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. PsychonautQQ

    Special Relativity kinetic energy

    Homework Statement In reference frame S', two protons, each moving at .500c, approach each other head-on. a) calculate the total kinetic energy of the two protons in frame S'. b) Calculate the total kinetic energy of the protons as seen in the reference frame S, which is moving with one of the...
  2. W

    Linear and Rotational Kinetic Energy

    Homework Statement A solid cylinder is rolling without slipping. What fraction of its kinetic energy is linear? Homework Equations Ke=\frac{mv^2}{2}+\frac{I(v/r)^2}{2} The Attempt at a SolutionKe=\frac{mv^2}{2}+\frac{(v/r)^2}{2}*\frac{(mr)^2}{2} Ke=\frac{3(mv)^2}{4} Linear Ke...
  3. V

    Kinetic friction or static friction

    Can anyone explain this case ? If a moving box is pulling with a force F and the kinetic friction is 0.4F, of course, it will move in the direction of applied force with an acceleration and the net force is 0.6F But what if the force suddenly decreases to 0.3F ? The box is still moving so the...
  4. U

    Coefficient of kinetic energy

    A box slides down a 30° ramp with an acceleration of 1.20 m/s^2. Determine the coefficient of kinetic friction between the box and the ramp ok so the only equations I know are μk= Fk(kinetic friction)/Fn(normal) and that Fn=mass x gravity so i don't know how to get Fk or mass... so help on...
  5. PhysicsKid0123

    Understanding the Difference Between Kinetic Energy and Momentum

    How can you tell the difference between kinetic energy and momentum? I know KE is a scalar and momentum is a vector but physically how do they differ? I don't seem to have a full intuitive understanding. I think I do, but I am unsure. I understand the difference quantitatively, KE= (1/2)mv^{2}...
  6. PhysicsKid0123

    Kinetic Energy of many-particle systems

    Okay, so I'm at conundrum with my physics textbook and I cannot seem to understand a few concepts. Allow me to elaborate. 1. Why is the total momentum of a firecracker sliding on ice the same before and after it explodes? Shouldn't the explosion cause the momentum to increase since the...
  7. O

    Kinetic Friction Word Problem

    Homework Statement An electric motor is used to pull a 125kg box across a floor using a long cable. The tension in the cable is 350N and the box accelerates at 1.2 m/s/s [fwd] for 5.0s. The cable breaks and the box slows down and stops a )Calculate the coefficient of kinetic friction...
  8. L

    Kinetic energy of electron

    Homework Statement (NOTE: I solved the first two parts of this problem, but I included it anyway so you have background knowledge for the third question) The average kinetic energy of an electron in a ground-state helium atom is 2.4x10^3 kJ/mol. A. What is the corresponding electron...
  9. H

    Coefficient of Kinetic Friction on Inclined Plane? [FORCES]

    Homework Statement There's no specific problem statement but I have to explain how I would find out the coefficient of kinetic friction given the angle (θ) of the inclined plane and mass of the object sliding down, nothing else. Homework Equations ƩF = ma μk = Fk (kinetic friction) / Fn...
  10. P

    Maximum and Minimum Kinetic Energies following decay

    Homework Statement An unstable particle of mass M = m1 + m2 decays into two particles of masses m1 and m2, releasing an amount of energy Q. Determine the kinetic energies of the two particles in the CM frame. Given that m1/m2 = 4, Q = 1 MeV, and that the unstable particle is moving in...
  11. H

    Kinetic Friction Problem [FORCES]

    Homework Statement A block is pushed across a horizontal surface with a coefficient of kinetic friction of 0.15 by applying a 150 N horizontal force. The block accelerates at 2.53 m/s^2. Find the mass of the block. Homework Equations ƩF = ma The Attempt at a Solution μk = 0.15...
  12. D

    Force from a Kinetic Energy Function

    Say you're given a function that represents the kinetic energy of some object, what would you have to do to derive the force from that function? I know that for motion along a straight line a conservative force F(x) is the negative derivative of its associated potential energy function U, but...
  13. D

    Force from a Kinetic Energy Function

    Say you're given a function that represents the kinetic energy of some object, what would you have to do to derive the force from that function? I know that for motion along a straight line a conservative force F(x) is the negative derivative of its associated potential energy function U, but...
  14. T

    Is Internal Kinetic Energy Directional at a Macroscopic Level?

    Hello, Please i have a question about the internal energy: The Internal Kinetic energy is the motion of molecules and it's a random motion. But at macroscopic level the whole object formed by those molecules move in a same direction? how it can be? In a flowing fluid not static? thank you
  15. P

    Why and How are Mechanical and Kinetic Energy different?

    Hey guys, I am in a teaching assistant in another 8th grade science class for my 2nd field experience in a title 1 school. The teacher is really struggling(*) to explain to the students the difference between Mechanical and Kinetic Energy. The book their using defines Mechanical Energy as...
  16. L

    What is the Coefficient of Kinetic Friction on an Inclined Plane?

    Homework Statement A block is projected up an inclined plane that makes an angle θ to the horizontal. It returns to its initial position with half its initial speed. What is the coefficient of kinetic friction between the block and the plane in terms of the angle of the incline...
  17. V

    Finding the kinetic energy of a proton in megaelectron volts?

    Homework Statement Charged particles such as protons are accelerated to high velocities and are allowed to collide with atomic nuclei to probe their internal structure. The electric potential outside of a point charge Ze. A) A lead nucleus (Z - 82) is approximately described as a sphere of...
  18. C

    Kinetic energy problem during collision

    Hi friends, I have an issue in solving a Kinetic energy problem during collision. Please Help me in solving this. Thank you all in advance. The problem is as: https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-prn2/1375160_1432382953655477_313625437_n.jpg Attempt...
  19. T

    Comparing Average Translational Kinetic Energy and Internal Energy of GD and GM

    Homework Statement Two closed containers A and B contain equal number of moles of ideal gases GD (diatomic)and GM (Monoatomic) ,respectively. Q .1 a) If the temperature of gases are same,which of them has more average translational kinetic energy ? b) Which of them has more...
  20. M

    Calculating Kinetic Energy of Cylinder with Clamped End

    Hi All, I have a very simple question on a calculation, I would not doubt it correctness if it were not for the fact I am told by a colleague it is wrong, and I cannot see why (good job college times are gone). We have a cylinder of length L, density $$\rho$$ constant along the cylinder, one...
  21. L

    Momentum vs. Kinetic Energy

    Homework Statement Two iceboats hold a race on a frictionless horizontal lake. The two iceboats have masses m and 2m. Each iceboats has an identical sail, so the wind exerts the same constant force F on each boat. The two ice boats start from rest and cross the finish line a distance s away...
  22. A

    Kinetic Energy in Reference Frames

    K'sys=1/2MtotalVcm2+1/2\muVreli2 I understand this equation represents the total kinetic energy in a reference frame. What I'm not getting out of this is the overall concept. I understand that the first part of the equation is supposed to represent the total kinetic energy required to...
  23. A

    What are my units when finding % diff. In kinetic energy?

    What are my units when finding % diff. In kinetic energy? Homework Statement Phrank (36.4kg) runs at 2.2 m/s and jumps onto a large stationary skateboard (2.3kg) on which Phrancine (22.7kg) is sitting. After the collision, Phrank, Phrancine, and the skateboard move at 1.3 m/s. Find the...
  24. T

    Determing the kinetic energy of an electron

    Homework Statement Homework Equations The Attempt at a Solution KEi + PEi = KEf 2.4 x 10-19 + e(1) = KEf KEf = 4.002 x 10-19 J So option c would be correct, right? Thank you in advance.
  25. N

    Kinetic Energy operator is hermitian

    The problem asks to show that the kinetic energy operator is Hermitian. The operator is given as T= -h^2/2mΔ but I know I can also write it as p^2/2m which would be (- ih∇)(-ih∇). My main question is if I can prove this in 1-D so that T=(-h^2/2m)d^2/dx^2 does that generalize to...
  26. P

    Calculating Coefficient of Kinetic Friction for a Hockey Puck

    A hockey puck on a frozen pond with an initial speed of 12.3 m/s stops after sliding a distance of 198.9 m. Calculate the average value of the coefficient of kinetic friction between the puck and the ice. So I started with finding the acceleration. I divided 12.3m/s by 198.9m then took the...
  27. M

    Kinetic friction ratio problem

    "A driver makes and emergency stop and locks up the brakes of the car, which skids to a stop on dry concrete. Consider the effect of rain on this scenario. If the coefficients of kinetic friction for rubber on dry and wet concrete are μk (dry)=0.80 and μk(wet)=0.50, how much farther would the...
  28. K

    Stopping Potential and kinetic potential

    [URGENT! Stopping Potential~~~~~~~#######~~~~~~~~~~~~~~~~ Homework Statement max Kinetic_energy= e*Vs=hf-work_function Vs is the stopping potential The attempt at a solution I am wondering, does this stopping potential refers the voltage difference of V_Emitter (where the...
  29. J

    Finding the Time: Kinetic Friction Lab Question

    Homework Statement A 4.5 kg block is on an incline with a coefficient of static friction of 0.50. The angle that the ramp makes with the horizontal is increased gradually, until the block begins to slide down the ramp. If you know that the coefficient of kinetic friction between the block and...
  30. A

    Rotating and kinetic energy kinematics

    Hello everyone, I've run across an interesting Newtonian physics problem that I'd like some input on. The problem begins with a rotating object. Let's assume it is a slender rod rotating about one end with a given mass (m), length (L) and rotational speed (ω). This results in the rod having...
  31. Y

    Thermodynamics- rms translational kinetic energy

    Homework Statement A container having a volume of 1m^3 holds 5 moles of helium gas at 50C. If the helium behaves like an ideal gas, the rms translational kinetic energy is? Homework Equations Total E= 3/2 nRTThe Attempt at a Solution I tried solving this question using this equation, but I did...
  32. M

    Partition between kinetic and potential energy?

    Hi, If I have a body which is freely vibrating with kinetic energy given by, say, Ekin=(1-1/10sin(ωt))cos2(ωt) what can be said about the potential energy? Of course the total energy should be constant but how big is it, in other words what is the potential energy? Thanks!
  33. C

    Compton effect and kinetic energy

    Homework Statement What is the maximum possible kinetic energy (keV) of a Compton electron and the corresponding minimum energy of a scattered photon resulting from scattering of a) 100 keV photon b) 1 MeV photon Homework Equations Δλ = 0.24(1-cosθ) The Attempt at a...
  34. C

    Quick question on coupled pendulum kinetic energy

    I have found that the kinetic energy term for each mass on a coupled (by a spring) pendulum has no \dot{y} terms, as in the the masses do not move in the y direction at all. What am I missing here? I know they are connected, but in the first normal mode, both masses should move like regular...
  35. G

    Basic question about kinetic energy and momentum

    Hi, I'm learning about momentum and kinetic energy and I wanted to know if the following relationships are true. I think they are true, but I am not sure: If kinetic energy goes down and mass stays constant, momentum goes down If kinetic energy stays constant and mass goes down, momentum...
  36. F

    Something I don't understand. [Kinetic & potential energy]

    I really don't understand this example: A feather or spring or whatever it's called got the stiffness 29.5 N/m. It is lying inside a horizontal tube (feather cannon). The feather /spring is connected in the very end of the tube. The feather / pring is as "long" as the tube if it's not being...
  37. S

    What is the velocity profile of a truck or tractor pull?

    Are there any fans of truck or tractor pulls out there? I am trying to understand what is happening to the coefficient of friction as the sled transitions from kinetic friction back to static friction. It is a fairly complicated model... The weight imposed on the front plate of the drag...
  38. M

    Difference between timespace expansion/contraction and kinetic motion?

    How could you tell in an experiment which was happening?
  39. D

    Kinetic Inductance: Combining Self-Inductance & Kinetic

    can self-inductance and kinetic inductance be combined in the same model?
  40. Q

    Show that kinetic energy of the earth after a collision can be ignored

    Show that the kinetic energy of the Earth after a collision can be ignored (stuck & desperately need help)? A ball of mass m2 is dropped and bounces off the floor. If the impact is viewed as an elastic collision between the ball and the earth, show that the kinetic energy of the Earth after the...
  41. J

    Why does work equal change in kinetic energy?

    If I hit a solid brick wall with a 2kg hammer moving 2m/s, the initial KE would be 4J. Which means the work done would be 4J. But W=F•d says the work is 0 since the hammer comes to a complete stop at the wall without any displacement. ?
  42. P

    Kinetic energy loss in a gravitational field.

    If I throw a ball vertically upwards from the surface of the earth, the upward motion slows due to gravity. At the ball's highest point, I have placed a repositioned platform for the ball to rest on; so it sits motionless, perched above the surface of the earth. When the ball began it's...
  43. B

    How Does a Variable Coefficient of Kinetic Friction Affect Block Acceleration?

    Homework Statement A block of mass m is at rest at the origin at t=0. It is pushed with constant force F0 from x=0 to x=L across a horizontal surface whose coefficient of kinetic friction is μk=μ0(1−x/L). That is, the coefficient of friction decreases from μ0 at x=0 to zero at x=L...
  44. K

    How Do You Calculate the Average Kinetic Energy of Multiple Helium Atoms?

    Homework Statement Write an expression E for the average kinetic energy of Helium atoms using the Boltzmann constant. Homework Equations PV = nRT PV = \frac{1}{3}mN\overline{c^2} k = \frac{R}{N_A} [P - Pressure ; V - Volume ; m - Mass of an atom ; N - Number of atoms ; others have...
  45. B

    Why Does Force Impact Momentum More Directly Than Kinetic Energy?

    why is force equal to rate of change in momentum rather than rate of change in kinetic energy. If we consider time is 1 sec,and if we consider an object is moving with particular velocity V1 and a force F is applied to change the velocity to V2, then according to Newton the force applied is...
  46. Q

    Box on ground, incline (with friction), ground: find Kinetic Energy

    Homework Statement A 15 kg block slides along a horizontal frictionless surface 3.0 m above the ground at a constant speed of 2.0 m/s. The block then slides down an incline that makes an angle of 35° with the horizontal and has a coefficient of kinetic friction equal to 0.30. After reaching...
  47. O

    Galilean invariance and kinetic energy

    I tried to look this up on the internet. I know there is a book about it but I forgot its title. I know that you can prove that the kinetic energy should be proportional to velocity squared by saying that this is the only Galilean invariant definition of kinetic energy. Can someone help me...
  48. D

    How Is Gas Pressure Derived in the Kinetic Theory of Gases?

    Hello! It's been around two months since I started to learn about the knetic theory of ideal gases. But I haven't managed to completely understand it. What I don't understand is how we derive the formula for the presure that the gas molecules exerts on a wall's surface. Here's what I've...
  49. S

    Average power given velocity, kinetic friction, and mass

    A 23.5-kg rock is sliding on a rough, horizontal surface at 9.15 m/s and eventually stops due to friction. The coefficient of kinetic friction between the rock and the surface is 0.180. What average power is produced by friction as the rock stops? I think that the way to solve it is use...
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