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

Newton's first law of motion states that when there is no net external force, an object will remain at rest or travel with a constant velocity. 
When an object is moving, there is a force of friction resisting the motion, and slowing it down. To maintain a constant velocity, a person must push with a force equal to that of friction, to counteract the frictional force. 
Since the pushing force cancels the friction force out, there is no net external force, and so the object moves at a constant velocity

Hope I helped! xx
P.S.: Think this question is more suited to be asked under the "Physics" section