Question

angular momentum

Answer 1

the velocity of a 2kg body moving in the xy plane is given by v=7i-2j m/s. Its position vector is r=2i+1j m. calculate its angular momentum about the origin in kgm^2/s

Answer 2

I know l=Iw and that v=rw so w=v/r

Answer 3

but Im not dividing the vectors properly I guess because Im not getting the right answer

Answer 4

What answer are you getting for omega?

Answer 5

w=3/2

Answer 6

hang on...

Answer 7

you are using cross product correct?

Answer 8

no I used dot

Answer 9

why is it cross?

Answer 10

v = omega cross r by definition ...it is a cross product not a dot product.

Answer 11

ok I need to change my notes then thank you. Can you walk me through the cross product then because when I tried that I got I=22k but the answer option is -22k

Answer 12

sure, what was your method to get 22k?

Answer 13

using foil
(7i-2j)(2i+1j)=14ii+7ij-4ji-2jj
0+7k+4k+0
7k+4k=11k
11*2=22

Answer 14

your method does appear valid....

Answer 15

Can you please check to make sure the that the vectors for v and r are correct from the original problem...

Answer 16

yes they are

Answer 17

is ti because I have to do the cross product of 1/r since w=v/r

Answer 18

Sorry for my typo...it should have said the method is not valid...you have dotted v and r

Answer 19

alright im still lost then

Answer 20

you should be expressing omega as follows ai + bJ + ck.
Then you know omega cross r gives is velocity. So you will then use the definition of the
cross product and solve for a, b and c

Answer 21

What book is this from?

Answer 22

I do not know. This is one of my past test questions that I got wrong, and I am trying to do it over again to understand what I did wrong, but Im not getting it.

Answer 23

Can you send me a photo of the question please?

Answer 24

I know the method I am using is correct but the answer I am getting doesn't make sense

Answer 25

how will a picture help?

Answer 26

Based on my math, the answer I am getting is not reasonable.
So perhaps if I see the question I might notice what the problem is.

Answer 27

the way I typed it is the way it looks there is no additional picture or anything

Answer 28

ok, do you know what the correct answer for omega is suppose to be?

Answer 29

no

Answer 30

ok hang on...

Answer 31

i am going to send you what I have done so you can see the problem

Answer 32

ok, lets use a scalar approach instead of vectors.
Consider the scalar equation v = omega*r
From what is provided you can find the magnitude of v and the magnitude or r and then sub in to find the magnitude of omega....go ahead

Answer 33

ok so then v=sqrt53 and r=sqrt5

Answer 34

so w=sqrt53/sqrt5

Answer 35

Yes, now you can use that to determine the angular momentum but it will be a scalr not a vector

Answer 36

so w~3 which times 2 is ~6

Answer 37

What are you using to calculate I for the body?

Answer 38

2kg. I see now that's wrong because I=mr^2 ill change that real quick

Answer 39

or would I because I assume it's suppose to be taken as a point mass

Answer 40

Fora point mass, it is mr^2

Answer 41

alright so then multiply by 5 since r=sqrt5 we get ~24

Answer 42

which is still positive

Answer 43

thank you for all your help but I think im just going to ask my teacher when I get a chance

Answer 44

You can get the correct sense of rotation by sketching the velocity vector and seeing whether you have clockwise of counterclockwise rotation which

Answer 45

the rotation will then be positive or negative based on sign convention

Answer 46

Sorry to interrupt, you can just work out L= r x mv using the fact that i x i = 0, j x j = 0, i x j = k and j x i = -k
L = m (2i + j) x (7i - 2j) = -22k

Answer 47

The minus sign means that as you look down on the xy plane from above the rotation is in a clockwise sense.