# Forces and Collisions

A force is a push or a pull that changes an object’s momentum. Whenever an object’s speed or direction changes, it is caused by a force (or multiple forces) pushing or pulling on that object.

Take a look at our air particle trapped in a box again. Every time the air particle collides with a wall, it changes direction. To change direction, a force has to be pushing or pulling on the particle. In this case, when the particle collides with a wall, the wall exerts a force on the particle and pushes it in the opposite direction.

But that's not all. Every time the air particle collides with a wall, the air particle exerts an equal force of its own on the wall and pushes the wall in the opposite direction.

So if the particle is pushing on the wall just as hard as the wall is pushing on the particle, why isn’t the wall also changing speed or direction every time the particle collides with it? Well, generally walls have a lot more mass than an air particle, and they are usually attached to the ground. This means that it would take a lot of force to move them.

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