What is Kinetic: 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. 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...
  2. T

    How Does Doubling the Force Affect Kinetic Friction Between Two Blocks?

    Homework Statement Block M1 is on top of block M2 which is on top of a horizontal frictionless surface. A horizontal force, F, is applied to M1. The coefficient of static friction is us and the coefficient of kinetic friction isuk. Find, a) the maximum force that can be applied to M1, such...
  3. 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...
  4. Dr. Who

    Conservation of Energy problem

    Question:- A bead slides without friction around a loop-the-loop (Please click on the below link for diagram). The bead is released from a height h = 3.50R. (a) What is its speed at point A? (b) How large is the normal force on it if its mass is 5.00 g...
  5. 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?
  6. 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...
  7. 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...
  8. 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)...
  9. 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...
  10. K

    Help me understand static and kinetic friction please

    Hi, I understand that the static coefficient of friction dictates how much force is required to initiate motion/slippage and kinetic(dynamic) coefficient of friction dictates how much force is required to maintain motion/slippage. So considering a clutch with a friction and drive plate, what is...
  11. 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 ...
  12. 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
  13. SpacemanRich

    Newtons' Second Law with Kinetic Friction Problem

    Homework Statement While moving in, a new homeowner is pushing a box across the floor at a constant velocity. The coefficient of kinetic friction between the box and the floor is 0.41. The pushing force is directed downward and an angle θ below the horizontal. When θ is greater than a...
  14. 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...
  15. 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?
  16. 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...
  17. rcgldr

    Kinetic friction and regenerative braking dilemma

    Assume a car has a smart regenerative braking system that optionally locks up the wheels with the tires skidding or allows the wheels to rotate while skidding, using the torque from the rotating and skidding tires to generate power into some type of kinetic energy recovery system. Assume the...
  18. 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...
  19. 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...
  20. R

    How to relate mass defect to kinetic energies?

    Homework Statement Just one more nuclear question if I may, Homework Equations None. The Attempt at a Solution I can do the first part of this question, but am unsure on the second part. I thought it may be related to the Q-factor (the difference in energy of the initial and final products...
  21. 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...
  22. 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...
  23. terryds

    Kinetic Theory of Gas: F=-2mv/2L/V

    F = Δp/Δt http://www.sumoware.com/images/temp/xzhhjttxobbodxgr.png (Sorry for bad sketch.. hehe) In the kinetic-molecular theory of gas, the Δp is -2mv (since it's supposed to be elastic collision) But, why the Δt is 2L/V ?? I think it must be a very small time (since the impact is done in a...
  24. 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.
  25. 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...
  26. 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...
  27. C

    Boltzmann equation and Kinetic equilibrium

    In section 3.1 of Dodelson's "Modern Cosmology", after introducing the Boltzmann equation, in the second paragraph of page 60 the author states that: "The first, most important realization is that scattering processes typically enforce kinetic equilibrium. That is, scattering takes place so...
  28. G

    At what angle of force is Kinetic friction least?

    Homework Statement The total force needed to drag a box at constant speed across a surface with coefficient of kinetic friction Uk is least when the force is applied at an angle theta such that a.sin(theta) = Uk. b.cos(theta) = Uk. c.tan(theta) = Uk. d.cot(theta) = Uk. e.sec(theta) = Uk...
  29. 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...
  30. C

    Boltzmann equation Kinetic equilibrium (Dodelson)

    In section 3.1 of Dodelson's "Modern Cosmology", after introducing the Boltzmann equation, in the second paragraph of page 60 the author states that: "The first, most important realization is that scattering processes typically enforce kinetic equilibrium. That is, scattering takes place so...
  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. C

    Determining Coefficient of Kinetic Friction w/m, t, and v

    Homework Statement A 2.0 kg block of wood slides over a horizontal surface and comes to rest in 2.5 s. If its initial velocity was 9.0 m/s, what is the coefficient of kinetic friction? Homework Equations rho=mv change in rho=F*change in t Force of friction = mu*Force normal Force normal=mg...
  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. G

    Finding the kinetic friction between the block and surface

    Homework Statement [/B] The block shown is pulled across the horizontal surface at a constant speed by the force shown. If M = 5.0 kg, F = 14 N and Theta= 35°, what is the coefficient of kinetic friction between the block and the horizontal surface? Homework Equations Fx = Fcos(theta) Ff = Fn...
  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. 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...
  40. 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...
  41. P

    Calculating the coefficient of kinetic friction

    Homework Statement Problem: A 10 kg box slides down a hill and has a constant velocity. The angle of the hill is 10° with respect to the horizontal. What is the coefficient of kinetic friction between the surface of the hill and the box? Homework Equations Fn = mg + ma ƒk = μk * Fn The Attempt...
  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. J

    Kinetic Theory of Gases Derivation

    Hi, I am struggling with a particular point on the derivation of the kinetic theory of gases. Between 5mins and 6mins 30seconds of the video below they discuss how to get a value for the average of the squared velocities of the molecules in the gas. The bit I don't get is why they divide by 'N'...
  45. 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...
  46. 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...
  47. Poetria

    Maximum kinetic friction-combined with a projectile problem

    Homework Statement A cannon at ground level is shooting emergency packets to people stranded on the roof of a flooded building of height H=125 meters. The corner of the building is located a distance D=50 meters from the cannon. It is desired that the incoming packets are flying tangent to the...
  48. **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...
  49. 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...
  50. S

    How Does Dropping Rocks from Different Heights Affect Kinetic Energy Ratios?

    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...
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