Intersting - bullet in to block of wood
Discussion
One for you physics guys...
If I were to have two identical blocks of wood on a bench with a slit in it......from beneath the bench I have two guns that each fire a bullet in to the blocks of wood.
Block of wood #1 has the bullet fired towards the centre of gravity (basically the middle of the wood), block #2 has the bullet fired 'off-centre' as shown by the pic below:
What happens is that both pieces of wood 'absorb' the bullet and they rise in to the air. Block #1 simply goes in to the air where as block #2 goes in to the air and rotates at the same time.
Question............. do both blocks reach the same height before falling back to earth?
If I were to have two identical blocks of wood on a bench with a slit in it......from beneath the bench I have two guns that each fire a bullet in to the blocks of wood.
Block of wood #1 has the bullet fired towards the centre of gravity (basically the middle of the wood), block #2 has the bullet fired 'off-centre' as shown by the pic below:
What happens is that both pieces of wood 'absorb' the bullet and they rise in to the air. Block #1 simply goes in to the air where as block #2 goes in to the air and rotates at the same time.
Question............. do both blocks reach the same height before falling back to earth?
I reckon no. The energy transferred into the left block will assuming it is hit dead centre on be forward motion. Some of the energy transferred into the right block will converted to rotary motion. Action and reaction will be equal but absorbed in different ways. I'm on the toilet at the moment so that is the best I can do. We need equations here.
tank slapper said:
They reach the same height because it is an inelastic collision and momentum is conserved. Energy is not conserved in an inelastic collision so the total kinetic energy of each block can be different, allowing for the difference in rotation.
But some of the momentum is transferred as angular momentum to block 2 so it should have lower linear momentum and thus travel less linear distance...Flibble said:
tank slapper said:
They reach the same height because it is an inelastic collision and momentum is conserved. Energy is not conserved in an inelastic collision so the total kinetic energy of each block can be different, allowing for the difference in rotation.
But some of the momentum is transferred as angular momentum to block 2 so it should have lower linear momentum and thus travel less linear distance...Well, according to a particular youtube science channel this question got a wide range of responses with even established people in the field arriving at an incorrect answer.
The blocks actually reach the same height (on average), meaning that there is no average difference in height reached between the two scenarios.
People who attempted to answer this question used a whole plethora of established methods/equations but the simplest route is often the best solution.
Basically it is the conservation of liner momentum that trumps the other methodologies.
As the bullet enters each block the bullet in block 1 goes further in to the wood dispersing its energy as material deformation & heat, where as the bullet in block 2 does the same thing but does not travel as far in to the wood due to the fact that the wood starts to rotate away from it with the bullet entry.
BUT, liner momentum is conserved, meaning that the upward momentum of the bullets is the same in both cases, so therefore the resultant upward momentum of bullet and wood in both cases will also be the same.
Linear momentum and angular momentum are not 'coupled', they are independent, in that momentum is only preserved in each case.
So basically we are not looking for the preservation of liner momentum to be found in resultant angular momentum.
The rotational energy is found from the fact that the bullet in block 2 does not travel as far in to the wood as in block 1; this difference in distance accounts for the energy transfer from bullet k.e. in to block rotational energy.
Here is the explainind video:
https://www.youtube.com/watch?v=BLYoyLcdGPc
The blocks actually reach the same height (on average), meaning that there is no average difference in height reached between the two scenarios.
People who attempted to answer this question used a whole plethora of established methods/equations but the simplest route is often the best solution.
Basically it is the conservation of liner momentum that trumps the other methodologies.
As the bullet enters each block the bullet in block 1 goes further in to the wood dispersing its energy as material deformation & heat, where as the bullet in block 2 does the same thing but does not travel as far in to the wood due to the fact that the wood starts to rotate away from it with the bullet entry.
BUT, liner momentum is conserved, meaning that the upward momentum of the bullets is the same in both cases, so therefore the resultant upward momentum of bullet and wood in both cases will also be the same.
Linear momentum and angular momentum are not 'coupled', they are independent, in that momentum is only preserved in each case.
So basically we are not looking for the preservation of liner momentum to be found in resultant angular momentum.
The rotational energy is found from the fact that the bullet in block 2 does not travel as far in to the wood as in block 1; this difference in distance accounts for the energy transfer from bullet k.e. in to block rotational energy.
Here is the explainind video:
https://www.youtube.com/watch?v=BLYoyLcdGPc
NDT said:
Utter cobblers.
Energy is conserved.
The second block cannot travel as high as the first while also spinning otherwise it would somehow have more energy than the first block.
This is really basic A-level maths / physics.
Not all energy is conserved as kinetic though, some goes into deforming the bullet and wood.Energy is conserved.
The second block cannot travel as high as the first while also spinning otherwise it would somehow have more energy than the first block.
This is really basic A-level maths / physics.
Simpo Two said:
Yes, so less force is going into 'up'.
Conservation of momentum is preserved as per the law of physics.So what happens is that the bullet enters the wood in both cases and the momentum of bullet and wood is then preserved, meaning that bullet and wood in both cases continue the upward momentum.
The difference being that one block is rotating.
This difference is noticed by the bullet not travelling as far in to the block as the other one does. So the difference in energy released by the bullet once inside the wood (oo err), is noticed by the rotation.
ie. bullet length in to wood describes its conversion of k.e. in to heat, deformation of wood material, sound etc.
....less length in to the wood means that some k.e. of that bullet travel distance has gone in to rotating the wood.
The video explains it better then I can.
AA999 said:
ie. bullet length in to wood describes its conversion of k.e. in to heat, deformation of wood material, sound etc.
....less length in to the wood means that some k.e. of that bullet travel distance has gone in to rotating the wood.
So some of the bullets k.e. has imparted a spin to the bit of wood, which surely leaves less k.e. to raise it?....less length in to the wood means that some k.e. of that bullet travel distance has gone in to rotating the wood.
To look at this another way: if both blocks of wood reach the same height but one is rotating and one is not, where did the extra energy come from to cause the rotation?
Mr2Mike said:
To look at this another way: if both blocks of wood reach the same height but one is rotating and one is not, where did the extra energy come from to cause the rotation?
All explained herehttps://www.youtube.com/watch?v=BLYoyLcdGPc
Mr2Mike said:
AA999 said:
ie. bullet length in to wood describes its conversion of k.e. in to heat, deformation of wood material, sound etc.
....less length in to the wood means that some k.e. of that bullet travel distance has gone in to rotating the wood.
So some of the bullets k.e. has imparted a spin to the bit of wood, which surely leaves less k.e. to raise it?....less length in to the wood means that some k.e. of that bullet travel distance has gone in to rotating the wood.
To look at this another way: if both blocks of wood reach the same height but one is rotating and one is not, where did the extra energy come from to cause the rotation?
The bullet that hits the side begins to lift the block earlier, as there is an uneven/less weight for it to deal with before it moves. Less of the force of the bullet is absorbed on impact, so more of it translates to upward movement, albeit with some rotating.
Have I read your explanations right? I'll watch the video now.
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