Setup: A block is pushed across a rough horizontal surface by a varying applied force.

Observations:

• The normal force n is always equal to the weight of the block mg since the block only moves horizontally, i.e. the block's acceleration in the y-direction is zero.
• Until the applied force F_app becomes larger than the maximum static friction m_smg, the static friction is equal to the applied force. In this case the block does not move and the net force F_net is zero.
• After the applied force is larger than the maximum static friction the net force of the block become positive and the block begins to accelerate. Note that the friction drops to lower value of kinetic friction m_kmg and remains constant until the block comes to a stop.
• As long as the block is moving the kinetic friction remains constant independent of the magnitude of the applied force or the block's speed or the block's acceleration.