# Problem with variables

1. Nov 7, 2004

### HobieDude16

ok, heres a problem that you have to work out with all variables, and i frankly have trouble doing that.... i got part a, and i did a lot of work on part b, but cant seem to come up with the right answer.... any help appreciated....

In Figure 11-32, a solid brass ball of mass m and radius r will roll without slipping along the loop-the-loop track when released from rest along the straight section. For the following answers use g for the acceleration due to gravity, and m, r, and R, as appropriate, where all quantities are in SI units.

Fig. 11-32

(a) From what minimum height h above the bottom of the track must the marble be released to ensure that it does not leave the track at the top of the loop? (The radius of the loop-the-loop is R. Assume R r.)

(b) If the marble is released from height 6R above the bottom of the track, what is the magnitude of the horizontal component of the force acting on it at point Q?

ok, for part a, i got 2.7R whichi is correct, so then i moved on to part b, and i was told by my TA to start with the formula mg6R = .5mv^2 + .5Iomega^2 + mgR.... so i did, and tried to solve for v. i used 2/5mR^2 for I and for omega, used v/R...... so if i do that, and solve for v, then i use a=v^2/r and then when i have that, F=ma so i have the idea, i just cant get it right. anybody have any ideas? thanks in advance
John

2. Nov 7, 2004

### HobieDude16

anybody have any ideas?

3. Nov 7, 2004

### HobieDude16

maybe i can find some help on another forum or something

4. Nov 7, 2004

Ok you work with conservation of energy and do, mgh=mgR+7mv^2/10. So at point Q you have F=mv^2/R=(50/7)mg

5. Nov 7, 2004

### HobieDude16

awesome, cant believe i didnt think of that, lol

6. Nov 7, 2004

### Phymath

btw u can find this one in almost every physics book it a classic

mg6R = 1/2mv^2+1/2Iw^2 + mgR

mg6R = 1/2mv^2 + 1/2 (2/5mr^2)w^2 + mgR

6gR = 1/2v^2 + 1/5r^2(v/r)^2+gR

5gR = 7/10 v^2

50/7gR = v^2

F = 50/7mg

Last edited: Nov 7, 2004
7. Nov 7, 2004

### HobieDude16

ah, really? its not in ours (since its asked as a question in ours... lol)

8. Nov 7, 2004

### physicsss

Help...

Hey HobieDude16,

Can you show me how you got part A? I'm learning angular momentum too and I have a similar problem except the ball is not on a ramp. Thanks.

9. Nov 7, 2004