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.

View More On Wikipedia.org
  1. Ennio

    Formulating the kinetic energy of an object - helical motion

    Hi guys, I need your support to formulate the kinetic energy of an object: - having mass m [Kg] - rotating with angular velocity o [rad/sec] referred to an axis t [m] distant (and parallel) to the symmetry axis of the object - moving along the direction of its symmetry axis with a costant...
  2. Exath

    Rotation of Rigid Bodies: Rotating stick with disc on top

    Homework Statement [/B] A thin cylindrical rod with the length of L = 24.0 cm and the mass m = 1.20 kg has a cylindrical disc attached to the other end as shown by the figure. The cylindrical disc has the radius R = 8.00 cm and the mass M 2.00 kg. The arrangement is originally straight up...
  3. T

    B Is the kinetic energy of a particle quantized?

    Is the kinetic energy of a particle quantized? Does it increase and decrease in discreet amounts?
  4. Monsterboy

    Kinetic energy and momentum in an elastic collision

    Homework Statement Mass m1 = 2kg traveling at v = 3 m/s Mass m2 = 3kg traveling at v = 2 m/s After an elastic collision (from opposite directions) what will be the momentum and velocities of each of the bodies ? Homework Equations [/B] Momentum = mass x velocity ##K.E = \frac {1}{2}.m.v^2 ##...
  5. S

    I Particle acceleration and kinetic energy

    Hi, I was reading about magnetic reconnection in astrophysical plasma and it says that magnetic reconnection transforms magnetic energy into heat via ohmic dissipation, particle acceleration and kinetic energy. It is not clear to me the difference between particle acceleration and kinetic...
  6. GabrielLight

    I About gravitational binding energy and kinetic energy

    If a celestial body's kinetic energy (say, the Moon's) surpassed the energy necessary to exceed the gravitational binding energy (GBE), would said body break apart because of it? For example, the Moon is currently orbiting the Earth at a speed of 1020 meters per second, giving it a kinetic...
  7. C

    I Total or kinetic energy in Bethe Bloch stopping power?

    The average loss of energy in a material per unit length of a particle (in particular an electron, which is stable) is described by the quantity ##dE/dx##. - for ionization it is given by the Bethe-Bloch formula $$-\left\langle {\frac {dE}{dx}}\right\rangle ={\frac {4\pi }{m_{e}c^{2}}}\cdot...
  8. A

    Do electromagnetic waves have potential and kinetic energy

    Do electromagnetic waves have potential and kinetic energy like springs, strings, etc. If so how are they calculated, inter-related? What is the total energy? Are the energies fluctuating over time? I am a physics hobbyist so generally the first answers should come with the least mathematics...
  9. M

    Kinetic Energy vs Flow Work: Exploring the Difference

    As per work energy principle, the energy spent in moving an object is stored as kinetic energy.Flow work is done to make the fluid move.Why is it not included in kinetic energy (i.e) why kinetic energy and flow work are considered seperately?
  10. C

    Maximum kinetic energy / cutoff potential problem

    Homework Statement Light with a wavelength of 630nm is directed at a metallic surface that has a work function of 1.5 eV. Find:a) The maximum kinetic energy of the emitted electrons. b) The cutoff potential required to stop the photoelectrons. Homework Equations Ek = hc / λ - W Ek = eVo...
  11. I

    Find the launch speed of a ball in a spring mechanism

    Homework Statement I have a question asking me to find the launch speed of a ball (mass 0.39kg) when released by a spring mechanism made of 2 springs each with force constant 25Nm^-2. they are pulled back 12 cm. the ball is initially at rest.Homework Equations [/B] v^2=u^2+2as f=ma f=kx...
  12. negative

    Relation between boltsmann/gas constant and temperature

    so i have been trying to calculate boltsmann constant by assuming the fact that for an ideal gas the equation : PV=nRT is true. i assume that for containing each molecule the wall needs to apply a force. now here is where it get's a little weird. each molocule should be only deflected in the...
  13. A

    Kinetic energy of a bouncing ball

    Homework Statement I'm trying to find the coefficient of restitution (COR) and energy loss upon impact of a bouncing ball, and I'm trying to understand how these relate to each other. Homework Equations COR=sqrt(2gh/2gH) suvat equations (possibly) 1/2mv^2=mgH (using m=0.0585kg, g=9.8) energy...
  14. Lukeblackhill

    Reduce Kinetic Energy Losses when Two Objects Collide

    I'd like to know which methods I could use to reduce the loss of kinetic energy when two iron balls collide, or when a plastic ball collides with the floor after a free fall. I have tried to cover the floor with a fabric that would make it more elastic, but it had a small effect (if it hat...
  15. J

    Mechanical energy for planet in elliptical orbit around star

    Homework Statement A planet is in an elliptical orbit around a star. Which of the following best represents the mechanical energy E_planet of just the planet and the mechanical energy Es_tar-planet of the star-planet system as functions of time for one complete orbit? Homework Equations...
  16. J

    Which sphere reaches the bottom of the inclined plane first

    Homework Statement Two spheres are placed side by side on an inclined plane and released at the same time. Both spheres roll down the inclined plane without slipping. (a) Using FBD, explain what force provides the torque allowing the sphere to roll down the inclined plane. (b) Which sphere...
  17. H

    Find the change in the Kinetic energy of an Ideal Gas

    Homework Statement Let 3/2kT be the kinetic energy of ideal gas per molecules. T the absolute temperature and N the avogadro number. Answer the following questions : 1) when the volume doubled at constant temperature. How many times the kinetic energy per molecule become greater than before...
  18. M

    Conservation of momentum - with understanding

    I just want to state that i DID solve the problem. I just seek understanding of it. I'd be really grateful if someone could answer two of my questions at the end of this post. The problem I've solved here is just to show what I'm dealing with. 1. Homework Statement Object 1 is moving towards...
  19. E

    How kinetic energy is proportional to velocity squared

    I hear somewhere explanation why kinetic energy is proportional to $v^2$ and not to $v^3$, for instance. https://www.physicsforums.com/threads/quadratic-forms-and-kinetic-energy.826884/ I wish to find reference for this. Now I find your thread with the same question and very similar answers...
  20. Suyash Singh

    Projectile motion, potential and kinetic energy

    Homework Statement https://www.physicsforums.com/attachments/upload_2018-4-29_12-40-30-png.224871/ Homework EquationsThe Attempt at a Solution 1/2mv^2=10 1/2m(ucos60)^2=10 1/2mu^2/4=10 mu^2=80 what to do now?
  21. M

    Angular momentum with a large wooden turntable

    Homework Statement Homework Equations Li = Lf L = I*omega K = (1/2)*(I)*(omega)^2 The Attempt at a Solution [/B] Given that there are no non-conservative forces in action, I am assuming that the two kinetic energies should be equal. However, as shown by my work above, the two values...
  22. E

    Finding the Kinetic Energy using the mass, radius and initial KE

    Information Given:Zero, a hypothetical planet, has a mass of 4.2 x 1023 kg, a radius of 2.8 x 106 m, and no atmosphere. A 10 kg space probe is to be launched vertically from its surface. Question: (a) If the probe is launched with an initial kinetic energy of 5.0 x 107 J, what will be its...
  23. T

    Kinetic energy of an alpha particle

    Homework Statement During the ##\alpha## decay of ##^{238}_{92}U## an ##\alpha## particle is fromed inside the nucleus. Given that the last 4 nucleons in ##^{238}_{92}U## have an average binding energy per nucleon of ##6.1MeV##, estimate the kinetic energy of the ##\alpha## particle when it is...
  24. D

    Change in the Kinetic Energy of a System

    The question is to find the change in kinetic energy of a system. the system in use is a pully system with a wooden block on a flat surface attached to a 100g mass. I have calculated force of friction between the block and desk, and using a ticker timer, I have individual speeds at every 0.1 of...
  25. A

    How close will body approach fixed point charge?

    Homework Statement Charged sphere with a mass of 15 mg and charge 2 nC moves with a speed of 15 cm/s towards a fixed point charge of 3 nC. How close will sphere approach charge? Homework Equations K=(1/2)*mv2 U=k*(Q1Q2/r) The Attempt at a Solution So I am not sure I approached correctly but...
  26. F

    Kinetic Energy Needed for Proton Acceleration

    Homework Statement a) Calculate the kinetic energy required to accelerate a single proton from a rest position to 0.9999c. The mass of a proton is 1.67*10^-27 kg. b) Find the ratio of the kinetic energy to the energy of a proton at rest. c) Explain why no particle accelerator will ever be able...
  27. Amitayas Banerjee

    Kinetic energy as seen from a different frame

    Homework Statement A mass m is initially at rest. A constant force $F$ (directed to the right) acts on it over a distance d. The increase in kinetic energy is therefore $Fd$. Consider the situation from the point of view of someone moving to the left at speed $V$ . Show explicitly that this...
  28. E

    Calculating Change in Kinetic Energy for a Collision on a Frictionless Air Table

    Homework Statement On a frictionless, horizontal air table, puck A (with mass 0.250 kg ) is moving toward puck B (with mass 0.360 kg ), that is initially at rest. After the collision, puck A has a velocity of 0.125 m/s to the left, and puck B has velocity 0.655 m/s to the right. I already...
  29. L

    Kinetic energy in electron volts

    Homework Statement How much kinetic energy is in electron volts? Homework Equations Kinetic energy = 1/2 mv2 1 electron volt = 1.6 x 10-19 J The Attempt at a Solution [/B] It's a bit unclear to me what the question is asking, since no context at all is given. It's one of the questions in...
  30. almosthavoc

    Very quick question regarding work and kinetic energy

    Homework Statement The question gave a force applied on a block as well as a distance. It also gave the work done by friction which was negative. The question was to find the change in kinetic energy.Homework Equations Wnet = change in KE The Attempt at a Solution I found the work done by the...
  31. T

    Kinetic & Potential Energy of 0° Ice/Water: Examined

    Hi. In all of my question, I mean degrees celsius when I say degree. Think we have a 0 degree ice. we give some energy to it (by a heater) and all of it get into a 0 degree water. How does the kinetic and potential energy of the molecules change? In the book in which I saw this question, it...
  32. M

    Kinetic Energy Equations in MS Excel

    Homework Statement I am having trouble with Microsoft Excel formulas to obtain the correct data for Kinetic Energy 2 initial, KE1 final, KE2f, KEi, KEf, total KE and total KE% Homework Equations Microsoft office uses equations such as =(A2+B2) and I need to determine what is the appropriate...
  33. K

    Lifting an object, kinetic energy, work and forces

    Homework Statement From a physics multiple choice test: "A car is raised to a height of 10 meters. The car is at rest both before and after it has been lifted. The mass of the car is 900 kg. The total work that has been done on the car during the move is" (The right answer is: zero). Wnet...
  34. S

    Help me find my error in a relativistic kinetic energy calculation

    Hi everyone. Given: an asteroid with the mass of 50,000,000 kg, which is moving with the velocity of oh-my-god particle -- 99.99999999999999999999951% of c. Due to relativistic effects, its total kinetic energy will be 1.44 E+36 Joules (Lorentz factor = 3.2 E+11). A hypothetical particle...
  35. lc99

    Is my reasoning correct? (rotational kinetic energy)

    Homework Statement Homework Equations L = IW RKE = .5IW^2 The Attempt at a Solution Since the two solid spheres have equal radii and same angular momenta, except that the mass differs, i think that the inertia is equal. However, since sphere A is more heavy, it would spin slower than sphere...
  36. G

    B How Is the Coefficient of Restitution Linked to Conservation of Energy?

    Where does the Coefficient of Restitution equation come from, in terms of the Conservation of Energy? The measure of efficiency is the velocity after the collision divided by the velocity before the collision, but how does it get to that point? I'm trying to view this focused on an object that...
  37. E

    Displacement of an object with certain Ek stopped by Frictional Force

    Please do excuse if certain terms are not the most technical ones as I am translating the problem from another language. Do feel free to kindly inform me of the more technical term that substitues the sloppy one I've used. (And do excuse the generally bad English overall :-)) 1. Homework...
  38. S

    Kinetic energy, size of object, and force applied

    Kinda looking to confirm what I already suspect. All things being equal, if two objects have the same kinetic energy, does the fact that one is larger and heavier than the other (although at a slower velocity) mean anything? Specifically when comparing bullets. One bullet .451" in diameter...
  39. How to Find the Potential Energy of a Particle # 9 - YouTube

    How to Find the Potential Energy of a Particle # 9 - YouTube

    How can one find the potential energy of a particle. The derivation of PE can be done by the fact that change in PE of a particle is equal to negative of work done by a force on the particle. This combined with Kinetic energy work theorem gives us an equation on conservation of mechanical energy.
  40. G

    Energy required to accelerate itself

    Hi. The energy required to accelerate an object of mass ##m## to a speed ##v## is ##E_k=\frac{m}{2} v^2##. But how much energy does an object need to accelerate itself to ##v##? Say, how much fuel (in energy units) does a car need to accelerate to ##v##, if there are no air resistance or...
  41. S

    How Do You Calculate Power in These Physics Problems?

    Homework Statement There are three problems: 1. How much power is required to lift a 10.00 kg barbell at a speed of 3.0 m/s? 2.What is the average power supplied by a 70.0 kg firefighter climbing up a ladder in 10.0 s? 3. A man pulls a wagon with a force directed 22.0˚ above the horizontal...
  42. Y

    Momentum vs kinetic energy help

    I have a question about momentum vs kinetic energy. For example, a block C with velocity Vc and mass=2m hit a block B with mass=m at stand still on a LEVELED frictionless track( no change of potential energy). The two block stick together and move at velocity Vcb. Find the relation of Vc vs...
  43. T

    HW question w/ rotational kinetic energy

    Homework Statement : [/B] A stationary horizontal platform is free to rotate about its vertical axis. The radius of the platform is R=1.6m and its moment of inertia is 660 kgm^2. A 43 kg boy jumps on the rim of the platform with the velocity 2.2 m/s tangential to the rim. What will be the...
  44. P

    Question about momentum and kinetic energy.

    Homework Statement Two blocks with masses 1 kg and 4 kg respectively are moving on a horizontal frictionless surface. The 1 kg block has a velocity of 12 m/s and the 4kg block is ahead of it, moving at 4 m/s. The 4 kg block has a massless spring attached to the end facing the 1 kg block. The...
  45. L

    Did i do this Kinetic Energy lost Collision Problem Right?

    Homework Statement Homework Equations Total Ke initial - Total Ke final = lost KE P= m1v1 + m2v2 The Attempt at a Solution Initial P : sqrt((0.08*50)^2 + (0.06+50)^2) Final P : (m1+m2)vf = 140vf Initial P = Final P and solve for vf , vf = 0.0357 KE initially : .5(0.080)(50^2) +...
  46. K

    Block-pulley system and kinetic energy

    Homework Statement A pulley with a radius of 3.0 cm and a rotational inertia of 4.5×10^-3 kg·m2 is suspended from the ceiling. A rope passes over it with a 2.0-kg block attached to one end and a 4.0-kg block attached to the other. The rope does not slip on the pulley. At any instant after the...
  47. R

    I Solving Integration by Parts for Relativistic Kinetic Energy

    Hi, I've been following a derivation of relativistic kinetic energy. I've seen other ways to get the end result but I'm interested in finding out where I've gone wrong here: I'm struggling with integrating by parts. The author goes from...
  48. D

    Combining conservation of momentum and kinetic energy

    Hi everyone, hopefully someone will be able to point me in the right direction with this problem, I get as far as combining the two equations together and no matter how I rearrange them they don't seem to cancel nicely and leave me with an awful quadratic, I can find vx, but the question...
  49. F

    What is the second term of the total kinetic energy in a rigid pendulum system?

    Homework Statement The bearing of a rigid pendulum of mass is forced to rotate uniformly with angular velocity (see Figure P. 1.32). The angle between the rotation axis and the pendulum is called θ.Neglect the inertia of the bearing and of the rod connecting it to the mass. Neglect friction...
  50. K

    B What is the ΔEp = ΔEk + W equation used for in thermodynamics?

    What is ΔEp = ΔEk + W I have come across this formula, but i haven't seen it before. What is it used for?
Back
Top