Questions about the irrelevance of energy (ft-lbs)

[HandgunHTR]

Yea I should have said rifle not shooter. The point was that the bullet receives more energy due to it having smaller mass.

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle weigh the same as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

 

[BigCountry49]

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle weigh the same as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

 

[waldo9190]

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

Well when you think about the fact that object A (bullet) weighs 150-ish grains, and object B (rifle) weighs 70,000-ish grains, its not so hard to fathom how one of them moves significantly faster than the other.

 

[Bluefish]

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle weigh the same as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

I think energy is being substituted for momentum. The momentum is conserved, energy is calculated from the velocity of the objects in motion. Since the larger object moves more slowly, energy is significantly less due to the velocity being squared in energy and not in momentum. Energy = 1/2 x mass x velocity ^2, momentum = mass x velocity.

 

[Caseknife]

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

Newton's Third Law of Motion, for every action there is an equal and opposite reaction. So, yes, @HandgunHTR statement is correct . If the rifle weighed the same as the projectile and powder, wouldn't want to pull the trigger standing behind it.

 

[BigCountry49]

Newton's Third Law of Motion, for every action there is an equal and opposite reaction. So, yes, @HandgunHTR statement is correct . If the rifle weighed the same as the projectile and powder, wouldn't want to pull the trigger standing behind it.

Newton’s third law applies to force and momentum, not energy. The rifle will receive the same amount of momentum, but not the same energy as the bullet.

 

[Dobermann]

I just spend time with a sniper who was deployed 4 times in Iraq. They did shoot .338 L FMJ BT.

Did they say how much knockdown power it had?

 

[HandgunHTR]

I think energy is being substituted for momentum. The momentum is conserved, energy is calculated from the velocity of the objects in motion. Since the larger object moves more slowly, energy is significantly less due to the velocity being squared in energy and not in momentum. Energy = 1/2 x mass x velocity ^2, momentum = mass x velocity.

I am well aware of the differences between energy and momentum. This is ultimately why it is difficult having this conversation with people who's understanding of physics comes from gun rags and ammo boxes (comment not aimed at you, just folks on gun forums in general).

The statement that I refuted was "the bullet receives more energy due to it having a smaller mass". That is simply not correct. Accuracy of wording is crucial when having these discussions, because as you have pointed out, there is a significant difference between energy, momentum, mass, weight, etc.

The amount of KE imparted to the bullet and the rifle are the same. However, in order to overcome inertia a lot more of it is used up to convert that energy to motion when talking about the rifle. So the amount of acceleration for the bullet is much higher than it is for the rifle (Newton's 2nd law). The result of this is that the bullet ends up with more momentum than the rifle, but energy imparted to both of those objects is the same.