# Angular momentum help

1. Dec 3, 2008

### sumitmanhas

angular momentum urgent help ...

1. The problem statement, all variables and given/known data
Which of the following is an accurate statement
1) The angular momentum of a moving particle depends on the specific origin with respect to which the angular momentum is calculated.
2) If the torque acting on a particle is zero about an arbitrary origin, then the angular momentum of the particle is also zero about that origin.
3) If the speed of a particle is constant, then the angular momentum of the particle about any specific origin must also be constant.
4) Consider a planet moving in a circular orbit about a star. Even if the planet is spinning it is not possible for its total angular momentum to be zero.
5) A particle moving in a straight line with constant speed necessarily has zero angular momentum.

2. Relevant equations
conservation of angular momentum and principles

3. The attempt at a solution

so for the first 1 ) this is likely to be true , which i never thought of because evrytime you calculate your angular momentum you choose the point of rotation and from there using right hand rule determine the direction (in or out)
2) makes no sense because if there is no torque acting on the origin then the angular momentum is constant
3) if speed is constant then angular momentum = mrv is same since the mass remain the same throughout
4)if an object keeps rotating doesn't it have an initial momentum and final momentum which are same , so, i mean it is constant then.
5) particle in straight line can have angular momentum , as in the case of a person jumping of the merry go round with horizontal speed doesnot have zero angular momentum.
i belive the first one is more accurate..
reply as soon as possible plz

2. Dec 4, 2008

### tiny-tim

Hi sumitmanhas!
Yes, it is true, but you have to prove it, using the definition.

That's sort-of right, but you haven't said whether or not that constant has to be zero.
No … what is r?
Yes, it is constant, but you haven't answered the question … can a planet like the Earth (which is both spinning once a day on its own axis and also moving round the Sun once a year) have zero total angular momentum?
Yes, but you should give an answer based on the definition.