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Box Against a Wall
Suppose we take a box and place it against a wall and remove our hand? The box will begin to travel downward, in the negative direction along the y axis. But why does this happen? If we examine the forces that are acting on the box, we will see that the only force acting on our object is the force of gravity pulling the box downward. Now, if we take the same box and push the box against the wall, the box will not move as long as our applied force is great enough. What keeps the box from falling back to the ground? According Newton's First Law of Motion, there must be some force that is opposing the force of gravity that is keeping the box up. This force is known as static friction and arises from the normal force that is exerted on the box as a result of your applied force. The pushing force that you apply onto the box creates an action-reaction force that the table exerts back on the box. This normal force in turn creates the static friction force that keeps the object stationary. Note that if your applied force is not great enough, the static friction force will be less than the gravitational force, in which case the object will fall.
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