# Homework Help: Pendulum friction problem

1. May 23, 2010

### cheerspens

1. The problem statement, all variables and given/known data
The attached figure shows a 2.0 kg metal ball suspended by two strings.
If the string on the left breaks anf friction can be neglected, how fast will the ball be moving at point A?

2. Relevant equations
GPE=mgh
KE=(1/2)mv2

3. The attempt at a solution
Is it possible to solve this problem without knowing the height from the ground that the ball is located?

#### Attached Files:

• ###### photo.jpg
File size:
36 KB
Views:
153
2. May 23, 2010

### Integral

Staff Emeritus
Re: Pendulum

Yes, all you need is the change in height.

3. May 24, 2010

### cheerspens

Re: Pendulum

Is there any way to solve without it because there was no height mentioned in the given problem?
Besides the fact that the ball is suspended 0.4 meters below its support and the string is 0.6 meters long.

Last edited: May 24, 2010
4. May 24, 2010

### jupiter13

Re: Pendulum

You are on the right track in using trig to solve for the angle (based on the information you added to the diagram). You only need to look at the y-component of the triangle that the string makes with the top surface. That is what your height change is.

5. May 24, 2010

### cheerspens

Re: Pendulum

So I have mgh=mgh+.5mv2 and I need to solve for v. How do I know what h (the height) is on either side of the problem? Is is 0.4 meters?

6. May 24, 2010

### sentient 6

Re: Pendulum

why is mgh = mgh + 0.5mv^2 ? at A the pendulum is at maximum K.E. while at the initial displacement and B the pendulum has max P.E. so 0.5mv^2 = mgh. you can find h as the length of the pendulum is 6 and the perpendicular height is 4 at the initial. So using that info. solve it all for v.