You push a heavy crate. at first it doesn't move. you push harder, and it finally starts to move, but you still have to exert some force to keep it moving at a constant velocity. explain.

Explanation:

From the first law of motion given by Newton, we understand that an object stays in the state of rest or motion unless and until an unbalanced external force acts on it. This force required to change the state depends on the mass of the body. This is also known as law of inertia.

Here, a heavy crate does not move until enough force is exerted which overcomes the inertia. At rest, static friction acts. So, the applied force must be able to overcome inertia as well as static friction.

While in motion, kinetic friction acts. Kinetic friction is less than static friction and hence in order to maintain a constant velocity, some force needs to be exerted to keep the crate moving.