# What is Kinetic energy theorem: Definition and 13 Discussions

In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest.
In classical mechanics, the kinetic energy of a non-rotating object of mass m traveling at a speed v is

1
2

m

v

2

{\displaystyle {\begin{smallmatrix}{\frac {1}{2}}mv^{2}\end{smallmatrix}}}
. In relativistic mechanics, this is a good approximation only when v is much less than the speed of light.
The standard unit of kinetic energy is the joule, while the English unit of kinetic energy is the foot-pound.

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1. ### Elastic potential energy - different methods, different results

Can someone please tell me where I am wrong? I tried to solve the problem using velocity equation; ##v_{f}^2= v_{i}^{2} + 2as## and got a= 50m/s^2, F= 50 000N and therefore F=kx -> k=50 000N/m because dx=1. But it's not correct. When I do it using conservation of energy I get 100 000N/m. Which...
2. ### Is Mechanical Energy Conserved for Non-Conservative Forces?

Hello, A generic force ##F##, which may be conservative or not, performs mechanical work which is always equal to $$W=\Delta KE=KE_{final}-KE{initial}$$ i.e. produce a change in the object's kinetic energy ##KE##. Work is essentially a way to inject or subtract kinetic energy from a system. If...
3. ### Dashpots and the Work - Kinetic Energy Theorem

Homework Statement I need to accommodate a dashpot in an intentionally simple work-kinetic energy analysis method. For example, for a box being dragged up a ramp via a rope while attached to a spring, I can deal with the work done by gravity, rope tension, spring force, and friction via the...
4. ### Physics of bullet impacts, I wonder if anyone can help?

If I have a bullet with 700 Joules KE, the mass is 0.008036m, velocity 417.39m/s, momentum p=3.35kg-m/s Kinetic energy is the ability to do work, Kinetic energy converts into FORCE...so yes KE will exert a force...which will do work, as to do work requires force and distance. Thus, when the...
5. ### Work-kinetic energy theorem - model rocket velocity/height

Homework Statement A student experimenting with model rockets measures the speed of a vertically-launched rocket to be 18.0 m/s when it is 75.0 m above the ground on the way up. The rocket engine fires from when the rocket is at ground level to when it is 8.75 m above the ground. If the rocket...
6. ### What about potential energy in the Work Kinetic Energy Theorem?

W_{total} = \delta K What about lifting a block upward? If you lift a 10kg block vertically and bring it to a rest, you are doing work on it but the velocity in the beg and the end is 0, thus the equation says the work done on it is 0. But isn't there potential energy? Does the equation not...
7. ### A rifle barrel question related with Kinetic Energy Theorem

Homework Statement A 15.0g bullet is accelerated from rest to a speed of 780m/s in a rifle barrel of length of 72.0cm a)Find the kinetic energy of the bullet as it leaves the barrel b)Use the work kinetic energy theorem to find the net work that is done on the bullet. (this problem had part...
8. ### Kinetic Energy - use a log scale or not?

Homework Statement Lets say we have a series of balls and an eggs. All items have equal mass but are traveling at different velocities. Thus they have differing kinetic energies. Now, for each 'ball' object we have, we want to find an egg object with a similar kinetic energy. Let's say...
9. ### Decelerating box on a truck and work

Homework Statement A truck is moving at constant velocity v0 with a box of mass m on it. Suddenly it starts decelerating until its velocity becomes zero. Between the box and the truck the is a force of friction and the deceleration of the truck is sufficiently slow to allow the box not to start...
10. ### Work - Kinetic energy theorem Need more fundamental understanding

I'm trying to find a good way to think about work. From what I can tell, it's what the object experiences, not really what the force does, correct? If a train is heading west and superman grabs it, stops it, and starts pushing it in the opposite direction until it's at the same speed as it...
11. ### Work and Kinetic Energy Theorem

Homework Statement A 4.5 kg box slides down a 5.1m high frictionless hill, starting from rest, across a 2.4m wide horizontal surface, then hits a horizontal spring with spring constant 540 N/m. The other end of the spring is anchored against a wall. The ground under the spring is...
12. ### Work kinetic energy theorem

Homework Statement A 2100 kg pile driver is used to drive a steel I-beam into the ground. The pile driver falls 5 m before coming into contact with the top of the beam. Then it driver the beam 12 cm farther into the ground as it comes to rest. Using energy considerations, calculate the average...
13. ### How Does the Kinetic Energy Theorem Apply to Bonny Blair's Skating Problem?

Bonny Blair skated 5.00 x 10^2 m with an average speed of 12.92 m/s. Suppose she corssed the finish line at this speed and then skated freely until her speed was 8.00 m/s. If her was mwas 55.0 kg, how much work was done by friction? Can anybody explain the kinetic energy theorem or help me...