Work-Kinetic Energy Contrasting Newton's Laws?

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SUMMARY

The discussion centers on the application of the work-kinetic energy theorem and Newton's Laws in calculating the kinetic energy and net force of a bullet accelerated in a rifle barrel. A 15 g bullet reaches a speed of 780 m/s, resulting in a final kinetic energy of 5.85 J, which was initially miscalculated due to an oversight in squaring the velocity. The average net force calculated using the work done over the displacement of 72 cm is approximately 8.125 N. The correct application of formulas reveals the significant acceleration experienced by the bullet, highlighting the importance of precise calculations in physics problems.

PREREQUISITES
  • Understanding of the work-kinetic energy theorem
  • Familiarity with Newton's Laws of Motion
  • Knowledge of basic physics equations: KE = 1/2mv^2, W = kf - ki
  • Ability to perform unit conversions (grams to kilograms, centimeters to meters)
NEXT STEPS
  • Review the work-kinetic energy theorem and its applications in motion problems
  • Study Newton's Laws of Motion in detail, focusing on their practical applications
  • Practice calculations involving kinetic energy and work done in various scenarios
  • Explore the implications of high acceleration in projectile motion
USEFUL FOR

Students studying physics, particularly those focusing on mechanics, as well as educators looking for practical examples of applying the work-kinetic energy theorem and Newton's Laws in real-world scenarios.

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Homework Statement




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.

Homework Equations


KE = 1/2mv^2, W = kf - ki, Fnet = ma


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!
 
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your KE final is wrong. it should be much bigger than that.
 
Last edited:
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!
 

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