# How to find the velocity of a pendulum need help

1. Feb 9, 2010

### grr_physics

How to find the velocity of a pendulum...need help!!

1. The problem statement, all variables and given/known data
A 3.60 m long pendulum with a 1.25 kg bob is pulled sideways until it is displaced 1.80 m horizontally from its rest position. If the bob is released, calculate the speed of the bob when it is 0.250 m about it's rest position.

2. Relevant equations

I found the potential energy when it passes through rest position= 5.91 J
and the velocity when it passes through rest position is 3.08 m/s. I don't know how to find it at 0.250 m & need help please!!

2. Feb 9, 2010

### grr_physics

Need help to find the velocity of a pendulum at a distance please!!

1. The problem statement, all variables and given/known data
A 3.60 m long pendulum with a 1.25 kg bob is pulled sideways until it is displaced 1.80 m horizontally from its rest position. If the bob is released, calculate the speed of the bob when it is 0.250 m about it's rest position.

2. Relevant equations

I found the potential energy when it passes through rest position= 5.91 J
and the velocity when it passes through rest position is 3.08 m/s. I don't know how to find it at 0.250 m & need help please!!

3. Feb 9, 2010

Re: How to find the velocity of a pendulum...need help!!

I think you mean you found the kinetic energy when it passes throught the rest position this being equal to the loss of potential energy.Use the same method,find the vertical distance moved when the horizontal displacement goes from 1.8m to 0.25m and use PE lost=KE gained.

4. Feb 9, 2010

### tiny-tim

Welcome to PF!

Hi grr_physics! Welcome to PF!
You don't need to do that …

just use conservation of energy (and a bit of elementary trig).

5. Feb 9, 2010

### grr_physics

Re: Need help to find the velocity of a pendulum at a distance please!!

so do i do this:

Ep = Ek
mgh = 1/2mv^2
gh = 1/2v^2
(9.81)(0.250)=1/2v^2

and keep going til i get the answer? where do i do trig?

6. Feb 9, 2010

### grr_physics

Re: How to find the velocity of a pendulum...need help!!

so do i do this:

Ep = Ek
mgh = 1/2mv^2
gh = 1/2v^2
(9.81)(0.250)=1/2v^2

and keep going til i get the answer?

7. Feb 9, 2010

### tiny-tim

That's right.

And you'll need the trig to find the height when the horizontal displacement is 1.80 m

8. Feb 9, 2010

### grr_physics

Re: Need help to find the velocity of a pendulum at a distance please!!

thanks! :)

9. Feb 9, 2010

### Staff: Mentor

Re: Need help to find the velocity of a pendulum at a distance please!!

Two threads merged into one. Please do not multiple post your questions, grr.

10. Feb 9, 2010

### grr_physics

Re: How to find the velocity of a pendulum...need help!!

sorry, promise i won't anymore!!