## angular impulse and angular momentum questions

Hi everyone,

I am stuck on two fairly easy questions that I hope someone will be able to help me with.

Questions:

1) How much angular impulse must be supplied by the hamstrings to bring a leg swinging at 8 rad/s to a stop, given that the leg's moment of inertia is 0.7 kg-m^2 ?

my solution:

angular impulse = change in angular momentum

Tt = Iw2 - Iw1 (the numbers 2 & 1 are supposed to be subscripts standing for final and initial; I=moment of inertia; w=angular velocity)

Tt = 0 - (.7)(8)
= -5.6 kg-m^2/s

what did I do wrong?

2) A 7.27kg shot makes seven complete revolutions during its 2.5 second flight. If its radius of gyration is 2.54 cm, what is its angular momentum?

my solution:

H = mk^2w (m=mass; k=radius of gyration; w=angular velocity; H = angular momentum)

7 revolutions = (360degrees x 7)/57.3

H = (7.27)(.0254^2)(43.979/2.5sec)
= .0825 kg-m^2/s

what did I do wrong?

thanks.
 Recognitions: Homework Help Maybe the given answers is incorrect.
 The correct answers according to the teextbook are: 1) 5.6 mg-m^2/s 2) 0.0817 kg-m^2/s

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## angular impulse and angular momentum questions

For question one it seems your textbook has defined the retarting torque force as negative.

As for question two, it just seems like a rounding error, the answers are very close to each other. If I was marking your work, I would mark both your answers correct. However, I'm not marking your work.

-Hoot

 Quote by Hootenanny For question one it seems your textbook has defined the retarting torque force as negative. As for question two, it just seems like a rounding error, the answers are very close to each other. If I was marking your work, I would mark both your answers correct. However, I'm not marking your work. -Hoot

But still how did they get "mg" for answer #1? how did they convert this?

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