# Homework Help: Work-Kinetic Energy Contrasting Newton's Laws?

1. Nov 13, 2007

### latitude

1. The problem statement, all variables and given/known data

This is a question that's supposed to prove to us that you can use the work-kinetic energy theorem as a second theory of motion. Except I keep getting different answers when I work it out first w/ KET and second with Newton's Laws. So I'm definitely NOT fulfilling the objective here...

In a rifle barrel, a 15 g bullet is accelerated from rest to a speed of 780 m/s.
a) Find KE, at rest AND when it reaches 780 m/s.
b) Find work done.
c) Barrel = 72 cm long. Find mag. of average net force with Fnet = W/delta'r'cos(theta), where delta'r'cos(theta) is the displacement.
And then you find acceleration w/ Newton's Laws and the net force.
2. Relevant equations
KE = 1/2mv^2, W = kf - ki, Fnet = ma

3. The attempt at a solution
At initial position, KE = 0
At final speed, KE = 5.85 J

Net force = 5.85 / (0.72m)(1) = 8.125 N (??)

Constant acceleration. This is so wrong.... 422500 m/s^2
Using vf^2 = vi^2 + 2a(x)

So my Fnet here = (0.015kg)(422500 m/s^2)...

Thanks!

2. Nov 13, 2007

### XJellieBX

your KE final is wrong. it should be much bigger than that.

Last edited: Nov 13, 2007
3. Nov 13, 2007

### latitude

D'oh! I totally forgot to square the velocity. Um. Yeah. Now everything works. Thanks...
Bullets have a friggin' huge acceleration! Zowee! Wouldn't want to get hit by one of those!