# Uniform circular motion - origin of speed

## Main Question or Discussion Point

I am having trouble understanding tangential speed as it applies to uniform circular motion and I would appreciate any help.

Say I have a ball on the end of a string that I hold with my hand and rotate at a constant speed. If the string provides the centripetal force and acceleration, where does the speed come from? what provides it?

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TSny
Homework Helper
Gold Member
Notice what your hand is doing while you are bringing the ball from rest up to its final speed.

As I am bringing it to its final speed, my hand is moving in arc. So my hand is also providing the speed?

TSny
Homework Helper
Gold Member
As I am bringing it to its final speed, my hand is moving in arc. So my hand is also providing the speed?
Yes. As the ball is first brought into motion, the string is not perpendicular to the velocity.

Look at this video at about time 45 seconds.

Here is a snapshot soon after the ball is first set into motion.

You can see that the string is not perpendicular to the velocity.

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Awesome. Thanks a lot, your vector pic. really helped.