# Help with physycs problems.

1. Oct 30, 2006

### draconicspectre

need some help with some physics homework, would really appreciate explanations for these problems and not just answers :D, neways here they are!

An automobile crankshaft transfers energy from the engine to the axle at the rate of 38.0 kW when rotating at a speed of 2900 rev/min. What torque does the crankshaft deliver?

If a 38.0 N·m torque on a wheel causes angular acceleration 22.9 rad/s2, what is the wheel's rotational inertia?

A disk, initially rotating at 150 rad/s, is slowed down with a constant angular acceleration of magnitude 3.4 rad/s2. (a) How much time does the disk take to stop? (b) Through what angle (rad) does the disk rotate during that time?

Thank you ahead of time! :D

2. Oct 30, 2006

### Office_Shredder

Staff Emeritus
I'm sure you've seen the expression P=FV, where P is power, F is force, and V is velocity. Can you think of an analogous situation for rotational motion?

Going back to linear mechanics again, I'm sure you've seen f=ma What's the analogous situation for rotational motion?

Can you think of a formula that, given initial velocity and deceleration, let's you calculate how long it takes for something moving in a straight line to stop? Try converting all of that into rotational terms. Then use a distance formula that, given initial velocity, time, and acceleration, let's you find distance, and try finding an analogous rotational equation

3. Oct 30, 2006

### draconicspectre

I'm sorry to be a hassle, but that didnt really help me at all :\ , can someone show me some formulas to help me solve these questions ?

4. Oct 30, 2006

### Hootenanny

Staff Emeritus
You must have met some formulae during your studies of rotational motion. Perhaps you could hazard a guess as to which would be appropriate here?

5. Oct 30, 2006

### draconicspectre

i have some formulas, but im not really sure how/what to use. Would u like me to start listing formulas that i have?
Once again, i'm sorry for any hassle. TY for help

6. Oct 30, 2006

### Hootenanny

Staff Emeritus
It is no hassle at all, Officeshredder gave you some pretty good hints with his post. Okay lets break the first one down.
Now, as Officeshredder said, the expression P=Fv, is valid for linear motion. Now, could you take a guess at what the rotational version would be? Or have a glance at your notes for something similar.

7. Oct 30, 2006

### draconicspectre

KE = 1/2 Iw^2
Would this one be suitable for our current problem?

8. Oct 30, 2006

### Hootenanny

Staff Emeritus
I was thinking more of $P=\tau\omega$.

9. Oct 30, 2006

### draconicspectre

Just realized i posted my problems backwards from the list of my HW, so i think that formula actually helps the last problem i listed.

Our angular velocity is 2900 rev / min.
Our power is 38 kW
so (not quite sure how to make that symbol) = 38/2900 would be our solution?

10. Oct 30, 2006

### Hootenanny

Staff Emeritus
We're almost there, but you must consider your units. Note that power is in kilowatts and our angular velocity is in rev/min, you must convert your values into SI units before doing the calculation; otherwise, you get yourself in all sorts of a mess. The SI units of power are watts and the SI units of angular velocity are radians per second.

11. Oct 30, 2006

### draconicspectre

38000w/
130(3.14)/sec

12. Oct 30, 2006

### Hootenanny

Staff Emeritus
Your power conversion is correct, but you may wish to check your velocity conversion.

13. Oct 30, 2006

### draconicspectre

2900 /60 = to seconds
48.33 *2(pie) = 96.67(pie)

14. Oct 30, 2006

### Hootenanny

Staff Emeritus
Looks alot better

15. Oct 30, 2006

### draconicspectre

would i use the same formula for the second problem?

16. Oct 30, 2006

### Hootenanny

Staff Emeritus
No, you need to find the moment of inertia (I). The formula you need is analogous to the linear equation F=ma.

17. Oct 30, 2006

### draconicspectre

i=mr^2 ?

18. Oct 30, 2006

### Hootenanny

Staff Emeritus
Not quite. Think about F=ma, m is the property of matter which gives it inertia....