# AP Physics Homework Problem

cepheid
Staff Emeritus
Gold Member
looking at the units, I need joules=Kg*m^2/s^2 . where will I get s^2 when I only have angular velocity

I think the issue you're having is because you had the equation for rotational KE slightly wrong, as I mentioned above.

By the way, I'm glad you looked at the units. Always keep track of the units. it's a great way to make sure your work is making sense, and to catch errors. If things aren't dimensionally consistent on both sides of an equation, then chances are you've made an error.

I'm pretty sure that the equation is KE = (1/2)Iω2

It's the rate of change of your angle with time. You go one rotation every 2 mins. How many radians is one rotation? How many seconds is 2 minutes? Therefore, how many radians per second is ω?

2∏/120 s.= 1/60 rad/sec was I right about constant GPE or was it a nice try? lol

I think the issue you're having is because you had the equation for rotational KE slightly wrong, as I mentioned above.

By the way, I'm glad you looked at the units. Always keep track of the units. it's a great way to make sure your work is making sense, and to catch errors. If things aren't dimensionally consistent on both sides of an equation, then chances are you've made an error.

oh cool! I finally get it. Thank you for helping me. I understand it too! amazing! lol

cepheid
Staff Emeritus
Gold Member
2∏/120 s.= 1/60 rad/sec was I right about constant GPE or was it a nice try? lol

I think you mean π/60 rad/s, but yeah. And yes you were right about the GPE, as I said in post #26 (we are once again posting too frequently to keep up with each other).

oh cool! I finally get it. Thank you for helping me. I understand it too! amazing! lol

You're welcome! One last thing I noticed: a few posts up, you said that I = mr2. This is true, provided that m is the total mass of all cars. But if m is the mass of just one car, then I = 36mr2, because you have to add up all of the individual mr2 terms in the summation that you posted before. I just wanted to make sure you were doing so.

I think you mean π/60 rad/s, but yeah. And yes you were right about the GPE, as I said in post #26 (we are once again posting too frequently to keep up with each other).

You're welcome! One last thing I noticed: a few posts up, you said that I = mr2. This is true, provided that m is the total mass of all cars. But if m is the mass of just one car, then I = 36mr2, because you have to add up all of the individual mr2 terms in the summation that you posted before. I just wanted to make sure you were doing so.

thank you cepheid and good night! : ) another successful physics night thanks to you. I hope your ok with me asking for your help until may 14th! haha. You help me understand so much better!

cepheid
Staff Emeritus