Question
A ball rolling on the ground slows down and eventually stops. Why doesn’t it keep rolling forever? What force is responsible?
Solution — Step by Step
The ball’s surface touches the ground as it rolls. Wherever two surfaces are in contact and there is relative motion between them, friction acts.
When a ball rolls, we call this rolling friction. The ball continuously presses into the ground surface slightly, and the ground pushes back — this resistance opposes the motion.
Rolling friction is weaker than sliding friction, which is why wheels were invented, but it is never zero for real surfaces.
The ball starts with kinetic energy (energy of motion). As rolling friction acts, it converts this kinetic energy into heat energy. You can feel this if you rub your palms together — that’s friction converting motion into heat.
Since energy is being lost from the ball, its speed decreases.
Friction keeps acting as long as the ball moves. The moment the ball slows to zero, there is no relative motion, so friction stops. The ball stays at rest.
The ball stops because friction continuously drains its kinetic energy until none remains.
Why This Works
Newton’s First Law says an object in motion stays in motion unless an external force acts on it. The ball would roll forever on a frictionless surface — this is not just theory. Ice surfaces or well-oiled ball bearings come close to this, which is why balls roll much farther on ice than on grass.
On a real ground surface, the irregularities (even microscopic ones) create resistance. Every tiny bump the ball has to climb over, every slight deformation of the ground — all of these take away a bit of kinetic energy.
The key insight is that friction here plays the role of an unbalanced force. It acts opposite to the direction of motion, decelerating the ball according to Newton’s Second Law: F = ma. Since F (friction) is negative relative to motion, a is negative — meaning deceleration.
Alternative Method
Think about it from an energy perspective instead of a force perspective.
The ball has kinetic energy:
Friction does negative work on the ball. Work done by friction = F × distance (in the opposite direction). This negative work reduces the ball’s kinetic energy.
When KE_final = 0, the ball stops. The energy hasn’t disappeared — it has been transferred to the ground and ball surfaces as heat, which is why the surfaces get slightly warm.
This energy approach is the same idea that comes back in Class 9 (Work and Energy) and in JEE thermodynamics. Friction always converts mechanical energy to heat — remember this connection.
Common Mistake
Many students write: “The ball stops because friction opposes motion.” This is incomplete. The examiner wants you to mention energy conversion — friction converts kinetic energy to heat energy. In CBSE Class 8 exams, answers that mention heat get full marks; answers that only say “friction opposes motion” often get partial marks.
Also, don’t confuse air resistance with ground friction here. Both act on a rolling ball, but ground friction (rolling friction) is the dominant reason. For a ball rolling on a smooth floor, air resistance is negligible at low speeds.