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.

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  1. mia_material_x1

    Optimized Kinetic Energy of Skier on Incline & Horizontal Surface

    http://imgur.com/luj32IE 1. Homework Statement A skier starts from rest at the top of a smooth incline of height 20 m as in Figure 2. At the bottom of the incline, the skier encounters a horizontal rough surface where the coecient of kinetic friction between the skis and the snow is 0.21. (a)...
  2. J

    Egg Drop Project and Kinetic Energy

    Homework Statement [/B] Our teacher has given us the weekend to go and work on the Egg Drop project. He's restricted us to 20g total mass and 6cm * 6cm * 30cm in size. Food, foam, bubble wrap is prohibited. Parachutes can be added but can not make it exceed 12cm * 12 cm * 30cm. So that's what...
  3. U

    Conservation of momentum or kinetic energy?

    Homework Statement A 100 kg person in space throws a 1kg sphere away from their body. The sphere starts from rest and ends up traveling 10 m/s away from the person. What is the kinetic energy of the person and the kinetic energy of the sphere? Is kinetic energy conserved?[/B]Homework Equations...
  4. Y

    A rifle barrel question related with Kinetic Energy Theorem

    Homework Statement A 15.0g bullet is accelerated from rest to a speed of 780m/s in a rifle barrel of length of 72.0cm a)Find the kinetic energy of the bullet as it leaves the barrel b)Use the work kinetic energy theorem to find the net work that is done on the bullet. (this problem had part...
  5. H

    B The Problem That Riddles Any Radical Thinker: WTH Is Energy

    Hello! I'm new to this forum but ok let's get this started, First I'll revisit the definition I know as physics; the study of nature in order to provide a reasonable(radical/rudimentary) explanation/reason. This I believe in is not quite how things are today, at school, with my teachers or at...
  6. P

    Finding combined kinetic energy

    1. Homework Statement Homework Equations In picture The Attempt at a Solution Is my setup correct for finding the combined kinetic energy in part a)
  7. P

    Magnetic Field and Charged Particle Motion

    Homework Statement Suppose a subatomic particle of mass m kg has kinetic energy K.E. a nJ and is moving southward toward a vertical wall. When the particle is distance d cm from the wall a magnetic field is turned on: it has magnitude b T and points upward. The particle turns westward so it...
  8. A

    Work function and kinetic energy

    Homework Statement The work function for potassium is 2 eV. Consider light with wave length 3.6*10^-7 m hitting the potassium surface. a) what is the stopping potential? b) what is the kinetic energy and velocity of the fastest electrons emitted? Homework Equations KE_max = hf - Φ q_e V_s =...
  9. moenste

    Distance traveled and the kinetic energy of a sphere

    Homework Statement A 'hammer' thrown in athletics consists of a metal sphere, mass 7.26 kg, with a wire handle attached, the mass of which can be neglected. In a certain attempt it is thrown with an initial velocity which makes an angle of 45 degrees with the horizontal and its flight takes 4...
  10. T

    Calculate the recoil momentum and kinetic energy of the atom

    Homework Statement Suppose an atom of iron at rest emits an X-ray photon of energy 6.4 keV. Calculate the “recoil” momentum and kinetic energy of the atom. (Hint: Do you expect to need classical or relativistic kinetic energy for the atom? Is the kinetic energy likely to be much smaller than...
  11. M

    Potential Energy and Kinetic Energy

    Homework Statement Two blocks are hung by a string draped over a pulley, a 1.5-kg block on the left and a 3.0-kg block on the right. The two blocks start out at rest and at the same height. A. What is the change in the gravitational potential energy of the system of blocks and Earth when the...
  12. B

    Understanding why change in Kinetic Energy is 0

    Homework Statement Find the minimum force ##\displaystyle F## required to move ##\displaystyle M_2##.Details and Assumptions: ##\displaystyle M_1 = 3Kg## ##\displaystyle M_2 = 5Kg## ##\displaystyle \mu_1 = 0.4## ##\displaystyle \mu_2 = 0.6## ##\displaystyle g = 9.8m/s^2## Homework...
  13. K

    Calculating Kinetic Energy and Maximum Speed for a Leaking Oil Tanker Truck

    Homework Statement A truck hauling a big tank of oil starts at position x=D (Chicago) and heads due west (–x direction) toward its destination at x = 0 (Des Moines). At Chicago, the total mass of the loaded truck is M and the mass of oil it is carrying is λM. (Thus M(1–λ) is the "tare" mass =...
  14. M

    Momentum and Kinetic Energy after an Inelastic Collision

    Homework Statement A small bullet is fired into a large piece of wood. After the bullet penetrates the wood, the assembly moves as one unit along a low-friction track in the direction of travel of the bullet. A. After the bullet is stuck in the piece of wood, is the momentum of the wood (not...
  15. M

    Kinetic Energy of uranium particles

    Homework Statement A uranium-238 atom can break up into athorium-234 atom and a particle called an alpha particle, α-4. The numbers indicate the inertias of the atoms and the alpha particle in atomic mass units (1 amu = 1.66 × 10^−27 kg). When an uranium atom initially at rest breaks up, the...
  16. ELiT.Maxwell

    Spring (with mass) kinetic energy -- velocity assumption

    why we assume that velocity decreases linearly in a spring (i.e. if one end is fixed, then velocity of a particle (of spring) at x from fixed =vx/l where v is the velocity of the free end) and why does it hold good too when the spring (linear mass density) is non uniform... EDIT: spring has...
  17. Stephanus

    Can kinetic energy lead to black hole?

    Dear PF Forum, I have a question in mind. But I'm not sure if this belong to SR forum, cosmology or classical physics. So I post it here. And perhaps as some of you might have known before or thought it over. It's about kinetic energy. Supposed this... A rocket, with a rest mass 1 ton. And the...
  18. A

    Calculating Kinetic Energy for an Object at an Angle

    I have a question regarding how to calculate kinetic energy. Let's say I was to find the kinetic energy of a ball shot at 180 m/s at a 45 degree angle. Do i simply sub in 180 as v (velocity) in the kinetic energy equation (1/2mv^2) or must it be split into components?
  19. D-son

    Is kinetic energy enough to propel objects in 3D space?

    I've recently created a 3D motion generator (Anti-Gravity Transportation (AGT)) in 2D/3D design. unfortunately my current physics knowledge is not advanced enough to take this machine to its maxium potential. i can concieve perpultion and sustainable energy coming from this idea. my question...
  20. vinicius0197

    Change in mechanical energy (gravitation)

    I got a little bit confused while studying gravitational potential energy, particularly the expression for long distances: (-GMm/r). An exercise asks me for the velocity, period and radial acceleration for a 1000kg satellite that I wish to put into orbit. That's okay. But after that, I'm...
  21. RyanH42

    Kinetic energy distribution in high speeds and large scales

    We know that If object is homogenius and small (like gold atom) we can use this equation to calculate total energy of object when it moves very slow due to speed of light.And the equation is ##1/2(m_1v^2+m_2v^2+m_3v^2...=1/2Mv^2##, ##(M=m_1+m_2+m_3...)## here v is small and we are talking about...
  22. Robin04

    How do we get to the concept of kinetic energy?

    Hi, I'm reading a high school textbook about mechanics. It's amazing how the author draws up the problems and solves them by introducing a new consistent concept. Now I'm reading about collisions. He writes the conservation of momentum but the problem is that we have two unknowns in one...
  23. E

    Quadratic forms and kinetic energy

    I heard that proportionality of kinetic energy with square of velocity, ##E_k\propto v^2##, can be derived with help of quadratic forms. It goes like: we guess that ##E_k\propto v^2## and we assume that momentum ##p\propto v##, then equation is valid in another inertial system. And so on. The...
  24. T

    Electric Potential and Kinetic Energy

    Homework Statement A charged particle (q = -8.0 mC), which moves in a region where the only force acting on the particle is an electric force, is released from rest at point A. At point B the kinetic energy of the particle is equal to 4.8 J. What is the electric potential difference...
  25. Soumalya

    Flow work and kinetic energy in fluid flow

    Considering horizontal flow in a pipe (under the action of pressure forces only) if we focus on a fluid element we have a pressure gradient across the same i.e, a net pressure force will always act over the element that essentially maintains the flow.Now, in a region in the flow field if the...
  26. fricke

    Do momentum and kinetic energy operators always commute for a free particle?

    For particle in the box wave function, it is the eigenfunction of kinetic energy operator but not the eigenfunction of momentum operator. So, do these two operators commute? (or it has nothing to do with commutator stuff?) How about for free particle? For free particle, the wave function is...
  27. Stephanus

    Exploring Kinetic Energy & Relativity: A New Look at an Old Phenomenon

    Dear PF Forum, I'd like to ask a question regarding a post in Relativity sub forum. The formula for kinetic energy is ##E_k = 0.5mv^2## Which makes sense. Because if we push/accelerate 1 kg object for 1 m/s2 for 8 meters, we'll spend ##E = N.m = 8 joules## How much time do we need to do that...
  28. C

    Momentum and Kinetic Energy, Elastic Collision

    Homework Statement [/B] A 2.0 kg ball moving with a speed of 3.0 m/s hits, elastically, an identical stationary ball as shown. If the first ball moves away with angle 30° to the original path, determine: a. the speed of the first ball after the collision. b. the speed and direction of the...
  29. toforfiltum

    Confused about solution to kinetic energy question

    Homework Statement 2. Homework Equations [/B] KE=1/2 mv2 The Attempt at a Solution To calculate the kinetic energy of the ball just after it leaves the surface, I use the ratio of the centre of gravities of the ball at the two different heights. My working is (0.41/0.76)×0.75 which gives me...
  30. J-dizzal

    Kinetic energy and work problem

    Homework Statement In the figure here, a block of ice slides down a frictionless ramp at angle θ=55.0 ˚ while an ice worker pulls on the block (via a rope) with a force that has a magnitude of 54.0 N and is directed up the ramp. As the block slides down through distance d = 0.400 m along the...
  31. S

    Conservation of Mechanical Energy

    Homework Statement This is my first time posting here guys so correct me if I am wrong. This question was on my test and I wanted to know if I did it right. There is a block of mass m and it is moving at a certain velocity before pressing against a spring which compresses distance x. Find...
  32. Q

    Kinetic energy, riddle me this

    Imagine two experiments A and B. In experiment A I load a gun with a cartridge and shoot a bullet. Total energy in this experiment will be E(heat) + E(projectile). I include recoil in E(projectile). E(heat) in its turn will heat the muzzle and some air. Done. In experiment B I do...
  33. scharry03

    Kinetic Energy Equation conceptual question?

    This is mostly a conceptual confusion I'm having, not a specific problem, so it didn't seem like it'd fit under homework problems. I'm having trouble understanding when potential energy should or shouldn't be used in a energy conservation equation. When looking at a problem with a skier going...
  34. J

    Newton's Cradle: Fascinating Momentum & Kinetic Energy

    I have always found the Newton's Cradle that my daughter bought for me fascinating. With the steel balls as identical as possible then the analysis of the process is simple. Both the momentum and kinetic energy are conserved (assuming negligible losses) on collision. The only way(?) this can...
  35. TenPin

    Momentum, Inertia, and Deflection

    Hi, new here, so I hope I'm doing this correctly. I'll keep this question as brief as possible, but it requires just a bit of setup. I'm not an engineer or physicist, but do have a more than decent layman's understanding. I've lately become fixated on the game of bowling (ten pin), and have...
  36. N

    Kinetic energy -- plot molecule frequency vs temperature

    hi I have been given an assignment at school to look at various ways heat is transferred through liquids. I'm interested in looking more into the kinetic energy of molecules and how their vibration causes heat, I would like to know if there is a device I could use to record the frequency of...
  37. Proz3m0

    How to derive kinetic energy equation for charged particles

    I know there are some threads discussing about the Lorentz force equation but this is something different I came across and I couldn't find anyway to derive that formula. To be specific I'm talking about a charged particles in a magnetic field, where F=vqB and everything that is moving has...
  38. A

    Calculate 8000 Megajoules of Water Tunnel Kinetic Energy

    There is 14 km of tunnel of 7.7 m diameter, full of water traveling at 3.75 m/s,that represents approximately 8000 Megajoules of kinetic energy. How? Calculate it to prove (Taken From Wikipedia under topic of "Water Hammer".
  39. Mysteryciel

    Kinetic Energy and Reference Frame

    I know that kinetic energy depends referance frame is there any article about this issue thanks ?
  40. L

    Kinetic Energy - use a log scale or not?

    Homework Statement Lets say we have a series of balls and an eggs. All items have equal mass but are traveling at different velocities. Thus they have differing kinetic energies. Now, for each 'ball' object we have, we want to find an egg object with a similar kinetic energy. Let's say...
  41. P

    How do I find rotational energy for a pulley system?

    Homework Statement So I have a horizontal pulley positioned at the edge of table with a mass of .2kg hanging down from a height of .76meters, the other end of the string is attached to a wooden block of mass .25kg that when the .2kg weight is dropped the wooden block is pulled towards the...
  42. K

    Calculating Kinetic Energy After a Turn on a Horizontal Plane

    Homework Statement A body is moving on a horizontal plane which gradually turns to another plane forming an angle ##α##. Friction coefficient is ##μ##. Find the kinetic energy of the body after the turn if in the beginning it was ##K_o##. Homework Equations Newton's laws. The Attempt at a...
  43. R

    Decay of Radium Finding Kinetic Energy of products

    Homework Statement Homework Equations Please see below. The Attempt at a Solution I'm pretty sure my method is correct but I'm getting small answers: $$ COM \\ \\ E_{6}= E_{2}+E_{\alpha} \\ \\ Minus \ E_{2} \ and \ square \ both\ sides\\ \\ E_{6}^{2}+E_{2}^{2}-2E_{2}E_{6}=E_{\alpha}^{2}\\...
  44. D

    Understanding Kinetic Energy and Momentum in Elastic Collisions

    Homework Statement Hi,I have been struggling on this problem for a whole day,looking for someone to sort it out! My question:(please see the attached diagram as well) Consider 2 spheres A and B,having different masses (m and M) but equal in size. A is moving to the right at a speed u, B is...
  45. S

    Average kinetic energy of particles of an ideal gas

    Homework Statement So first the task: Determine the average value of the kinetic energy of the particles of an ideal gas at 0.0 C and at 100 C (b) What is the kinetic energy per mole of an Ideal gas at these temperatures. I took the above right out of the pdf we got from our professor. I know...
  46. physicsisgreat

    Decelerating box on a truck and work

    Homework Statement A truck is moving at constant velocity v0 with a box of mass m on it. Suddenly it starts decelerating until its velocity becomes zero. Between the box and the truck the is a force of friction and the deceleration of the truck is sufficiently slow to allow the box not to start...
  47. D

    Equation used to find kinetic energy of proton

    I am having trouble deciding when to use which equation. If you're given the wavelength of a proton, let's say 100 fm, and have to find the kinetic energy of it, how would you do this? Here are my ideas: Idea 1: Use lambda = h/p, where p = sqrt(2Em). Idea 2: Use E=(pc)^2+(mc^2)^2 =...
  48. goonking

    Kinetic energy to thermal energy

    Homework Statement Homework Equations Q = c m ΔT The Attempt at a Solution so converting 55 mph to meters per sec gives me a velocity of 24.58 m/s kinetic energy of car is 1/2 m v2 i set 1/2 m v2 = c m ΔT plugging in v and c and masses of both ΔT = 134.56 is that correct?
  49. moenste

    Speed of bullet and kinetic energy lost in the impact

    Homework Statement A bullet of mass 2.0 * 10-3 kg is fired horizontally into a free-standing block of wood of mass 4.98 * 10-1 kg, which it knocks forward with an initial speed of 1.2 m s-1. (a) Estimate the speed of the bullet. (b) How much kinetic energy is lost in the impact? (c) What...
  50. T

    How to use kinetic energy for this problem?

    Homework Statement The problem is that my teacher asked to solve this using kinetic energy but i doubt it uses kinetic energy... i thought it uses potential energy. [/B] You put your little sister (mass m) on a swing whose chains have length L and pull slowly back until the swing makes an...
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