# Question about Momentum vs. Kinetic Energy vs. Deforming In Collisions

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• Physicist-Writer
Physicist-Writer
I'm currently reading Jason Thalken's Fight like a Physicist, and I think so far they gave one of the most intuitive explanations of the practical difference between Momentum and Kinetic Energy, but I'd like to just conceptually reinforce it and make sure I have it right.

So from what I understand momentum is completely unconcerned with the stiffness or other material properties that compose the colliding masses are made of, as well as shape and size etc, the only thing that matters in determining whether one object will bulk translate another object in the collision are the mass and velocity. So it doesn't matter if it's gold clashing with iron etc, if the Iron mass is less than than the gold mass ( which it typically is since gold is denser than iron), and the gold was moving faster, it should always overpower the iron and make the iron move in the other direction. This is why, unlike in Hollywood, a bullet shouldn't move a heavy person much on impact if the recoil of firing the bullet in the first place didn't push back the shooter.

However, the Kinetic Energy is what determines the damage. The bullet exiting the gun has the same magnitude but opposite momentum as the recoil of the gun, but different kinetic energy since it was much lighter. This kinetic energy difference is part of the reason that the target can be injured significantly by the bullet, while the shoulder of the shooter is less harmed in comparison. The other part comes down to shape, surface area and the hardness/softness of the bullet and the target, which influence how energy is transfered and the pressure at contact.

So , the material properties necessarily dictate the magnitude of the forces and pressures at impact, but newton's third law still applies equal and opposite forces to both masses, so that whenever a block of iron clashes with the block of gold, the Force generated at the collision would deform the gold more than the iron in the clash, even if the gold still always pushes the iron away.

Is this accurate? Or are there some other potential factors I could be missing? Or is there a more precise way to think about this.

Physicist-Writer said:
So from what I understand momentum is completely unconcerned with the stiffness or other material properties that compose the colliding masses are ...
The amount of momentum transferred to the target does depend on whether the projectile bounces back, gets stuck or exits on the other side. And that in turn depends on material properties etc.

Physicist-Writer said:
This kinetic energy difference is part of the reason that the target can be injured significantly by the bullet, while the shoulder of the shooter is less harmed in comparison.
There is no fundamental physical reason why dissipating the bullet's energy on impact would do more damage than applying it on firing. If the targeted person had a bullet-catching-device that decelerates the bullet in exactly the inverse way as it was accelerated in the barrel, the pressure profile at his shoulder would be the same as for the shooter.

So the damage issue boils down to the second part, which is rather engineering than some fundamental physics reason:
Physicist-Writer said:
The other part comes down to shape, surface area and the hardness/softness of the bullet and the target, which influence how energy is transfered and the pressure at contact.

Physicist-Writer and berkeman
A.T. said:
The amount of momentum transferred to the target does depend on whether the projectile bounces back, gets stuck or exits on the other side. And that in turn depends on material properties etc.
So material properties determine the amount of force created, the lighter object gets affected more in terms of bulk acceleration by whatever newton third law pairs are formed, but damage done is completely dependent on how much pressure is generated and whether that's enough to overcome the stiffness of the objects, which is not directly related to mass.

A.T. said:
There is no fundamental physical reason why dissipating the bullet's energy on impact would do more damage than applying it on firing. If the targeted person had a bullet-catching-device that decelerates the bullet in exactly the inverse way as it was accelerated in the barrel, the pressure profile at his shoulder would be the same as for the shooter.
Yeah, I meant that gun is much heavier than the bullet so if equal force magnitude was applied to both it's impossible for the gun to get as much kinetic energy as the bullet which is part of why the shooter is not harmed as much by the recoil of his or her own gun.

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