Here's why:

* Forces are vectors: Forces have both magnitude (strength) and direction.

* Cancellation: When two or more forces act on an object in opposite directions, their magnitudes can cancel each other out, resulting in a net force of zero. This means the object will either remain at rest or continue moving at a constant velocity.

Example:

Imagine you push a box to the right with a force of 10 Newtons, and your friend pushes the box to the left with a force of 10 Newtons. These forces cancel each other out, and the box will not move.

Key Point: Forces only cancel each other out if they act in opposite directions and have equal magnitudes. If the forces are not equal or not in opposite directions, there will be a net force, causing the object to accelerate.