Why does a ball on a string fly off when you let go?

Whirl a ball around and around, then suddenly let go. It rockets away — but in which direction? Almost everyone guesses wrong. Let's whirl one, make a guess, then cut the string and watch what really happens.

1The two things going on

A moving thing wants to go straight

There are only two ideas here. Watch each one on its own:

Things keep going straight

A moving thing travels in a straight line at the same speed, unless something pushes or pulls it. Nothing pulls it? It just keeps going straight. (Grown-ups call this inertia.)

The string pulls inward

The string always pulls the ball toward your hand — inward, to the center. That tug is the only thing bending the ball off its straight path and into a circle.

2The big question

So when you let go… which way?

Here are the two ideas people have. We're not saying which is right yet — that's for you to test.

guess A

Straight outward

The ball was being held in, so the second it's free it shoots straight away from the center, like it was being thrown outward.

guess B

Straight sideways

The ball keeps going the way it was already moving — sideways, along the edge of the circle — and carries on in that straight line.

3Your turn — be the hand

Whirl the ball yourself

Slide to set how fast you whirl it. Watch the violet string pull the ball inward every moment, bending its path into a circle.

Whirl speed: medium
SLOWFAST
the ball the string (pulls inward)

4Now cut the string

Guess first, then snip ✂️

You're going to cut the string mid-whirl and watch where the ball really goes. But guess before you snip — no peeking!

Guess before you cut

The ball is whirling in a circle. You cut the string. Which way does the ball fly?

5But wait — two honest catches

Two true things that make it tricky

It really feels outward

Your hand does have to pull hard to hold a fast ball, and it feels like the ball is yanking outward.

The truth: that hard pull is your hand pulling the ball inward. The outward feeling is just the ball trying to go straight while the string keeps tugging it back.
"Straight" only lasts a blink

The instant after you cut it, the ball flies in a perfectly straight line sideways.

The catch: then gravity starts curving it downward and air slows it — so it's a straight launch, not a forever-straight line.

Nothing throws the ball outward. The string was pulling it inward the whole time — so the moment you cut it, the ball just keeps going straight sideways, the way it was already moving.

Psst, grown-ups: circular motion needs a continuous centripetal (center-pointing) force to supply the inward acceleration v²/r — here, the string's tension. There is no real outward force on the ball; the "centrifugal force" you feel is a fictitious force that appears only in the rotating frame. Remove the tension and Newton's first law takes over: the ball departs tangentially at its instantaneous velocity (after which gravity and drag act on it).