Why does a ball on a string fly off the second you let go?
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Why does a ball on a string fly off the second you let go?
The short answer
When you cut the string on a whirling ball, it does not shoot straight outward — it flies off sideways, in a straight line along the direction it was already moving (the tangent). The string had been pulling the ball inward the whole time to bend its path into a circle, so the moment that pull is gone, the ball just keeps going straight.
How it works
A moving object keeps traveling in a straight line at a steady speed unless a force pushes or pulls it — that is inertia, Newton's first law. To make the ball go in a circle, the string constantly pulls it inward, toward your hand. That inward pull is the only real force bending the ball's straight-line motion into a curve. At every instant the ball is actually trying to head straight along the tangent. Cut the string and there is no longer anything bending its path, so it leaves on that tangent and continues in a straight line (until gravity curves it down and air slows it).
What people get wrong
Many people think a spinning thing is flung outward by an outward force, so they expect the ball to fly straight away from the center. There is no outward force on the ball. The 'centrifugal' push you feel is your body sensing the ball's resistance to being pulled inward, not a real force throwing it out. The only real force is the string pulling inward, and when it is gone the ball goes sideways, not outward.
The catch
It is true that the ball feels like it pulls outward, and your hand really does have to pull hard to hold it — that hard pull is the inward force doing its job. And the ball flies perfectly straight only for the first instant after release; in the real world gravity then bends its path downward and air resistance slows it, so it is a straight launch rather than a straight line that lasts forever.
Questions kids ask
Which way does a ball on a string actually go when the string snaps?
It flies off sideways along the tangent — the exact direction it was moving at the instant the string broke — in a straight line. It does not shoot straight outward from the center.
If nothing pushes the ball outward, why does it feel like it pulls away from my hand?
What you feel is the ball resisting being pulled inward. Your hand pulls the ball toward you to keep bending its path into a circle, and that pull is what you feel as a tug. The ball is not being pushed outward by a force; it is just trying to go straight while the string keeps redirecting it.
What is the string actually doing while the ball spins?
The string pulls the ball inward, toward the center, the entire time. That steady inward pull is what curves the ball's natural straight-line motion into a circle. Without it the ball would simply travel in a straight line.
Does the ball keep going straight forever after the string is cut?
Only for an instant. With no string, no other sideways force acts, so it leaves in a straight line. But gravity soon curves its path downward and air resistance slows it, so on Earth it traces an arc as it flies away.
For grown-ups
Circular motion requires a continuous centripetal (center-pointing) force to supply the inward acceleration v²/r. Here that force is the string's tension. There is no real outward force acting on the ball; the 'centrifugal force' is a fictitious force that appears only in the rotating reference frame. When the tension is removed, Newton's first law governs: the ball departs tangentially at its instantaneous velocity, after which gravity and drag act on it.