Does a moving object need a force to keep moving?
Short answer: Newton's first law: force changes motion. Constant velocity needs zero net force.
The trap
The wrong idea
“A moving object needs a constant force to keep moving.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
A puck slides on perfectly frictionless ice. To keep it moving at constant velocity you need:
- A constant push
- No force at all
- An increasing push
Show the answer
B. No force at all
Newton's first law: force changes motion. Constant velocity needs zero net force.
The click
This was Aristotle's idea, and daily life seems to back it: stop pedalling and a bicycle slows to a stop. But it stops because friction and air drag are forces acting on it, slowing it down. Your pedalling only cancels them.
Newton's first law says an object stays at rest or keeps moving in a straight line at constant speed unless a net external force acts on it. Force is what changes velocity (F = ma), not what maintains it. If the net force is zero, the acceleration is zero, so the velocity stays constant, whatever that velocity is.
On perfectly frictionless ice nothing opposes the puck's motion, so it needs no push at all to keep sliding at constant velocity. A constant push would make it accelerate steadily, and an increasing push would make its acceleration grow.
The correct reading: constant velocity needs zero net force, not zero forces. A car cruising at steady speed has the forward push from the road on its tyres exactly balancing friction and air drag. Spacecraft such as Voyager 1 have coasted through space for decades without firing their engines to keep moving, because there is almost nothing out there to slow them. Ask "what changes the motion?" and look for the force there.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.