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Find final angular velocity.

by ScienceGeek24
Tags: angular, final, velocity
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ScienceGeek24
#19
Mar5-12, 11:42 PM
P: 158
http://www.real-world-physics-proble...ollision_8.png like this, i know all my momentum. but 150 does not equal -150 un less you set the equation to 0=150-150 which equals 0. But what happened to the angular velocity??? don't i need to find like a magnitude in angular in velocity??
gneill
#20
Mar5-12, 11:48 PM
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P: 11,676
Quote Quote by ScienceGeek24 View Post
http://www.real-world-physics-proble...ollision_8.png like this, i know all my momentum. but 150 does not equal -150 un less you set the equation to 0=150-150 which equals 0. But what happened to the angular velocity??? don't i need to find like a magnitude in angular in velocity??
Zero is a perfectly respectable magnitude

If you have two equal mass objects traveling with the same speeds in a head-on collision and they stick together, what's their final velocity?

By the way, the equation shown in your link is for elastic collisions, not inelastic. In a perfectly inelastic collision the two objects fuse -- m1+m2.
ScienceGeek24
#21
Mar5-12, 11:52 PM
P: 158
0? but this 2 objects had different speeds! does i mean that doesn't matter how fast the objects are moving , when they stick together they both have 0 velocity??
gneill
#22
Mar5-12, 11:54 PM
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Quote Quote by ScienceGeek24 View Post
0? but this 2 objects had different speeds! does i mean that doesn't matter how fast the objects are moving , when they stick together they both have 0 velocity??
Conservation of mometum. They had different rotational speeds but they had equal and opposite momenta. In the linear example I just gave, suppose you doubled the speed of one object but cut its mass in half. It would have the same momentum as before. Same result of zero net momentum and zero velocity for the stuck pair.
ScienceGeek24
#23
Mar5-12, 11:59 PM
P: 158
oh so it was only finding the moments of inertia to calculate angular momentum and use conservation of energy of inelastic collision??? that was it? ok now for part 2 i know the answer is 338 J
ScienceGeek24
#24
Mar6-12, 12:01 AM
P: 158
If i sum up both of my KE rot they give me the answer, but here is my doubt why do i have to sum it up? don't I have to find the KE total and subtract that from KE rot f to get the kinetic energy lost??
gneill
#25
Mar6-12, 12:03 AM
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P: 11,676
Quote Quote by ScienceGeek24 View Post
If i sum up both of my KE rot they give me the answer, but here is my doubt why do i have to sum it up? don't I have to find the KE total and subtract that from KE rot f to get the kinetic energy lost??
Yes. So what's the initial total KE and the final total KE?
ScienceGeek24
#26
Mar6-12, 12:07 AM
P: 158
KE total is = 225J+112.5J

KEf=0 ???
gneill
#27
Mar6-12, 12:10 AM
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P: 11,676
Quote Quote by ScienceGeek24 View Post
KE total is = 225J+112.5J

KEf=0 ???
Is there a problem?
ScienceGeek24
#28
Mar6-12, 12:17 AM
P: 158
LOL no. So the KEf is 0 after all? I thought it was 0 because after the collision they both have no kinetic energy because they have no velocity. They both come to a stop. Don't know if my reason is indeed correct.
gneill
#29
Mar6-12, 12:20 AM
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P: 11,676
Quote Quote by ScienceGeek24 View Post
LOL no. So the KEf is 0 after all? I thought it was 0 because after the collision they both have no kinetic energy because they have no velocity. They both come to a stop. Don't know if my reason is indeed correct.
Of course. No momentum for a single object (the now stuck together disks) means no motion. No motion → no kinetic energy.
ScienceGeek24
#30
Mar6-12, 12:21 AM
P: 158
thanks man! :D


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