Question

In which case of a moving body, force is not needed?

• A. To increase the speed of the body
• B. To decrease the momentum of the body
• C. To change the direction of motion
• D. To keep the body in uniform velocity

Answer 1

The correct option is D To keep the body in uniform velocity

According to law of inertia or Newton's first law of motion, a body in state of rest or of uniform motion remains in state of rest or of uniform motion unless it is acted upon by an external force. Hence a body in uniform motion required no force to keep them in uniform motion.
Given that force = mass x acceleration
Constant speed means there is no acceleration, therefore if acceleration = 0 force is also 0.
So a force is not necessary to maintain constant speed, assuming no friction.
In deep space, for example, objects will zoom along at a constant speed with no force on them whatsoever. That results from Newton's First law of motion, which states a body in motion tends to stay in motion unless acted on by a net force.
But in most cases, when v = constant and it requires a force F to maintain that constant speed, it's because there is a braking force of friction or air drag trying to stop the body from moving. And that's why a push or pull is necessary to maintain a constant speed, to offset those braking forces.