What is Kinetic energy: Definition and 1000 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. Glenboro

    How Does Kinetic Energy Change in a Baseball After Being Hit?

    Homework Statement After being hit, a 0.145 kg baseball has a speed of 45.6 m/s. 1. What is the kinetic energy of the baseball after it has been hit? 2. How much work is done on the baseball by the baseball bat? 3. In moving through the air, the baseball slows down to a speed of 30.0 m/s...
  2. W

    Electric potential, Can Someone Check This?

    Homework Statement A particle charge of 7.6μC is in a uniform electric field directred to the left. Another force, in addition to the electric force, acts on the particle so that when it is released from rest it moves to the right. After it has moved 8cm, the additional force has done...
  3. Sampad Saha

    How does Potential Energy converted into Kinetic Energy? ?

    As we know that every object that has kinetic energy could also have potential energy. Now suppose an object is in rest at a distance of h meter from ground so it has some potential energy and no kinetic energy, now if it starts to fall towards the ground can we say that its total potential...
  4. Buzz Bloom

    How does a particle's kinetic energy vary with a(t)?

    I am curious about how a particle's kinetic energy changes with the explansion of the unverse. I calculated that for a matter dominated universe it would vary as 1/a2, while the photon temperature varies as 1/a. Is this correct? Is this also true in general? If so, the implication would be...
  5. L

    Average values of operators of potential and kinetic energy

    In case of quantum LHO in eigen state of the system ##|n \rangle## \langle \hat{T} \rangle=\langle \hat{U} \rangle=\frac{1}{2}(n+\frac{1}{2})\hbar \omega What will happened in some superposition of states? Does Ehrenfest theorem can tell me something more general? Is it possible to say that...
  6. shanepitts

    Average kinetic energy of harmonic oscillator

    I was wondering why the average kinetic energy is calculated in this manner. They are dividing the kinetic energy function by the period (noted here as To) and integrating it with limits from 0→To. Why? (This is a tiny portion of a larger textbook example, not an actual assignment.) Sorry for...
  7. Mr Davis 97

    Does v represent speed in kinetic energy formula?

    Since kinetic energy is in is a scalar quantity in joules, then is the v in (1/2)mv2 speed or velocity?
  8. Y

    How to calculte Kinetic Energy of Scissor Lift system

    Hi, here i need to calculate the tilt angle of the scissor lift. I had done some reviews for designing a scissor lifter, some source state that, tip-over will happen only when the kinetic energy of the system is less than the Potential energy after impact. Hence, i was trying to use this...
  9. Mr Davis 97

    Data for a potential and kinetic energy experiment

    I have performed an experiment testing how mechanical energy is conserved. I conducted this experiment by first creating a ramp. I gathered data by first calculating the gravitational potential energy of a marble on various points on the ramp (depending on height). Next I rolled the ball down...
  10. A

    Relativistic Kinetic Energy as a solution to a DE

    Has anyone ever seen KE derived this way? The only non-classical assumption is that mass = Energy/c^2 (I know, that's a big one) and that it accumulates as mass as the object is accelerated. (sorry for the plain text formatting, I need to learn how to do it right, I may reply with my first try)...
  11. J

    Temperature and Average kinetic energy of moleces

    1. The temperature of air wil ... if we add a hydrogen molecule of a speed higher than 1980 m s/-1. 2. The temperature of air will ... if we take away a hydrogen molecule of a speed highr than 1980 m s /-1. 3. The temperature of air will ... if we take away a hydrogen molecule of a speed lower...
  12. aladinlamp

    Kinetic Energy vs Momentum vs Power

    Imagine this object as a big tanker ship, moving to the right weight 500 000 metric tones linear velocity 0.01 m/s momentum cca 5 000 000 kg m/s kinetic energy 25 000 J, which is 25 kilowatt-secondshow much power could i extract from this object in order to slow it down to complete stop ...
  13. Quarlep

    Does a Moving Sphere's Kinetic Energy Affect Its Atomic Particles?

    Lets think we have a sphere and it moves a constant veloctity v.So it will have a kinetic energy.Is this kinetic energy efectts spheres particle energy.(sphere made up but atoms) Thanks
  14. P

    Can a Flywheel Store 1 MW of Power for 50 Seconds with Only 33% Energy Loss?

    Hi fellow scientists and engineers, I have a question regarding the design of a flywheel. I know that the actual engineering of a flywheel is very complex and in no way I should expect actual practical answers from this simple question. I'm just trying to get some estimates for a renewable...
  15. B

    Comparing force of mass at rest vs falling mass

    What equations can be used to compare the force of a resting X mass to the force of a smaller Y mass falling Z distance. Specifically, how much resting mass would equal the force of 300 pounds being dropped from 10 feet up?
  16. L

    Kinetic energy of Earth and ball system

    Homework Statement There is a system consists of a ball and the earth. Explain why you can consider the kinetic energy of the system equals to the kinetic energy of the ball. Homework Equations ##F=-\frac{m_{1}m_{2}}{{r}^{2}}## The Attempt at a Solution The magnitude of the force acting on...
  17. manogyana25

    Average kinetic energy of the molecules in a cold liquid less?

    Why is the average kinetic energy of the molecules a cold liquid less? As the temperature of a liquid decreases, the average kinetic energy of its molecules reduce. What is the reason behind this?
  18. F

    Do you consider both regular and rotational kinetic energy?

    Homework Statement If I have a ball moving in a circular path (ball is connected to a string), as shown in this picture: http://w3.shorecrest.org/~Lisa_Peck/Physics/syllabus/mechanics/circularmotion/Images/cent_force_on_ball.gif should I say that the energy of the ball is both its kinetic...
  19. E

    Work, Force, Energy and power question about moving car.

    Homework Statement A car has a power output of 45kW and travels at a constant speed of 30m/s. What force is the engine applying on the car? Homework Equations F=m*a W=ΔKE+ΔGPE KE=½mv^2 GPE=mgh W=F*d P=ΔE/ΔT gh_1+½v_1^2=gh_2+½v_2^2 The Attempt at a Solution I really have no clue. Since there...
  20. R

    Speed parameter β and Lorentz factor γ

    Homework Statement Find the speed parameter β and Lorentz factor γ for an electron that has a kinetic energy of: a) 6.00 keV b) 2.00 MeV c) 4.00 GeV Homework Equations KE=(γ-1)*511 KeV SQRT(1/(1-β^2))=γ The Attempt at a Solution a) 6 KeV=(y-1)*511 y=1.0117 1.0117=SQRT(1/(1-β^2)) β=0.152 b)...
  21. Helmet

    Developing impact protection, kinetic energy diffusion

    Hi, I'm just an average guy here, mostly self educated, and am looking for experts to verify my theories on developing revolutionary impact protection. If I disrupt direct impact to the head using a highly elastic layer of material, does that create an inelastic impact interruption? A helmet...
  22. K

    Spring pendulum, Kinetic Energy

    In the included picture, I don't get how they get to the kinetic energy part. I would say, that the traveled distance is equal to (l + x(t))*θ. Then I would take the time derivative, resulting in dx(t)/dt * θ + (l + x(t))* dθ/dt. Then I would square this result and multiply that with 1/2 m. But...
  23. CAH

    Electrons absorb exact energy photons so how is Ek possible

    Hello! I've read that electrons can only absorb photons of exactly the right amount of energy to move to a higher energy level, if its to little or too much then it doesn't absorb it at all, so my question: How can electrons be liberated from an atom with Kinetic energy when they can't absorb...
  24. P

    Obtaining kinetic energy equation.

    The question is about 2 sticks standing above each other, but i have a more general question. The position of my mass is described by: (2rsin(a) - rsin(b) , 2rcos(a)+rcos(b)) the goal is to get a formula for the kinetic energy, so i need the speed of the mass. If i take the derivatives of...
  25. Chrono G. Xay

    Kinetic Energy of Double Compound Pendulum and Parallel Axis

    Kinetic Energy of Double Compound Pendulum and Parallel Axis Theorem Hello, there. I have a project I'm working on where I need to be able to calculate the kinetic energy of what basically amounts a double compound pendulum. However, the pivot point of the second pendulum is not at the center...
  26. Dadface

    Is 'Carrying' the Right Term for Describing Kinetic Energy in Moving Objects?

    I think its correct to state that: "Kinetic energy is a property of a moving object." But would it be correct to state that: "Kinetic energy is carried by a moving object"? If not is there a better alternative terminology to use? Thank you.
  27. xxyourstruly

    How do I calculate percent elasticity?

    Homework Statement I was given a chart to figure out the different momentum and kinetic energies of different collisions (elastic and inelastic) and in the lab he posed the question "Calculate the percent elasticity for each of the collisions." Homework Equations conservation of momentum...
  28. T

    I have one question regarding Kinetic Energy

    The question is. There are two particles, one with charge -q and the other with charge Q, and -q particle rotates around the Q particle at a radius r1. What is the Energy at this point? The answer is like this: E= KE + U So now I know that U= -kQq/r1 But according to the answer: KE= kQq/2r1...
  29. G

    How much kinetic energy is released

    Homework Statement A 6.0-kg object, initially at rest in free space, “explodes” into three segments of equal mass. Two of these segments are observed to be moving with equal speeds of 20 m/s with an angle of 60° between their directions of motion. How much kinetic energy is released in this...
  30. Phynos

    Relativistic kinetic energy - particle decay

    Homework Statement A pion at rest (mπ = 273me) decays into a muon (mμ = 207me) and an antineutrino (mn ≈ 0). Find (a) the kinetic energy of the muon and (b) the energy of the antineutrino in electron volts. Homework Equations K = (γ-1)mc2 E = γmc2 ER = mc2 E2 = p2c2 + (mc2)2 I didn't...
  31. G

    Energy question about kinetic energy and potential

    Homework Statement [/B] Objects A and B, of mass M and 2M respectively, are each pushed a distance d straight up an inclined plane by a force F parallel to the plane. The coefficient of kinetic friction between each mass and the plane has the same value...
  32. 1

    Conservation of Angular Momentum & Energy question

    Question: A streetcar is freely coasting (no friction) around a large circular track. It is then switched to a small circular track. When coasting on the smaller circle its speed is: a) greater b) less c) unchanged Relevant Formulas: w = v/r KE = 1/2mv2 My teacher said the normal force from...
  33. B

    Kinetic energy of object free fall

    If an object is dropped from height, h, then gravitational energy is E = mgh, but kinetic energy when object hits the ground is E=(1/2)mv^2. Conservation of energy states that E(g)=E(k). Why does kinetic energy not depend on height object was dropped?
  34. goonking

    Solving for Height of a Falling Cube Using Kinetic Energy

    Homework Statement Homework Equations 1/2 mv^2 = mgh The Attempt at a Solution first, a side question, if p = mv, can't we just place p into 1/2 mv^2 so it becomes 1/2 p^2? anyway, on to the question, at the point where the cube falls off, the Fnormal should just about equal 0, correct? is...
  35. C

    Prove Kinetic Energy equation (without calculus)

    Homework Statement Prove kinetic energy is relationship between mass and velocity using E(g)=mgh (no calculus, momentum, kinematics). Homework Equations gravitational energy: E=mgh kinetic energy: E=(1/2)mv^2 The Attempt at a Solution I know it can be derived using the gravitational energy...
  36. B

    Box on Inclined Plane (with Friction)

    Homework Statement A crate of mass 9.2 kg is pulled up a rough incline with an initial speed of 1.52 m/s. The pulling force is 110 N parallel to the incline, which makes an angle of 20.6° with the horizontal. The coefficient of kinetic friction is 0.400, and the crate is pulled 4.94 m. (e)...
  37. C

    Kinetic Energy Time Derivative

    Homework Statement So the first part asks to prove the time derivative of kinetic energy is dT/dt=F dot product v which I did not problem. but then the second part of the problem asks to prove that if the mass is changing with time then the time derivative of d(mT)/dt=F dot product m and I'm...
  38. davidbenari

    Does kinetic energy increase the inertial mass?

    In class we had a conceptual problem: There will be a race between two springs. Both will receive the same initial impulse, but one has been agitated and is therefore oscillating in some way, the other is just still. Who arrives first? The answer is that the still spring arrives first because...
  39. M

    Bungee jump height/spring energy

    Homework Statement Imagine that you have been given the job of desiging a new bungee jumping platform. Customers of mass 53.0 kg will step off a platform, attached to a rope of unstretched length 52.6 m and spring constant 17.0 n/m. How far below the platform will the end of end of the rope...
  40. N

    Does electromagnetic radiation actually carry kinetic energy?

    Hi all, Does electromagnetic radiation actually carry kinetic energy? Looking around the web gives different answers. I have seen explanations like radiation pressure experiments as evidence, but this has been argued being effects from energy absorbed by the recipient. In case it does not...
  41. K

    I with rotational kinetic energy problems

    I tried to use this formula: KE=(1/2)(I)(w)^2, and work=change in KEFor the first question, i tried to plug the number into the rotation kinetic energy formula: 3600rpm=376.8rad/s (0.5)(2,000kg)(0.125m)^2(376.8rad/s)^2 I found out this was the answer 1.1 x 106 J, but I got 2218410J For the...
  42. T

    Work, Kinetic Energy, Spring, Friction Problem

    A block slides from rest, along a track with an elevated left end, a flat central part, into a relaxed spring, as shown in the figure. The curved portion of the track is frictionless, as well as the first portion of the flat part of L = 10 cm. The coefficient of kinetic friction between the...
  43. H

    A spring and two blocks with kinetic energy

    1. I have two blocks, one that is 1.6 kg moving left at 3.00 m/s and the other is 2.1 kg moving right at -1.74 m/s, and they are compressing a block with a spring constant of k=600 N/m. I'm supposed to be finding the distance that the spring is compressed at that instant, and I know that...
  44. G

    How to calculate the change in kinetic energy on a plane?

    Homework Statement A crate is pulled by a force (parallel to the incline) up a rough incline. The crate has an initial speed shown in the figure below. The crate is pulled a distance of 9.33 m on the incline by a 150 N force. The acceleration of gravity is 9.8 m/s 2 . What is the change in...
  45. Suraj M

    Derive the equation for kinetic energy

    question: I came across a question which said: Derive the equation for kinetic energy of a body of mass m and velocity v. What do they mean? Attempt: I actually just wrote that by work energy theorem diff. in KE = work done so Fx = KE $$ max = m \frac {v²-u²}{2x} x = ½mv² - ½mu² $$ each of...
  46. **Mariam**

    Kinetic Energy and Work problem

    Homework Statement [/B] A horizontal force of 50 N is applied to a 2.0 kg trolley, initially at rest, and it moves a distance of 4.0 m along a level, frictionless track. The force then changes to 20 N and acts for an additional distance of 2.0 m. (a) What is the final kinetic energy of the...
  47. B

    Calculating Kinetic Energy of a Gas at Different Temperatures

    Homework Statement A stationary gas has a kinetic energy of 500 J at 30 C. What is its kinetic energy at 60 C? Homework Equations KE = 3/2 RT The Attempt at a Solution I first converted T to Kelvin = 333K. I substituted all the values into find KE but my answer is around 40K, which is not...
  48. S

    Kinetic Energy, What is K2/K1

    Homework Statement A rock is dropped from a distance RE above the surface of the Earth, and is observed to have a kinetic energy of K1, when it hits the ground. An identical rock is dropped from 2RE above the surface and has kinetic energy K2 when it hits the ground. Re is the radius of the...
  49. H

    Total Kinetic Energy: Moment of Inertia & CM Axis

    K = Ktranslational + Krotational My book says that in order for this equation to be valid the moment of inertia must be taken about an axis through the center of mass. Why is this true?
  50. irre1evantt

    Relativistic Kinetic energy, momentum, speed.

    Homework Statement An Electron (rest mass=9.11*10^-31kg) is accelerated to an energy (mass energy+kinetic energy) of 30*10^6 eV (30 MeV). What is its kinetic energy? Its momentum? Its speed? (Note: 1 eV = 1.602*10^-19 Joules; c=2.998 * 10^8 ms^-1) Homework EquationsThe Attempt at a Solution...
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