# Homework Help: Pendulum - energy

1. Apr 29, 2010

### Patrickas

1. The problem statement, all variables and given/known data
80kg mass man is swinging on a swing. His amplitude is 1m. in 60 seconds he does 15 swings. Find the potential and kinetic energy after 1/12 period.

2. Relevant equationsW(p)=mgh or kx^2/2 W(k)=mv^2/2 W(max)=Fr^2mg/2l

3. The attempt at a solution i Found out that maximum energy is 100J. But when i am calculating energy of petencial or kinetic i get 1/3 or 2/3 of maximum...or something even more horrid. =]

2. Apr 29, 2010

### Patrickas

oh c'mon!!! there must be someone out there knowing how to do this...

3. Apr 29, 2010

### tiny-tim

Hi Patrickas!

(try using the X2 tag just above the Reply box )
erm … you can't post around 2am New York time and expect an answer 4 hours later!!

anwyay, show us your full calculations, and then we'll see what went wrong, and we'll know how to help.

4. Apr 29, 2010

### Patrickas

yea, but there are alot more people out there not from new york, who might know.

5. May 1, 2010

### Patrickas

hmm. Anyone..?

6. May 1, 2010

### tiny-tim

Show us your full calculations, and then we'll see what went wrong, and we'll know how to help.

7. May 1, 2010

### Patrickas

T=60/15=4s
t=4/12=1/3s

So i know that W=kx^2/2 Well then i tried getting k, k=mg/l , now i needed the lengh of swings so i figured mv^2/r=mgr/l l=r^2g/v^2, now i need speed =D. v=s/t is s= 4*r? , or s=2*3.14*r? and t=1/3. Anyways i sence that something here is wrong. The numbers don't add up properly.

Assuming that s=2*3.14*r i can get T=2*3.14*$$\sqrt{\frac{l}{g}}$$ and get that l(t=4s)=4m Then i get that maximum energy is 100J. Now what...

Ahh... i used to calculate s=4r and not 2*3.14*r. I think it works out.

W=$$\frac{mgt2*3.14r}{l2T}$$

When i put t=1/3 i get that potencial or kinetic(don't know which..) is 25J !!! Which means that other is 75J Now which is which...?

Last edited: May 1, 2010
8. May 1, 2010

### Patrickas

Hey thanks for the help! I have been trying to solve this one for days! hehe

9. May 1, 2010

### tiny-tim

Hi Patrickas!

You're making this very complicated.

You know T = 4, A = 1, and so distance x = 1*cos(2πt/4).

So you can find v … then KE = 1/2 mv2, and PE = … ?

10. May 1, 2010

### Patrickas

O yea! Thanks! It should be much faster and easier to solve those problems now.