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. O

    Distinction between Kinetic Energy and Momentum

    Hello, I would like to provide my thoughts on the distinction between KE and p and would like to know if this reasoning is sound and objective. I will give an example to start: A bowling ball is dropped from some height into a pool of honey. It is observed that the bowling ball has a velocity V...
  2. Gh. Soleimani

    My Case of Total Translational Kinetic Energy of Ideal Gas

    I have really problem with equation of the root – mean – square (rms) speed of the gas molecules when we use the universal gas constant (R) mentioned in all references. Accordance to all reference books, we have below equation for the total translational kinetic energy: Vrms = (3RT/ M)^0.5...
  3. B

    Calculate the coefficient of static & kinetic friction

    Homework Statement You are trying to slide a sofa across a horizontal floor. The mass of the sofa is 200 kg, and you need to exert a force of 350 N to make it begin to move. a) Calculate the coefficient of static friction between the floor and sofa b) after it starts moving, the sofa reaches a...
  4. hackhard

    Why were momemtum, kinetic energy and work introduced?

    why were quantities like momentum, force , potential energy, kinetic energy,work ,etc needed to be introduced in physics? and why were they defined the way they are defined?. would it not be possible to explain nature without defining these quantities or by using alternate physical quantities ?
  5. N

    Escape velocity and kinetic energy

    is it right to say, "when all the potential energy is converted in kinetic energy the object is moving at the escapevelocity. and "when the change in potential energy and kinetic energy is constant at the same time it is laying still on the ground or in a perfect circulair orbit. and the last...
  6. T

    Inelastic Collision and kinetic energy?

    Homework Statement A piece of taffy slams into and sticks to another identical piece of taffy that is at rest. The momentum of the two pieces stuck together after the collision is the same as it was before the collision, but this is not true of the kinetic energy, which is partly turned into...
  7. slymme

    Swinging, equilibrium, kinetic energy problem

    Homework Statement This is a more than one part question that depends on answers from the previous parts, which I have so I will not rewrite those questions. I will write the question I am facing difficulty with: Answers from previous parts: Length = 1.95 m (From top of chains to center, its...
  8. Elena14

    Maximum kinetic energy in projectile motion

    Finding the maximum kinetic energy in a simple parabolic projectile motion requires us to find the maximum velocity. I know that the velocity will be maximum at two points, the point of projection and point of return. So there must be two points where the kinetic energy is maximum. But my...
  9. D

    Kinetic energy of two particles

    If light of certain wavelength falls on two particles say electron and a neutron(isolated) then since they are absorbing the same amount of energy their kinetic energy must be same. But using the formula: $$ \lambda = \frac{h}{\sqrt{2mE}} $$ we get $$ E = \frac{h^2}{2m \lambda^2}$$ which states...
  10. D

    Kinetic Energy of neutron and electron

    Question in my textbook:- The wavelength of light from the spectral emmision line of sodium is 589 nm. Find the kinetic energies of electron and neutron at which they both have same de broglie wavelength. Logically, since light of same energy is falling on both then their kinetic energies must...
  11. A

    Rotational Kinetic Energy and Conservation of Momentum

    I am having trouble wrapping my head around a physics concept. If we roll solid sphere down an inclined plane it will reach the bottom at a different time than if we were to say, roll a hoop down the same inclined plane. This is because they have different rotational inertias, and so more of...
  12. M

    Velocity after collision and Kinetic Energy.

    Homework Statement A trolley of mass 0.5 kg rolls along a level, frictionless ramp at a velocity of 6.0 m s-1. It collides with a second trolley of mass 1.0 kg which is initially at rest. The first trolley rebounds with a speed of 2.0 m s-1 . a) Stating clearly any principles involved, find...
  13. RedDelicious

    Relativistic Kinetic Energy Derivation

    Homework Statement This problem comes from an intermediate step in the textbook's derivation of relativistic energy. It states that E_k\:=\:\int _0^u\frac{d\left(\gamma mu\right)}{dt}dx then leaves the following intermediate calculation as an exercise to the reader: Show that...
  14. Ontophobe

    Does Kinetic Energy Indicate Temperature or Velocity Based on Scale?

    On very small scales, an object's kinetic energy tells you its temperature, and on larger scales, an object's kinetic energy tells you its velocity. Where's the cut off? I mean, velocity is relativistic, but temperature... isn't... is it? Also, black body radiation decreases a body's temperature...
  15. Ontophobe

    Black Body Radiation and Kinetic Energy

    I see that black body radiation slowly depletes a body's thermal energy, which is just another way of saying that black body radiation depletes the kinetic energy of a body's constituent particles. But does black body radiation also cut into the kinetic energy of the body as a whole, such that a...
  16. Delta what

    Kinetic vs thermodynamics control

    So I understand that thermodynamics of a chemical reaction is the change in Gibbs free energy and that kinetics mainly deal with the activation energy (or production of a high energy intermediate). My question comes from something that my organic chemistry professor stated in class. He said...
  17. P

    Large Mass But Low Kinetic Energy

    Can anyone please let me know an example where a body has large mass but low Kinetic Energy? Please provide some reasons for same.
  18. StonedPhysicist

    Kinetic friction in a two mass, one pulley problem

    Homework Statement I have solved the problem where one mass m1 is on a horizontal frictional table connected to a mass m2 which is hanging off the table by a string over a pulley (assuming pulley massless), assuming that m2 is greater than m1 there is a net acceleration causing the mass 2 to...
  19. F

    What Is the Kinetic Energy of a Proton at 1/4 the Speed of Light?

    Homework Statement Calculate the deBroglie wavelength of a proton moving at 1/4 the speed of light. How does the kinetic energy of this proton compare to its p^2 /2m? What does p^2 / 2m anyway conceptually (isn't it one of the triangle thingies in the lower version of E=mc^2)? Homework...
  20. Cathr

    How Does Energy Conservation Apply in a Pendulum-Object Collision?

    1. A ball hanging on a pendulum hits an object standing on the table. The interaction is elastic and linear. After that, the object falls on the floor. Homework Equations From state 1 to 2, we have the conservation of the potential energy of the pendulum to its kinetic energy, right before...
  21. LarryS

    SR Kinetic Energy Mass Increase for Uncharged Particles?

    The mass of a particle will increase as its velocity increases according to special relativity. This has been confirmed in countless experiments in which particle accelerators accelerate charged particles. What type of experiments have confirmed this for uncharged particles? Thanks in advance.
  22. 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...
  23. 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...
  24. 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...
  25. 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...
  26. 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 +...
  27. A

    Does Kinetic and Static Friction Act Simultaneously on a Moving Body?

    When a body is in motion , does kinetic and static friction act simultaneously
  28. 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...
  29. 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...
  30. 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...
  31. 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.
  32. P

    Rolling Fricition, Kinetic Friction

    Does rolling friction depend on the surface area? In other words, the size of the object placed? If I had a block and sphere of the same mass. Without static and kinetic friction coming into play. Which would experience greater rolling friction if they were on the same incline and of the same...
  33. E

    Kinetic Theory of Gases- no. of collisions

    Homework Statement One mole of oxygen at 27°C and at one atmospheric pressure is enclosed in a vessel. Assuming the molecules to be moving with Vrms, find the number of collisions per second which the molecules make with 1 m2 area of the vessel wall. Homework Equations vrms = √(3RT/M) PV = nRT...
  34. 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...
  35. 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...
  36. 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...
  37. lucphysics

    Kinetic and potential energy (satellite low orbit)

    Hello, I have this problem statement : "Due to air drag, the radius of a satellite’s circular orbit decreases from r to r - △r, where the positive quantity △r is much less than r. The mass of the satellite is m. Show that the increase in orbital speed is △v = +(△r/2)[(GM/r^3)^1/2]; that the...
  38. 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...
  39. Lagraaaange

    Kinetic Theory: How many molecules enter pinhole in vacuum?

    Homework Statement Pressure in vacuum is 10^-3 Torr. External Pressure is 1atm and 300K. Pinhole of 10^-10cm^2 made in vacuum. Assuming all molecules striking hole pass through, how many molecules leak into vacuum in 1hour? Answer: 10^17 molecules Homework Equations Flux = <v>*N/4V Rate = Flux...
  40. 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...
  41. 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)...
  42. 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...
  43. 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...
  44. 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...
  45. 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?
  46. S

    Find the kinetic friction of an object at an angle

    A box weighing 87 lb is being pulled with a rope that exerts a force of 60. lb at an angle of 15 degrees with the horizontal. If the coefficient of kinetic friction is .35, what is the kinetic friction?
  47. J

    Kinetic Gas Theory Calculations

    Homework Statement Questions are attached to this thread. Homework Equations P =mv (P is momentum) P = F/A (P is pressure) F = deltaP/delta t (P is momentum) The Attempt at a Solution I have managed to do questions a-e which I was very happy with but then moving onto the extension has made me...
  48. 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...
  49. Lagraaaange

    Assumptions of Kinetic Theory: How big should a cubical element be?

    Homework Statement Assuming uniform distribution. What must be the size of a cubical element of volume in the container so that the number of particles in each volume element may vary by 0.1% when the gas is as standard conditions. Probable deviation is given by N^(1/2) where N is the number of...
  50. Lagraaaange

    Basic Kinetic theory: How many molecules are there?

    Homework Statement A closed vessel contains liquid water at equilibrium with its vapor at 100C and 1 atm. 1 gram of water vapor occupies 1670cm^3. Hvap = 2250Jg^-1. How many molecules are there per cm^3 of vapor?How many vapor molecules strike each cm^2 of liquid surface per second? Homework...
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