# Homework Help: Physics 12 lab (work)

1. Jul 2, 2013

### gdhillon

1. This lab is based on 'newtons cradle' () I cant link to the actual lab video because that requires my password and username....not that I dont trust you guys/gals but its a public site. Anyway, he lets the first ball go at 3.0 cm above it's equilibrium position and asks for me to calculate the potential energy (the ball on the side reaches a height of 2.6cm from the initial bounce). I found that to be 1.5*10^-2J. the equation I used was ep=mgh...=.05(9.8).03. Then I am asked to Use the law of conservation of energy to calculate its velocity before impact. This where im confused as to what numbers I should use.

I tried Eki+epi=Ekf+Epf, I plugged in the numbers;

1/2(.05)v^2+(.05*9.8*03)=0
.025v^2=-.0147
v=-2.424.......I set this equation to equal zero b/c they ask me for the velocity before impact, and before there was an impact there was no energy transferred. Am I correct in this thinking? If im not please correct me. The negative velocity I got doesn't quite make sense to me. And I am a little stuck, pleas help!

Thanks!

Gavin

Last edited by a moderator: Sep 25, 2014
2. Jul 2, 2013

### CWatters

is correct but you then made a mistake rearranging it.

The initial KE at release Eki=0
The final PE at the bottom Epf=0

so you are left with

Epi=Ekf
or
mgh = 0.5mV2

The mass then cancels and you can solve for V.

3. Jul 8, 2013

### gdhillon

okay thank you, I got 7.7x10^-1 J

Now for question 5) they ask for the velocity of the ball on the other side after the collision. So I would use the formula :Eki+epi=Ekf+Epf. And for the height I would have .026m. So the initial PE would this time be zero and the final KE would be zero to.
And I would have Eki=Epf. Plugging numbers in I would have 1/2v^2=9.8(.026) and I would end up with 7.1x10^-1J . Does that sound right? It sounds right to me because the energy in is very close to the energy out (I assume a little bit of energy was lost to sound and heat)

Thanks,

Gavin

4. Jul 12, 2013

### gdhillon

Can anyone chime in?

5. Jul 12, 2013

### pgardn

The reasoning seems correct to me. But did they ask for v or E?

You have to know the mass of ball bearings to find E?

You gave an answer in Joules. Where are the m/s?

Last edited: Jul 12, 2013
6. Jul 12, 2013

### pgardn

Should the 0.77 be in m/s ?

7. Jul 13, 2013

### haruspex

You mean .71, right?

8. Jul 13, 2013

### pgardn

okay thank you, I got 7.7x10^-1 J

I was referring to his first answer.