# Q & A: bullet shock

Q:
------------------------------------------------------------------------ On Mythbusters via high speed cameras we see bullets leaving guns and hitting objects. In both cases there is around a millisecond delay before we see the gun move to the bullet reaction and again after the bullet hits an object again there is about 1 millisecond before the object moves after being hit. What is this delay due to, as I wolud expect the reaction would evoke some immediate movement? Hope you can give me an answer as the problem is rattling around my head without resolution. Cheers Graeme Daw Australia.
- Graeme Daw (age 69)
Melbourne Victoria Australia
A:
The fastest that any mechanical disturbance will propagate is the speed of sound in the material. Typical sound speeds are around 1000 m/s. So in 1ms, the disturbance could propagate about 1 m. Is that roughly the size of the objects?

Mike W.

(published on 05/04/2010)

## Follow-Up #1: recoil time

Q:
Hi Mike, Thanks for the explanation regarding disturbance propagation. The gun fires the bullet from handgun is about 30 cm from the muzzel before the recoil barrel recoil starts, with a bullet velocity of around 300 metre/s. The propagation throughout the handgun to start the recoil has taken 1 millisecond and through a 15 cm device puts the propagation speed around 500 metre/s. Would you expect the propagation time to be so low? The same applies to the bullet striking a suspened handgun. In closing it would seem that before the propagation of the recoil has reacted on the gun mass, the gun or the suspended handgun have infinite masses. This would seem to be the case as the striking bullet completly and instantly dissintergrated well within the millisecond when striking the suspended handgun and well before its starts to move. With the above, is the infine mass a reasonable assumption? Look forward to your comments. Graeme Daw
- Graeme Daw (age 69)
Malmsbury Victoria Australia
A:
I'm not positive I'm getting the picture, but maybe it's something like this.

The recoil delay may occur because the departing bullet leaves a trail of hot exhaust moving backward in the barrel. It would take a little time for it to transfer its momentum to the gun. The relevant speed of sound may be in the gas, not the solid.

I really don't understand why the momentum transfer to the target is not faster.

Mike W.

I don't understand it either.    LeeH

(published on 05/09/2010)

## Follow-Up #2: momentum puzzle

Q:
Mike, With the bullet striking the suspended gun, the bullet disintergrates and the particles move on but still the gun takes about 1 millisecond before it starts to move. No gas involved here just energy transfer which seems to take about 1 millisecond, in a body where the sound conuction time is only microseconds. I supose you could set up an experiment with the swinging balls on strings where one ball is released and the energy is progressivly transfered through each ball and the last one is is moved in the direction of the first swinging ball. A high speed camera could then determine the delay between each ball. Then the experiment could then be conducted with much larger balls. The energy transfer delay could then be compared between large and small balls perhaps giving some insight into what exactly is happening. Perhaps there is some other factor in creating the delay in this energy transfer. Interesting.
- Graeme Daw
Australia
A:
Yeah, this seems like a real puzzle. Once the bullet has left the target, the target momentum is essentially set. So how can there be a delay before the target moves? It's not too hard to explain if some portion of the target can be moving, not very visibly, before the rest catches up. But that doesn't sound possible here.