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# Angular momentum/moment of inertia watch

1. I've been working through a question today and whilst I have finished it , am not sure if i've been doing it correctly.

The question is: A 2kg particle is moving at constant speed of 3.5 m/s in a clockwise direction around a circle of radius 4.0m.
a)What is the angular momentum of the particle about the center of the circle.
b)What is the particles moment of inertia about an axis through the centre of the circle
c)what is the angular velocity of the particle?

so for a) i used L= r x mv = 28 kgm^2/s

b) i used I= 1/2 MR^2 = 16kgm^2

and for c) i used L=Iw thus w = 1.75 rads

I felt that it was abit too easy to be correct so if anyone could see somewhere ive gone wrong that would be appreciated
2. (Original post by santeria133)
I've been working through a question today and whilst I have finished it , am not sure if i've been doing it correctly.

The question is: A 2kg particle is moving at constant speed in a clockwise direction around a circle of radius 4.0m.
a)What is the angular momentum of the particle about the center of the circle.
b)What is the particles moment of inertia about an axis through the centre of the circle
c)what is the angular velocity of the particle?

so for a) i used L= r x mv = 28 kgm^2/s

b) i used I= 1/2 MR^2 = 16kgm^2

and for c) i used L=Iw thus w = 1.75 rads

I felt that it was abit too easy to be correct so if anyone could see somewhere ive gone wrong that would be appreciated
There seems to be something missing in your question.
You have only been given a value of m and r
Where did you get the value of v from?
3. (Original post by Stonebridge)
There seems to be something missing in your question.
You have only been given a value of m and r
Where did you get the value of v from?
my bad , it was meant to say at a constant speed of 3.5m/s . I'll edit now
4. a) is ok
b) check the formula
c) you've calculated angular momentum. (Was this what the Qu. asked for? Angular velocity is just v/r
5. (Original post by Stonebridge)
a) is ok
b) check the formula
c) you've calculated angular momentum. (Was this what the Qu. asked for? Angular velocity is just v/r
well for b, i saw that if the axis goes through the circle the formula is 1/2 MR^2 and thats what i understood from the question

and for c the formula was L=Iw so i just used the value of L and I already calculated to work out w if that makes sense?
6. (Original post by santeria133)
well for b, i saw that if the axis goes through the circle the formula is 1/2 MR^2 and thats what i understood from the question

and for c the formula was L=Iw so i just used the value of L and I already calculated to work out w if that makes sense?
in b) you have the wrong formula.
Check this out.
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

In c) as you are given v and r you should calculate w from w = v/r using the data given in the question.
7. (Original post by Stonebridge)
in b) you have the wrong formula.
Check this out.
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

In c) as you are given v and r you should calculate w from w = v/r using the data given in the question.
you make a good point. but the only formula i see on there that uses I is L=Iw but that would mean id have to calculate the angular velocity with the method you used but that wouldnt make sense since it asks that in part c ?
8. (Original post by Stonebridge)
in b) you have the wrong formula.
Check this out.
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

In c) as you are given v and r you should calculate w from w = v/r using the data given in the question.
Sorry I didnt see the information below , i still cant see where I've gone wrong. Shouldn't it be some fraction or integration of MR^2? Which one should i use and why?
9. (Original post by santeria133)
Sorry I didnt see the information below , i still cant see where I've gone wrong. Shouldn't it be some fraction or integration of MR^2? Which one should i use and why?
It states the mass is a particle. This is not an extended mass, it's the simplest case. There is no integration. You only need that for extended masses such as discs, rings and spheres etc.

Its angular momentum about the centre of a circle is simply mvr
That is, its linear momentum mv times r (definition)

Its moment of inertia is mr2 by definition.
That formula is given in the link I gave you.

The angular velocity of the mass is simply v/r

That's how you approach this question. Then it makes more sense.
You have over complicated it.

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