Question

a horse needs continuos force in order to move a cart with constant speed . why ?
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Answer 1

See, This is an practical situation.
Here practically friction comes into play.
ACCORDING TO LAWS OF FRICTION, THE FRICTIONAL FORCE IS MAXIMUM AT THE POINT WHEN THE BODY TENDS TO MOVE OR SLIDE.
That point where the body tends to movie is called limiting friction
so,
A horse need continuous force until it crosses the limiting friction to move the cart