# Homework Help: Velocity Vector question

1. Dec 9, 2009

### itr

The velocity of a .5kg body, for t=1s to t=35s, is given by v=5t^2 i + 3/t j -t k

v has - over it....and the i, j, and k have the hats on them (^)

For this interval, What is the body's angular momentum?
What is the torque exerted on the body?
what is the acceleration of the body?
what is the foce on the body?
is either angular momentum or linear momentum conserved?
As t goes to infinity, what happens to the position of hte particle?

I am not sure what the interval means, what each part stands for....
I believe that you are gonna hvae to start to differentiate the interval to come up with some of the terms...i know force is the acceleration/2. I am pretty clueless...i feel like a stupid physics student. If you could help me out on the problem that would be cool...ty

2. Dec 9, 2009

### putongren

hi itr,

I'm not good at physics also ... but i think i know how to address a couple of your questions. Here's my attempt, since I am learning also. Also, future commentators, feel free to correct me if I make a mistake. If you look at the wiki on angular momentum, the vector is given as L =r x mv . r is the position vector, L is the angular momentum vector, and v is the velocity vector. We are given the velocity vector, but not the position vector. How do we get the position vector from the velocity vector? If we know the velocity vector and position vector, then we can calculate the angular momentum vector by doing a cross product.

As for torque, differentiate the angular momentum with respect to time since you have calculated the angular momentum.

3. Dec 9, 2009

### itr

Thank you for posting, i am sure i will be using some of your inforamtion to solve the problem :D

4. Dec 9, 2009

### blaze33

u get position vector by integrating the velocity vector.

5. Dec 9, 2009

### itr

sweet thanks...its intigrate not differentiate?

6. Dec 9, 2009

### blaze33

if u differentiate the velocity vector u get the acceleration vector.
if u integrate the velocity vector u get the position vector.
u know how integration works right?

7. Dec 9, 2009

### cocoon

L = m* (r x v)
find r by integrating v
then find L

t = r x F = r x m*a
find a by taking the derivative of v
solve for t using r from before

no, because t and F are clearly non-zero

lim r = <inf, inf, -inf>

8. Dec 10, 2009

### itr

thank you. and yes i can intigrate and differentiate

9. Dec 10, 2009

### itr

I have one additional question...when you have v=5t^2 i + 3/t j - t k......
what is the i stand for, what does j stand for, what does k stand for......
ALso, what does the 5t^2 mean the 3/t mean....and the -t

10. Dec 11, 2009

### itr

Can somebody please explain f) better for me, i dont really understand it.....lim r = inf, inf, -inf

11. Dec 11, 2009

### cocoon

I don't understand that either. I didn't write that. I wrote lim r = <inf, inf, -inf>

Variables in bold mean vectors. I got that from my Linear Algebra book. And inf is short for infinity. <x, y, z> is a vector with x, y, and z components.

12. Dec 13, 2009

### itr

I got everything i need...i did cross product on everything got my torque and angular momentum vectors....how do i utilize the time factors in here...for instance....for the acceleration ....do i plug in 1 and plug in 35 into the velocity vector....subtract the 2....and then divide by 34?

for my questions listed above in the first post....which ones need that time? does the angular momentum need it based over the time....or is the question just asking what is the angular momentum vector?

Last edited: Dec 13, 2009
13. Dec 13, 2009

### itr

it is urgent please if you can help me that would be greattttt

14. Dec 14, 2009

anyone??