Calculating Potential Energy and Velocity in Newton's Cradle Lab

[gdhillon]

1. This lab is based on 'Newtons cradle' () I can't link to the actual lab video because that requires my password and username...not that I don't trust you guys/gals but its a public site. Anyway, he let's 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 I am 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 I am 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

 

[CWatters]

Your equation...

Eki+epi=Ekf+Epf

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.

 

[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

 

[gdhillon]

Can anyone chime in?

 

[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?

 

[pgardn]

Should the 0.77 be in m/s ?

 

[haruspex]

Should the 0.77 be in m/s ?

You mean .71, right?

 

[pgardn]

You mean .71, right?

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

I was referring to his first answer.