[jason0802]

Tie an object such as a pencil eraser to a string. Use the string to swing the object around your head at a constant speed. Can the object be swung so that the string is parallel to the horizontal, flat, and even floor on which you stand?

I don't know how to explain in physics, please help!

[ebaines]

Think about the forces acting on the object: there is a component downard (due to gravity) and a component outward (due to the centripetal acceleration of the object). These two forces add together to form a total force which the string must resist. Since a string is incapable of resisting anything but tension (can't resist bending forces or compression forces) this means that the directoin of the string must be exactly in line with the sum of the forces that are pulling on it. So the question boils down to this: if you add the forces of gravity and centripetal aceleration on the object, can the resultant force ever be in a direction parallel to the floor?

Jason: Post back with what you think the answer is.

[jason0802]

The answer is no!
Thank you so much ebaines, it is big help!