Physics problem

[saahmed]

Im stuck on this problem and Ive gotten to the point where Im just staring at my computer, so I figured Id give you guys a go at it.

I got Part A which was fairly simple. But Ive tried everything but cant seem to get Part B. I must be missing something. This is one of those online assignments where you get as many attempts as you want. I keep getting Part A, but cant get Part B. Please explain how you did Part B if you get it. I have the correct answers too.

In Fig. 11-52, a 7.3 g bullet is fired into a 0.46 kg block attached to the end of a 0.12 m nonuniform rod of mass 0.88 kg. The block-rod-bullet system then rotates in the plane of the figure, about a fixed axis at A. The rotational inertia of the rod alone about A is 0.038 kg·m2. Treat the block as a particle. (a) What then is the rotational inertia of the block-rod-bullet system about point A? (b) If the angular speed of the system about A just after impact is 7.2 rad/s, what is the bullet's speed just before impact?

Fig. 11-52

 

[Fenixgoon]

you know momentum is conserved. in this case, mv (bullet) = I w (system after impact). the units aren't the same, so you need to convert. w = v/r, I = mr^2

then you get MV (bullet) = MV (system) and can solve

 

[saahmed]

Yeh I think converting to correct units is where Im having the problem, I dont think Im doing it right. I already got I which was .0447. w is given. And I know that linear mv= Iw. I tried converting the w to meters per second but I must not be doing it right, or maybe thats not how I should do it?

basically just took 7.2 * circumference. took that times I. and divided by mass of bullet to get velocity. But im not getting the correct answer.

EDIT: Also tried getting velocity of entire system using mv=Iw. then mv(bullet)=mv(system). I get 32.7. Correct answer is 367.

 

[Conky]

Umm, do your own homework. I took "Honors" :roll: Physics when I was attending the University and got an A.

I would do your homework but I am about to go to sleep. If you can't figure this out by the time I wake up then email or PM me and I will tell you the answer. 😉

 

[saahmed]

Originally posted by: Crazyfool
Umm, do your own homework. I took "Honors" :roll: Physics when I was attending the University and got an A.

I would do your homework but I am about to go to sleep. If you can't figure this out by the time I wake up then email or PM me and I will tell you the answer. 😉

I did my own homework. This is one problem that I cant figure out and its probably due to some stupid oversight on my part because I think I know the concepts. BTW- I already have the answer to this problem with these specific numbers. I need to know how to do it so that I can solve the problem when I click to get a fresh set of numbers. So essentially I will be doing the problem on my own, someone might just help me with how to solve it without the actual values I will be using.

 

[Fenixgoon]

Originally posted by: saahmed
Yeh I think converting to correct units is where Im having the problem, I dont think Im doing it right. I already got I which was .0447. w is given. And I know that linear mv= Iw. I tried converting the w to meters per second but I must not be doing it right, or maybe thats not how I should do it?

basically just took 7.2 * circumference. took that times I. and divided by mass of bullet to get velocity. But im not getting the correct answer.

EDIT: Also tried getting velocity of entire system using mv=Iw. then mv(bullet)=mv(system). I get 32.7. Correct answer is 367.

because you're trying to equate angular velocity and linear. you need to convert from angular TO linear, then solve. i told you how

you have MASSbullet VELbullet = Isystem Wsystem. the units aren't the same, you need to convert.

I = MR^2, W = V/R

MR^2 x V/R = MV.

MASSbullet Vbullet = MR^2 V/R

 

[Random Variable]

Originally posted by: Crazyfool
Umm, do your own homework. I took "Honors" :roll: Physics when I was attending the University and got an A.

I would do your homework but I am about to go to sleep. If you can't figure this out by the time I wake up then email or PM me and I will tell you the answer. 😉

What the heck is honors physics?

 

[saahmed]

Originally posted by: Fenixgoon

Originally posted by: saahmed
Yeh I think converting to correct units is where Im having the problem, I dont think Im doing it right. I already got I which was .0447. w is given. And I know that linear mv= Iw. I tried converting the w to meters per second but I must not be doing it right, or maybe thats not how I should do it?

basically just took 7.2 * circumference. took that times I. and divided by mass of bullet to get velocity. But im not getting the correct answer.

EDIT: Also tried getting velocity of entire system using mv=Iw. then mv(bullet)=mv(system). I get 32.7. Correct answer is 367.

because you're trying to equate angular velocity and linear. you need to convert from angular TO linear, then solve. i told you how

you have MASSbullet VELbullet = Isystem Wsystem. the units aren't the same, you need to convert.

I = MR^2, W = V/R

MR^2 x V/R = MV.

MASSbullet Vbullet = MR^2 V/R

Ohhhh. I wasnt thinking straight sorry. Yeh, that makes sense now. Im a dumbass, the whole time it was one stupid error. I just had to divide the rad/s by radius.

Thanks a lot.

 

[saahmed]

Originally posted by: Random Variable

Originally posted by: Crazyfool
Umm, do your own homework. I took "Honors" :roll: Physics when I was attending the University and got an A.

I would do your homework but I am about to go to sleep. If you can't figure this out by the time I wake up then email or PM me and I will tell you the answer. 😉

What the heck is honors physics?

Its for "smart" people. He just has to show off 😉. I took honors rhetoric (basically English class) and calculus last semester.