i About this experiment — click to learn the physics ▼
What you're looking at
A mass is whirled around a fixed centre on a string — like a ball on a rope or a stone in a sling. It moves at a steady speed, yet it is constantly accelerating, because its direction keeps changing. The orange arrow is the velocity (always tangent to the circle); the gold arrow is the inward force that bends the path into a circle.
Why a force is needed
Newton's first law says a moving object travels in a straight line unless a force acts on it. To keep the mass on a circle, something must continually pull it toward the centre — the centripetal ("centre-seeking") force. Here it's the string's tension; for a car it's friction, for a planet it's gravity. Cut the string and the force vanishes, so the mass flies off along the tangent — proof that the inward force was doing all the work of turning it.
Centripetal acceleration and force
Even at constant speed, changing direction is acceleration, directed toward the centre with magnitude:
By Newton's second law, the force required is mass times this acceleration:
Notice the force grows with the square of the speed and falls off with radius — doubling the speed needs four times the force, which is why fast tight turns are so demanding.
Angular speed and period
The angular speed ω = v/r is how fast the angle sweeps, and the period T = 2πr/v is the time for one full lap. The velocity vector always stays perpendicular to the radius.
Things to try
Raise the speed and watch the centripetal force shoot up (it's a square law). Increase the radius at fixed speed and the force eases. Then cut the string at different points and notice the mass always leaves along the tangent, never straight outward.