# Homework Help: The child whirls a stone

1. Feb 15, 2005

### waterchan

I just began university physics, and one homework problem is driving me nuts!

A child whirls a stone in a horizontal circle 1.9 m above the ground by means of a string 1.4 m long. The string breaks, and the stone flies off horizontally, striking the ground 11 m away. What was the centripetal acceleration of the stone while in circular motion? (Neglect air resistance.)

I assume the major challenge involves projectile motion, and once the initial velocity of the stone when the string snapped is found, the rest is a matter of plugging in that and the radius of the circle in the centripetal acceleration equation. I have tried drawing diagrams, using all the projectile motion equations to form enough equations to solve for the unknowns, but my intermediate steps turn out to be inconsistent.

I've been pondering this problem for 2 hours straight now, and my pages are smeared with eraser marks. Please help!

2. Feb 15, 2005

### learningphysics

Lets look at the motion once the string breaks.

What is the equation for vertical displacement?

What is the equation for horizontal displacement?

These two should allow you to determine the initial velocity. Note that it was traveling in a horizontal circle. So the vertical component of initial velocity is 0.

You should be able to get two equations with two unknowns (t - time, and vh - horizontal velocity).