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Robittybob1
12th April 2012 - 04:49 AM
I started a thread on the Sciforums site but all they are keen to discuss over there are religious topics. The real scientists are here and on Physics Forum.

What I've got in my thought experiment is two blocks of plasticine weighing 10kg opposite each other on this rotating frictionless weightless table each block is 1 meter from the axis. "Radius = 1 meter"

And then we will fire bullets into the blocks and they will lodge in them. Higher energy bullets fired on one side and lower energy bullets on the other. The energy is in the form of velocity (KE = (1/2) mv^2) Mass of the projectile is the same on both sides and weighs 10g.

When the bullets lodge in the plasticine they transfer their momentum and kinetic energy to the whole set-up table plus the 2 blocks 20kg total mass. So the mass of the blocks will increase with the accumulation of spent bullets embedded in the plasticine.
The momentum in the bullet is converted to angular momentum with the radius "R = 1 meter"
The momentum of a 10g bullet moving 1000 m/sec fired into a single 10 kg block will be 10 kg.m/sec, the combined mass of both blocks and the two extra bullets (20.02 kg) to give a tangential velocity of 0.4995005
m/sec.
High speed side
The momentum of a 10g bullet moving 1100 m/sec fired into a single 10 kg block will be 10 kg.m/sec, the combined mass of both blocks and the two extra bullets (20.02 kg) to give a tangential velocity of 0.549450549
m/sec.

Low speed side
The momentum of a 10g bullet moving 900 m/sec fired into a single 10 kg block will be 10 kgm/sec, the combined mass of both blocks and the two extra bullets (20.02 kg) to give a tangential velocity of 0.44955045
m/sec.
A difference of 0.0999001 m/sec. So I take it that the block will start to rotate.

Does that momentum imbalance remain even when the table starts spinning ever faster?

Why I ask this is the impulse seems to lessen on the side where the bullet and the block move in the same direction and correspondingly increase on the other side where the motions oppose.

Can someone help me with the maths of this situation please?
Robittybob1
12th April 2012 - 08:02 PM
Is the problem too hard or not fully understood? I just can't understand why there is no one willing to help out on this one.
flyingbuttressman
12th April 2012 - 08:09 PM
QUOTE (Robittybob1+Apr 12 2012, 12:49 AM)
Can someone help me with the maths of this situation please?

I believe you can just add the relative velocity of the blocks to the bullets.
If the oncoming block is moving at 10 m/s relative to the bullet, you can increase the velocity of the bullet by 10 m/s. For the receding block, you can subtract the same 10 m/s from that block's bullet.
Robittybob1
12th April 2012 - 09:26 PM
QUOTE (flyingbuttressman+Apr 12 2012, 08:09 PM)
I believe you can just add the relative velocity of the blocks to the bullets.
If the oncoming block is moving at 10 m/s relative to the bullet, you can increase the velocity of the bullet by 10 m/s. For the receding block, you can subtract the same 10 m/s from that block's bullet.

So if the differnece in velocity of the bullets is 100 m/sec, that means once the table plus blocks is spinning with a tangential velocity of 50 m/sec the relative velocity (closing speed) of the bullets 1050 m/sec.
So in your opinion once this limit is reached the increase in velocity is zero.

I think that is a good answer.
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