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. K

    Is Momentum Conserved When a Spring Unloads Between Two Masses?

    Let's say there are two masses, attached together by a string, and there's a compressed spring in between them. When the string in between is cut off, the spring unloads, pushing both masses in opposite directions. My thinking: 1. Their momentums will be equal to each other. Their momentum...
  2. L

    Kinetic energy of a hollow cylinder

    Hello 1. Homework Statement In a uniform solid cylinder of radius r, mass m and height h we emptied a cylindrical cavity of radius r/4 tangent and parallel to the symmetrical axis z of the original cylinder. The cylinder is rolling without slipping on a horizontal plane. Find the kinetic energy...
  3. J

    Can the kinetic energy be a function of the position vector?

    Hi guys, I got one confusion when reading Goldstein's Classical Mechanics (page 20, third edition). After getting the equation, then it says that Note that in a system of Cartesian coordinates the partial derivative of T with respect to qj vanishes. Thus, speaking in the language of...
  4. C

    Calculating Kinetic Energy in English Units

    Homework Statement An object has a mass of 10 lbm. b. Find the change in kinetic energy ft·lbf when its velocity decreases from 50 mph to 15 mph. Homework Equations KE=1/2mv^2 Ke=(ft*lbf) 1MPH=1.4667 ft/s The Attempt at a Solution When calculating this, does it need to be in slugs, lbm, or...
  5. irhum31

    Two colliding particles, finding ratio of M/m

    Homework Statement A particle of mass m and initial velocity v0 collides elastically with a particle of unknown mass M coming from the opposite direction as shown in the left-hand sketch on the next page. After the collision, m has velocity v0/2 at right angles to the incident direction, and M...
  6. I

    I Energy needed to launch a spaceship into Space

    A given planet has a mass, M = 6.42*10^23 kg, and radius, R = 3.39 * 10^6 m. The gravitational konstant is G = 6.67*10^-11 A spaceship with mass m = 4000 kg is launched from the planets surface. How much energy is needed to send the spaceship to a height h = 100 km above the planets surface?
  7. Clarky48

    Dirac notation - expectation value of kinetic energy

    It's my first post so big thanks in advance :) 1. Homework Statement So the question states "By interpreting <pxΨ|pxΨ> in terms of an integral over x, express <Ekin> in terms of an integral involving |∂Ψ/∂x|. Confirm explicitly that your answer cannot be negative in value." ##The 'px's should...
  8. X

    Maximum Kinetic Energy from a Physical Pendulum

    Homework Statement Determine the maximum kinetic energy of a uniform rod of mass 0.5Kg and length 0.75 that has an angular displacement of 5 degrees. Homework Equations y = rsin (x) where x is the angular displacement The Attempt at a Solution Using conservation of energy ETotal = EMech +...
  9. G

    Deriving hamiltonian operator for rotational kinetic energy.

    Homework Statement I am trying to get the hamiltonain operator equality for a rigid rotor. But I don't get it. Please see the red text in the bottom for my direct problem. The rest is just the derivation I used from classical mechanics. Homework Equations By using algebra we obtain: By...
  10. W

    Finding the total translational KE of 1kg of Neon

    Homework Statement The screen shot I have attached is the original question, I think I'm ok up until the Ne part. My answer just seems like a ridiculous number, I haven't had much experience with this so don't really know what an 'normal' KE for 1kg of gas would be. Homework EquationsThe...
  11. C

    How do I figure out the final position of the block?

    Homework Statement Three blocks of identical mass are placed on a frictionless table as shown. The center block is at rest, whereas the other two blocks are moving directly towards it at identical speeds v. The center block is initially closer to the left block than the right one. All motion...
  12. Z

    Work and Kinetic Energy of a Sled

    Homework Statement A man pulls a 15-kg sled containing a 5.0-kg dog along a straight path on a horizontal surface. He exerts a force of 55 N on the sled at an angle of 20.° above the horizontal, as shown in the figure above. The coefficient of friction between the sled and the surface is 0.22...
  13. M

    Rearranging change in kinetic energy for mass

    Homework Statement I need to rearrange the change in kinetic energy equation for mass Homework Equations Change in Kinetic energy = 0.5mv^2-0.5mu^2 The Attempt at a Solution I factorised the right hand side to give: Change in kinetic energy = m(0.5v^2-0.5u^2) Then: m = Change in...
  14. E

    Variation on 3-ball elastic collision

    Homework Statement hello! so i am trying to figure out how to calculate the resultant velocities and directions(angles/vectors) that two perfectly elastic spheres might travel in if they were to be hit simultaneously by a third sphere at an angle. all the spheres are of equal mass, initial...
  15. A

    Calculate the electron kinetic energy ejected from cathode?

    I would like to calculate the electron kinetic energy ejected from a cathode as a function of the electric field and pressure without known of the temperature and velocity.
  16. G

    Momentum or Kinetic Energy to find Acceleration?

    Hi all, I have charged particles (protons) accelerated by an electric field. If I add up all the momentum and find the change in momentum per sec I can find the acceleration of my craft by dp/dt /M = a If I add up all the kinetic energy of the particles and find the KE per sec, I can find the...
  17. 5P@N

    How much kinetic energy to propel an object

    Homework Statement A Physics textbook weighing 3 kg needs must be launched into a nearby trash can. The velocity necessary to accomplish this feat is 4 meters per second when unleashed in its flight. Assume no wind resistance. What energy is necessary to accomplish this deed? Homework...
  18. M

    Speed of Toy Car on Curved Track: Exploring Kinetic Energy

    Homework Statement A toy car coasts along the curved track shown. The car has initial speed VA when it is at point A at the top of the track, and the car leaves the track at point B with speed VB at an angle θ above the horizontal. Assume that energy loss due to friction is negligible...
  19. S

    Requesting help with Potential and Kinetic energy

    Hello PF. I have a question on a concept of Potential and Kinetic energy. So for this let's take a ramp and a ball at the bottom. If I apply a force on the ball to make it go up the ramp, when it starts to move it will gain kinetic and potential gravitational ( I think ) but what about at the...
  20. B

    Amplitude vs Location when KE=PE?

    Homework Statement A 0.50-kg object is attached to an ideal massless spring of spring constant 20 N/m along a horizontal, frictionless surface. The object oscillates in simple harmonic motion and has a speed of 1.5 m/s at the equilibrium position. (a) What is the amplitude of vibration? (b) At...
  21. N

    Classical mechanics (find trajectory and kinetic energy)

    Homework Statement Given the force ## \vec{ F }(x) = (-12x + 6) \vec{i} ## ; find kinetic energy ## T## at the point ##x=2## and trajectory of a particle ## \vec{r}(t) ##, given that ## \vec{r}(t=0)=\vec{0}## and ##\dot{\vec{r}}(t=0)=\vec{0}## . 3. The Attempt at a Solution Since...
  22. N

    Kinetic energy lost during a collision?

    Homework Statement A 1kg object traveling at 1 m/s collides head-on with a 2 kg object initially at rest. If the collision is completely inelastic, determine a) the velocity of the objects after impact and b) how much kinetic energy was lost during the collision. Homework Equations Part a)...
  23. David Earnsure

    Kinetic energy ignoring friction

    Homework Statement 2 trucks, one with twice as much mass than the other, are at rest on separate tracks. A man pushes each truck for 5 seconds. If you ignore friction and assume equal force is exerted on both trucks - which truck will have the most kinetic energy afterwards? Homework Equations...
  24. B

    Special Relativity -- Kinetic Energy

    Homework Statement An electron e− and positron e+ moving at the same speed in the Earth reference frame collide head-on and produce a proton p and an antiproton p¯. The electron and positron have the same mass. The proton and antiproton also have the same mass. The mass of the proton is 1836.15...
  25. vetgirl1990

    Total kinetic energy for rolling hoop

    Homework Statement A hoop has a mass of 200g and a radius of 25cm (ICM = MR2 ). It rolls without sliding along level ground at VCM = 5m/s. What is it's total kinetic energy? Homework Equations K = 1/2Iω2 ICM = MR2 v = rω The Attempt at a Solution The answer is 5J, but I'm getting 2.5J. ICM...
  26. CMATT

    Spring Force and kinetic energy

    A novelty clock has a 0.0109 kg mass object bouncing on a spring that has a force constant of 1.34 N/m. How many joules of kinetic energy does the object have at its maximum velocity if the object bounces 3.49 cm above and below its equilibrium position?
  27. M

    How Do You Calculate Car Mass from Kinetic Energy Change?

    Homework Statement A) A car moving at 18 m/s is accelerated to 35 m/s. If the change in kinetic energy is 154 kJ, determine the mass of the car. Homework Equations KE = 0.5mv^2 Kinetic energy = change in momentum = mv2-mv1 The Attempt at a Solution Im not sure how to go about this...
  28. T

    Kinetic energy of fluid + collision

    Hello, When a fluid is flowing, it has of course a kinetic energy and thus we can define kinetic power equal to ρAv v is the velocity and A is the surface of a solid for example. I have a question (maybe it is a strange one) : When fluid collides with a solid, do all the molecules collide with...
  29. R

    At what angle does the normal force go to zero?

    Homework Statement An ice cube is placed on top of an overturned spherical bowl of radius r, as indicated in the figure. If the ice cube slides downward from rest at the top of the bowl, at what angle θdoes it separate from the bowl? In other words, at what angle does the normal force between...
  30. P

    Power and Change of Kinetic Energy in Variable Mass System

    I am thinking about something that is getting me quite confused. To illustrate what is confusing me let's have a cart moving in the horizontal direction on a track without friction, and a motor which adjusts to keep the kart at a constant speed V in the positive direction. Now, it's snowing so...
  31. N

    Calculating Kinetic Energy of a Ball: Is it Possible?

    Homework Statement A ball with a mass of 2.3 kg is thrown into the air. If the ball rises to a maximum height of 15 m before stopping and coming back down, how much kinetic energy did the ball have initially? Homework Equations KE=0.5*mass*velocity^2 The Attempt at a Solution I don't think...
  32. R

    Calculating Uncertainty for Gravitational Potential and Kinetic Energy Equations

    Homework Statement I am writing a lab report and I need to calculate the uncertainty error for Gravitational Potential Energy and Kinetic Energy. The Professor told us that the margin of uncertainty for time is +- 0.025 seconds, margin for height is +- 0.001 meters, margin for mass is +-0.001...
  33. L

    Kinetic energy of a spinning body

    Sorry if this question has already been addressed. I know that, at least for a point mass or a rigid body traveling at constant velocity, in special relativity kinetic energy T is defined as: T = E - Mc2 where E is the body's total energy and M its invariant mass. I have a problem in...
  34. W

    Unsure about formula for kinetic energy

    Imagine the flagpole and ball with mass m fall together. Can I here use the formula mg*l*sin(theta)=(1/2)*mv^2 If I can, in what direction will the velocity vector be - is it going to be tangential to the flagpole?
  35. W

    Can't use formula for kinetic energy 1/2*mv^2?

    Question number 1; find the angular speed. I imagine a point mass at the very top, it has potential energy mgL, this is converted to kinetic energy 1/2*mv^2. I put them equal to each other and say that v=ωL. This gives me ω=sqrt(2g/L), the answer is supposed to be ω=sqrt(3g/L). Why can't I...
  36. gracy

    Kinetic energy and potential energy relationship

    I know,kinetic energy=##\frac{1}{2}####mv^2## But I saw one other equation it is kinetic energy=##\frac{1}{2}####|P.E|## Then I started looking for in which specific condition is it true?Because I know it is not always applicable .But I could not find that particular condition when it is true.So...
  37. M

    Working out kinetic energy and speed

    Homework Statement Homework EquationsThe Attempt at a Solution [/B] a) I think I just have to work out the potential energy and the maximum kinetic energy would is the same? c) Once I've found the kinetic energy, I'm able to work out velocity. Would the velocity be the maximum speed? c)...
  38. M

    Working out speed, kinetic energy and resistive force

    Homework Statement Homework Equations [/B] The Attempt at a Solution a) I'm not sure how to calculate speed without having both distance and time but I believe it will revolve around calculating gravitational potential energy and kinetic energy. bi) I think this is just simply using the...
  39. Jonski

    Work Energy and deflection Problem

    Homework Statement The 2 kg piece of putty is dropped 2m onto the 18 kg block initially at rest on the two springs, each with a stiffness k =1.2kN/m . Calculate the additional deflection of the springs due to the impact of the putty, which adheres to the block upon contact. Homework Equations...
  40. Alettix

    Angular velocity of a rod - translational KE?

    Hello! :) When solving a problem, I had to calculate the angular velocity of a homogenius rod when it comes to vertical position, after being released from a horisontal position (the rod is fixed at one end). This is as usually done with energy conservation, using the rotational energy of the...
  41. Yousufshad

    Comparing Velocities through Kinetic Energy

    Homework Statement An oxygen atom is 3.00 times as massive as a helium atom. In an experiment, a helium atom and an oxygen atom have the same kinetic energy. What is the ratio VHe/VO? Homework Equations KE = 1/2 mv^2 The Attempt at a Solution I know KE for helium is = 1/2 mv^2 and KE for...
  42. H

    When do I use Kinetic Energy and Momentum?

    Homework Statement A billiard ball (Ball #1) moving at 5.00 m/s strikes a stationary ball (Ball #2) of the same mass. After the collision, Ball #1 moves at a speed of 4.35 m/s. Find the speed of Ball #2 after the collision. Homework Equations Conservation of Kinetic Energy Conservation of...
  43. W

    Approximate Values of Kinetic Energy and Potential Energy

    Homework Statement https://s2.lite.msu.edu/res/msu/perl_author/MI3/CH7/IMAGES/MI3_7P32.png The figure shows a potential energy curve for the interaction of two neutral atoms. The two-atom system is in a vibrational state (i.e., a total energy state) indicated by the heavy solid horizontal line...
  44. M

    Is energy conserved in an object sliding down an inclined plane on ice?

    1. A ball falls to the ground through the air. Is the energy of the air conserved?2. None3. When a ball falls to the ground a transformation of energy occurs. The previously stored potential energy gradually becomes kinetic energy during its fall. The energy conserved in the air is the slight...
  45. I

    Law of conservation of energy

    Does the law of conservation of energy make valid predictions when energy is converted from gravitational potential energy into kinetic energy?
  46. F

    Confused...calculating energy in a spring before launch of a ball

    Homework Statement So a little background before stating the problem: During a lecture a couple weeks prior to lab, the instructor did an example where we were told to find the max height of a ball launched upwards by compressing a spring. In order to calculate that, he showed that E1 is the...
  47. Elvis 123456789

    Spring/ Kinetic Energy Problem

    Homework Statement At a waterpark, sleds with riders are sent along a slippery, horizontal surface by the release of a large, compressed spring. The spring with a force constant k = 4300 N/m and negligible mass rests on the frictionless horizontal surface. One end is in contact with a...
  48. Tiggy B

    Find velocity of an accelerated proton using kinetic energy

    Homework Statement So I'm trying to find the final velocity of a proton that is being accelerated by a particle accelerator, just before it collides with a particle. All I have is its kinetic energy just before collision: 4.7066×10^(-13) J. I thought I should use KE = 1/2 mv^2 but then...
  49. yeshuamo

    Kinetic energy of electron: Quantization of angular momentum

    Homework Statement Show that the quantization of angular momentum implies that the kinetic energy of the electron is quantized as K=nhforb/2, where forb is the frequency of rotation. Assume circular orbit. Homework Equations Radial acceleration: arad = v2/r = (4π2r/T = 2*π*v/Tr = nħ KE =...
  50. Guapa

    Does Kinetic Energy Apply to Bullets in Ballistics?

    Homework Statement Just a simple question, does Kinnetic enrgy applies to a bullet that has being shot?Homework Equations K=1/2 * m * v^2 The Attempt at a Solution N/A
Back
Top