Work-Kinetic Energy Contrasting Newton's Laws?

In summary, the conversation is about a question requiring the use of the work-kinetic energy theorem as a second theory of motion. The person is having trouble getting consistent answers when using both the theorem and Newton's Laws. They provide information about a rifle barrel and a bullet's acceleration, and ask for help in finding the average net force and acceleration using different equations.
  • #1
latitude
56
0

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!
 
Physics news on Phys.org
  • #2
your KE final is wrong. it should be much bigger than that.
 
Last edited:
  • #3
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!
 

FAQ: Work-Kinetic Energy Contrasting Newton's Laws?

What is work?

Work is defined as the product of force and displacement in the direction of that force. In simpler terms, it is the energy that is required to move an object a certain distance.

What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is directly proportional to the mass of the object and the square of its velocity.

How do Newton's laws relate to work and kinetic energy?

Newton's laws of motion explain how forces affect the motion of an object. The first law states that an object at rest stays at rest and an object in motion stays in motion with a constant velocity, unless acted upon by an external force. The second law relates force and acceleration, which in turn affect the work and kinetic energy of an object. The third law states that every action has an equal and opposite reaction, which can also affect work and kinetic energy.

How does work change kinetic energy?

Work can change the kinetic energy of an object by either increasing or decreasing its speed. When work is done on an object, the object gains kinetic energy. On the other hand, when work is done by an object, its kinetic energy decreases.

What is the difference between conservative and non-conservative forces in relation to work and kinetic energy?

Conservative forces, such as gravity and elastic forces, do not dissipate energy and therefore do not change the total mechanical energy (the sum of kinetic and potential energy) of an object. Non-conservative forces, such as friction and air resistance, dissipate energy and therefore decrease the total mechanical energy of an object. This can affect the work and kinetic energy of the object as well.

Back
Top