# Basic acceleration question

1. Mar 19, 2009

### Lord Patrick

I have a very simple test question about acceleration. A ball is moving at a constant speed inside of a hollow sphere. The question is whether or not the ball is accelerating. I said that it was not accelerating because the ball is always moving forward, even though it is traveling in a circular path. My teacher said that it was accelerating but the only explanation that I was given was that the ball is moving in a circle. This is a very simple question, but it is bothering the heck out of me.

2. Mar 19, 2009

Staff Emeritus
What is the definition of acceleration?

What is the difference between speed and velocity?

3. Mar 19, 2009

### ImAnEngineer

If a ball with a mass m makes a circular motion, the direction of the velocity is constantly changing. In order to change the velocity of an object, a force is needed, and F=ma. If there were no acceleration, the ball wouldn't go in a circle, but simply in a straight line.

The acceleration we're talking about when some object goes in a circle is the centripetal acceleration. So you could say that the ball is accelerated to the center by the centripetal force.

4. Mar 19, 2009

### Bob S

Are you taking about a moving ball inside a stationary hollow sphere (in an inertial coordinate system)? Are you talking about a ball with a constant speed (a scalar) and not a constant velocity (a vector)? Is the ball rolling without slipping on the inside surface of the hollow sphere? Is the ball rolling without slipping on the inside surface of the sphere under the influence of gravity? Is the ball solid, with uniform density? Are you talking about the ball rolling in a horizontal plane only (no vertical motion)?

5. Mar 19, 2009

### tiny-tim

Welcome to PF!

Hi Lord Patrick! Welcome to PF!
Anything whose velocity is changing is accelerating …

so is its velocity changing?