Suppose there is a crate in contact with the ground when an external force directed to the right is applied to it. I understand that if this external force does not exceed the maximum static frictional force: 1. the crate will not move and 2. the static frictional force will equal the external force being applied.
However, lets say that the external force is greater than the maximum static frictional force and the force is applied at a time t = 0s. It is clear now that at time t = 0s the crate will have some acceleration to the right.
My confusion is that since we know the crate will move, if a free body diagram of the forces acting on the crate at time t = 0 was drawn would the forces acting in the horizontal direction be:
- the max static friction force acting to the left and the external force to the right or
- since we know the crate will move, the kinetic friction force to the left and the external force to the right.
My reasoning is that since at time t = 0s, the crate is still at rest, we cannot use kinetic friction since the surfaces (ground and crate) are not sliding over one another. Using this reasoning the forces acting on the crate at t = 0 must be the max static friction force and the external force. Is this correct?