My Math Forum rate of angular momentum of a point

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 April 23rd, 2017, 05:06 AM #1 Newbie   Joined: Apr 2017 From: UK Posts: 4 Thanks: 0 rate of angular momentum of a point Hi, Does anyone know how to calculate the rate of angular momentum of a point,Q? Google is not helping me right now. Thanks
 April 23rd, 2017, 05:33 AM #2 Math Team   Joined: Jul 2011 From: Texas Posts: 2,701 Thanks: 1359 Depends on the problem statement ... you have a specific one in mind?
 April 23rd, 2017, 06:05 AM #3 Newbie   Joined: Apr 2017 From: UK Posts: 4 Thanks: 0 I need to find the rate of angular momentum of the centre of mass. Its for a biped robot.
April 23rd, 2017, 07:17 AM   #4
Math Team

Joined: Jul 2011
From: Texas

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Quote:
 Originally Posted by BrainFart I need to find the rate of angular momentum of the centre of mass. Its for a biped robot.
this is still a rather general statement ... angular momentum is usually measured relative to an axis of rotation or a specific position in space. In general, the angular momentum of a point mass relative to some other point is the cross product of the point mass's position vector and its linear momentum.

$\vec{L} = \vec{r} \times \vec{p} = \vec{r} \times m\vec{v}$

Also, you do understand that angular momentum and rate of change of angular momentum are two different things?

 April 23rd, 2017, 08:44 AM #5 Newbie   Joined: Apr 2017 From: UK Posts: 4 Thanks: 0 I beg your pardon, I do mean the rate of change. Am I right in thinking to get the rate of change you could take the derivative with regards to time of your above equation?
 April 23rd, 2017, 09:16 AM #6 Math Team   Joined: Jul 2011 From: Texas Posts: 2,701 Thanks: 1359 the time rate of change of angular momentum is net torque ... $\dfrac{dL}{dt}=\tau_{net}$ Thanks from BrainFart
 April 23rd, 2017, 11:19 AM #7 Newbie   Joined: Apr 2017 From: UK Posts: 4 Thanks: 0 Thank you

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