Answer:
This is because the heavy carton requires a net force to move.
Explanation:
When the carton is initially at rest, its acceleration is zero. A force F which is greater than the static frictional force, f on it has to be exerted on it to produce a net force which will thus produce a net acceleration.
So, F - f = ma. So, F = f + ma.
So, the force applied, equals the net force plus the frictional force on the heavy carton.
At constant velocity, the acceleration is zero and thus the net force required to keep the object in motion just balances the kinetic friction on it. If F is the force applied and f the kinetic friction, F - f = ma = 0. So, F = f which is less than calculated above.
So, a heavy carton is more difficult to move from rest than to keep pushing it with a constant velocity because, a net force is required to start moving the heavy carton from rest,and no net force is required to keep pushing it at constant velocity.
