# Bullet penetration

1. Feb 23, 2014

### oneamp

If F=ma, why does a bullet that's going a constant velocity, have enough force to penetrate an object?

2. Feb 23, 2014

### Staff: Mentor

It doesn't go at a constant velocity. A bullet decelerates very rapidly.

3. Feb 23, 2014

### Staff: Mentor

A moving bullet doesn't have 'force', it has momentum and energy. Under the right conditions, that energy may be sufficient to allow the bullet to penetrate an object. During the collision, forces are generated that slow down and deform the bullet.

[STRIKE]And as DaleSpam stated, [/STRIKE]a bullet doesn't move with constant velocity. The air exerts a retarding force on it.

Last edited: Feb 24, 2014
4. Feb 23, 2014

### oneamp

Thanks...

5. Feb 23, 2014

### Staff: Mentor

I'm pretty sure that's not what he meant. I'm pretty sure that he meant that in the case being described by the OP - when the bullet hits something - it isn't going at a constant velocity, it is decelerating very rapidly, which involves a very large force.

...We've gotten almost this exact question several times in the past few days...

6. Feb 23, 2014

### LikesIntuition

You could look at the bullet's behavior in a vacuum, in which case there would be no air friction. Then it would travel at a constant velocity, and when it hit something, some sort of impulse would occur, which involves forces over some period of time that change the momentum of both the bullet and whatever it's hitting, but conserve the total momentum.

This is exactly what's happening in the air, too. It's traveling through a "vacuum" of space, that also happens to be filled with air molecules. So when the bullet runs into those molecules, the interaction I described above happens. Sorry if that just sounds like rambling, but I figured another take on it might help you glean some more insight!

And if your question is specifically about how a thing moving at a constant velocity can deliver a force, then it's like the posters above were saying: when the bullet makes contact with anything else, it does accelerate (changes velocity) and that acceleration is the one in the F=ma equation, not its lack of acceleration BEFORE the impact. The bullet has no "force" if it isn't interacting with something. Hope that helps a bit!

Last edited: Feb 23, 2014
7. Feb 24, 2014

### Staff: Mentor

Ah, OK. I was wondering, since I would not have said that a bullet decellerates rapidly through the air. :uhh:

8. Feb 26, 2014

penetration of bullet

It penetrates not due to its velocity but by the deceleration provided by the body into which it penetrates. hence by action reaction force is exerted and the bullet penetrates

hope this helps.
if there is need of further comprehension please feel free to reply.

regards

9. Feb 26, 2014

$F=\frac{mv-mu}{t}$
$F=\frac{0.2*0-0.2*500}{0.5}=-200N$