# Uniform Circular Motion

1. Homework Statement
A 0.150-kg ball on the end of a 1.10-m-long cord (negligible mass) is swung in a vertical circle.

2. Homework Equations
Total net force = mass*centripetal acceleration = mass*(velocity squared)/radius

3. The Attempt at a Solution

The question I have is that... I'm looking at the book Physics Volume I, 6th Edition...And for the radius, shouldn't it be 1.10m/2 to get the radius? On the book, they just have 1.10m for r...So I'm confused here since I thought 1.10 is the diameter of the cord, not the radius. Could someone clarify that?

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Hootenanny
Staff Emeritus
Gold Member
1. Homework Statement
A 0.150-kg ball on the end of a 1.10-m-long cord (negligible mass) is swung in a vertical circle.

2. Homework Equations
Total net force = mass*centripetal acceleration = mass*(velocity squared)/radius

3. The Attempt at a Solution

The question I have is that... I'm looking at the book Physics Volume I, 6th Edition...And for the radius, shouldn't it be 1.10m/2 to get the radius? On the book, they just have 1.10m for r...So I'm confused here since I thought 1.10 is the diameter of the cord, not the radius. Could someone clarify that?
The radius is the distance from the centre of a circle to any point on the circumference. Now, when the ball is being swung in a vertical circle it is *always* on the circumference of the circular path, agreed? So, what is the distance between the ball and the centre of the circle?

The radius is the distance from the centre of a circle to any point on the circumference. Now, when the ball is being swung in a vertical circle it is *always* on the circumference of the circular path, agreed? So, what is the distance between the ball and the centre of the circle?
Shouldn't it still be half the length of cord still since the center of the circle would be at the middle, so r should be half of the length....no?

Hootenanny
Staff Emeritus
Gold Member
Shouldn't it still be half the length of cord still since the center of the circle would be at the middle, so r should be half of the length....no?
If you were swinging a ball around on a piece of string, at which point would you hold the string?

If you were swinging a ball around on a piece of string, at which point would you hold the string?
AHA, that makes a lot more sense. So r would be the same even if the ball was in a horizontal circular motion, right?

Hootenanny
Staff Emeritus