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. L

    Kinetic energy of rotation and parallel axis theorem problem

    Homework Statement A circular disc of radius 25 cm and mass 0.5 kg is revolving in its plane with an angular velocity of 4 radians per second. Find A) its kinetic energy of rotation, and B) its new angular velocity if a mass of 10 kg is suddenly fixed on the rim of the disc. Homework...
  2. C

    Increase in kinetic energy in current carrying conductor

    Homework Statement If I have two current carrying conductor carrying current is same direction, they move towards each other so kinetiisc energy increases. So which force does work here? Homework EquationsThe Attempt at a Solution I know magnetic force can't do work, is work done by some...
  3. M

    Finding the speed of the block for one and two springs

    Homework Statement The spring in the figure (Figure 1) is compressed by length Δx. It launches the block across a frictionless surface with speed v0. The two springs in the figure (Figure 2) are identical to the spring of figure 1. They are compressed by the same length Δx and used to launch...
  4. K

    Side kick on a falling object, mechanical energy and work

    If an object with 1kg mass falls free (no air drag), from 100 meters high, and the gravitational acceleration is 9.81, then the mechanical energy will be: 1*9.81*100 = 981 joules. The work the gravitational force does on the object will be: 9.81N*100m = 981 joules. Another object with same...
  5. F

    Constant rotational kinetic energy

    A mass m is attached at the end of the string. The mass moves on a frictionless table, and the string passes through a hole in the table, under which someone is pulling on the string to make it taut at all times. Initially , the mass moves in a circle, with kinetic energy [E][/0] . The string is...
  6. H

    Ranking Kinetic Energy: Why Does Mass Matter More Than Amplitude?

    Homework Statement Rank the cases from maximum Kinetic Energy (KE) to lowest KE. State any ties. Briefly, explain your ranking. Case 1 has mass M and amplitude A Case 2 has mass 2M and amplitude A Case 3 has mass M and amplitude 2A Homework Equations 1. E=\dfrac{1}{2}kA^{2} 2...
  7. P

    Time of Electron/Proton to Radiate Away Half of Their Energy

    Homework Statement It is shown in more advanced courses that charged particles in circular orbits radiate electromagnetic waves, called cyclotron radiation. As a result, a particle undergoing cyclotron motion with speed v is actually losing kinetic energy at the rate $$\frac{dK}{dt} = -...
  8. starstruck_

    Kinetic energy of solid sphere that is rolling

    Homework Statement A solid sphere of mass m=2.5 kg is rolling at v=5.3 m/s. Calculate the transitional kinetic energy, rotational kinetic energy, and the ratio of the two (Rotational/ Transitional). Homework Equations [/B] Inertia of solid sphere = 2/5 mR^2 (where R is the radius and m is the...
  9. L

    Kinetic Energy and Newton Forces - distance/speed

    Homework Statement A sled of mass m is given a kick on a frozen pond. The kick imparts to it an initial speed of 2.20 m/s. The coefficient of kinetic friction between sled and ice is 0.150. Use energy considerations to find the distance the sled moves before it stops Homework Equations...
  10. L

    Kinetic Energy & Work Problem -- Tractor Pull Competition

    Homework Statement Homework Equations W = ΔKE W = KEf-KEi The Attempt at a Solution W = .5mvf2 -.5mvi2 I know that final KE for A is 4 times the KE of B because for A ... KEf = 1/2 m (2v)^2 4KE = 1/2 m v ^2 for A I don't know what initial KE for both of them are... so how do i know if that...
  11. Alexander350

    Escape velocity and kinetic energy of the Earth

    If you had two masses, m_{1} and m_{2}, and you released them in space infinitely far apart, their kinetic energies would satisfy \frac{1}{2}m_{1}v_{1}^2+\frac{1}{2}m_{2}v_{2}^2=\frac{Gm_{1}m_{2}}{r} if they met with a distance r between their centres of mass. This equation therefore tells you...
  12. T

    Kinetic energy of recoil atom in the photoelectric effect

    Homework Statement In the photoelectric effect it is generalley assumed that all the energy is given as kinetic energy to the electron while the atom is neglected. Do not neglect the recoil kinetic energy of the atom and calculate the kinetic energy of the photoelectron and the ionized atom and...
  13. D

    How to define kinetic energy of a compound system

    A system is defined as two connected masses in an atwood machine and the earth. How do I find total kinetic energy in a system where more than one object is moving (or is only one object actually moving since the two masses are connected with a string? idk) This is an isolated system so... ΔK...
  14. D

    Kinetic energy of a compound system (atwood machine)

    A system is defined as two connected masses in an atwood machine and the earth. How do I find total kinetic energy in a system where more than one object is moving (or is only one object actually moving since the two masses are connected with a string? idk) This is an isolated system so... ΔK...
  15. Jamie_Pi

    Starting height of marble rolling around a loop the loop

    Homework Statement The marble rolls down a track and around a loop-the-loop of radius R. The marble has mass m and radius r. What minimum height h must the track have for the marble to make it around the loop-the-loop without falling off? (Use any variable or symbol stated above along with the...
  16. R

    Kinetic energy of an electron on the β spectrum?

    Homework Statement A 191Os nucleus decays via β- emission to an excited state of 191Ir at 171 keV. What is the maximum kinetic energy of the electron on the β spectrum? a) 0.134 MeV b) 0.143 MeV c) 0.171 MeV d) 1.859 MeV Homework Equations 191 Os = 190.960928u 191 Ir = 190.960591u...
  17. Anonymous123451234

    Inelastic collision, determine ratio KEf/KEi

    Consider the following extreme cases for a completely inelastic collision. In each case, determine the approximate ratio KEf/KEi. a) A moving train collides with a stationary car. b) A moving car collides with a stationary train. I do not understand this question, I'm not given masses or...
  18. L

    Why are the objects the same speed at the finish line?

    I have the problem below. I know that net work is equal to the change in kinetic energy which is Wnet= 0.5mv^2-0.5mvi^2.I also know that the work by force of gravity is the same for both objects. I don't understand why the objects would be the same speed at the finish line.
  19. A

    How Does Gravity Affect Kinetic Energy in a Glider System?

    Homework Statement This is a lab I did for physics. In this experiment, a glider starting from rest on an air track is attached to a string. The string is drawn over a pulley with a mass hanger tied to the other end. Work is done by gravity which results in a gain of kinetic energy of the...
  20. M

    How to calculate the force of a charge through its kinetic Energy?

    Good Afternoon Is it correct to calculate the force of a point charge by relating the force to the kinetic energy and this with its derivative? I have the graphic V/r (Velocity Vs Space), want to calculate force, relate work with force in this way: W=ΔEk, in my case the kinetic energy initial is...
  21. A

    What is the equation for kinetic energy of a bicycle?

    Homework Statement How did they get mv^2? Where did the 1/2 go? I tried working out the equation below the question and I ended up getting (1/2)mv^2. Look at the image to see. It's the rotational kinetic energy formula. Homework Equations It's in part 1 The Attempt at a Solution It's in part 1
  22. M

    Why does kinetic energy depend on the frame of reference?

    I'm having this discussion with my engineering peers: A ball is sitting on top of a train traveling at, say, 10m/s. The ball has mass of 2kg, for simplicity. The ball's kinetic energy KE relative to ground zero is 1/2 m v^2, or 100J. A person riding on the train picks up the ball and tosses...
  23. G

    Electric Potential Energy - Final Velocity

    Homework Statement Four protons (each with mass 1.7 10-27 kg and charge 1.6 10-19 C) are initially held at the corners of a square that is 7.1 10-9 m on a side. They are then released from rest. What is the speed of each proton when the protons are very far apart? (You may assume that the...
  24. C

    Question Regarding Changes in Kinetic Energy

    Homework Statement 1. [/B]The only force acting on a 4.0 kg body as it moves along the x-axis varies as shown in the figure. The velocity of the body at x = 0 is 7.0 m/s. c) What is the maximum kinetic energy attained by the body between x = 0 and x = 8 m? Homework Equations Work = delta K K =...
  25. O

    Pulling an object at an angle with friction -- find the Kinetic Energy

    Homework Statement A boy pulls a sled (mass m=59.2 kg) along the ground using a force of magnitude F = 586 N inclined at an angle of θ=37.3° to the horizontal. There is friction between the sled and the ground with a coefficient of µk = 0.205. If the sled starts at rest, how much kinetic energy...
  26. L

    Object on a vertical Ring with spring attached

    picture: https://ibb.co/k5P0GG Two objects slide without friction on a circular ring of radius R, oriented in a vertical plane. The heavier object (of mass 3m) is attached to a spring with an unstretched length of zero (admittedly an unphysical assumption) and spring constant k. The fixed end of...
  27. H

    Kinetic energy of the system of 2 masses....

    Homework Statement Velocity of centre of mass of two particles is v and the sum of the masses of two particles is m. Kinetic energy of the system - (A) will be equal to ½mv² (B) will always be less than ½mv² (C) will be greater than or equal to ½mv² (D) will always be greater than ½mv²...
  28. L

    [College Chem] kinetic energy problem

    Homework Statement [/B] atom traveling at 1.00% the speed of light has a KE of 2.016x10-16 kJ. Identify the element. Homework Equations [/B] KE = .5mv^2 Hv= binding energy + KE The Attempt at a Solution [/B] Ok. So I plugged into velocity and me to find my mass and got like 4.48E-26 g. So I...
  29. R

    B Exciting an H-Atom: Effects on Kinetic Energy

    If we give an hydrogen-like atom some energy in the form of photon like less than or more than the energy difference between the two energy states of H-atom .then would it increase the kinetic energy of electron
  30. NoahCygnus

    Work done on the centre of mass and kinetic energy of system

    If we have two objects forming an isolated system and their centre of mass is ##X_{com}##, we know by work energy theorem that work done on centre of mass will be ##\int F_{ext}.X_{com}= 0## as no external force is acting on the system. However, if there is internal forces between the objects...
  31. Irfan Nafi

    Which ball has more energy halfway through the track?

    2 frictionless tracks have equal length bumps with one going up and the other down. Halfway through the track which one has more energy? Assuming that they started at the same height, I thought that they had the same amount of energy, but since Ball B ends up having a greater velocity at the...
  32. C

    Oscillation and kinetic energy

    Homework Statement Homework EquationsThe Attempt at a Solution What do I do next to work out kinetic energy?
  33. S

    Kinetic energy of an electron or the whole atom to produce heat?

    Does electron really vibrate to produce heat energy or is it atom in whole to produce heat?
  34. A

    GPE and Escape Velocity: Understanding the Relationship for Moving Objects

    Is it necessary to be the kinetic energy greater than gpe to move ( I don't talk about in orbits) Example : Is it will be harder to move an object has a bigger gpe ( same mass but bigger hight from ground). And thanks.
  35. G

    Rotational kinetic energy decreases, internal increases?

    If rotational kinetic energy of a closed system decreases, another form of energy must increase for the conservation of energy of a closed system. We assume this system (a person in rotation) has these forms of energy: ΔE=ΔK (only rotational around an axis) + ΔUi (internal) + ΔEt (thermal) with...
  36. J

    A On the formation of a black hole due to high kinetic energy

    Hello! If energy bends spacetime, then an object moving at high velocity will bend spacetime a lot around it due to its really big kinetic energy. It follows, that an object can become a black hole at extremely high enough velocities. But, since velocity is relative, we can find an observer for...
  37. J

    How Do You Graph Kinetic Energy Against Time and Distance?

    <Moderator's note: Moved from a technical forum and thus no template.> A horizontal force of 80 N acts on a mass of 6 kg resting on a horizontal surface. The mass is initially at rest and covers a distance of 5 m in 0.92 s under the action of the force. Assuming there are no energy losses due...
  38. M

    Kinetic energy to electrical energy generator

    I am going back over the topic of electricity and energy. I am just wondering within a kinetic energy generator what is the relationship between the speed of the rotating shaft and the overall electrical output of the generator. Thanks
  39. J

    Calculate change in KE and momentum (impulse)

    Homework Statement I have attached the problem. A 14,000 kg tractor traveling north at 21 km/h turns west and travels 26 km/h. Calculate the change in the tractor's a. kinetic energy b. linear momentum (magnitude AND direction) Homework Equations [/B] delta KE = (1/2)mv^2 - (1/2)mvi^2...
  40. J

    Final speed and direction after a collision (elastic+inelastic)

    Homework Statement A billiard ball moves at a speed of 4.00 m/s and collides ELASTICALLY with an identical stationary ball. As a result, the stationary ball flies away at a speed of 1.69 m/s. Determine a. the final speed and direction of the incoming ball after the collision b. the direction...
  41. E

    Change in Potential energy equals change in Kinetic energy?

    Homework Statement I'm trying to digest the concept of change in potential energy being set equal to a change in kinetic energy. Does this relationship always hold? Please see below for more details. Homework Equations PE = mgh KE = .5mv^2 W = ΔKE = -ΔPE W = f*d The Attempt at a Solution...
  42. P

    Significant Figures with Kinetic Energy Formula?

    Homework Statement An object of mass m = 2.3±0.1 kg is moving at a speed of v = 1.25±0.03 m/s. Calculate the kinetic energy (K = 1 /2mv2 ) of the object. What is the uncertainty in K? I am not exactly sure if I used the error equation correctly when I start using Δ(v2). Could someone verify my...
  43. K

    Can anyone help with this magnetism problem?

    A 2.53*10^(-6) C charged particle with kinetic energy of 0.0929 J is fired into a uniform magnetic field of magnitude 0.147 T. If the particle moves in a circular path of radius 3.38 m, determine its mass in kg. (answer should be within 2*10^-14 kg) The formula r=mv/qb comes to mind where r is...
  44. G

    A Usage of kinetic energy in proof for E=mc^2

    http://bildr.no/view/aWc4dW95 Above is a question that I posted on a school help site. This is the answer I got: https://www.scribd.com/document/352723366/problem-about-usage-of-E-mc-2-for-v-much-smaller-then-c-ans Can you use this answer to show that the type of calculation that I tried in...
  45. M

    How Does Box Size Affect Kinetic Energy Calculations in Gas Molecules?

    In definition and proof of kinetic energy, it uses the second law of Newton. The length of the box is L and it uses this length to define the time of applying the force on the wall. Actually this time should be only time that molecule reaches to the wall and changes its velocity. My question...
  46. Kiwigami

    Understanding Drag Force against a non-linear Force

    Homework Statement I'd like to understand Drag Force better; but school always ignores it. Thus, I'm asking this purely out of obssession. I'm picturing a scenario where a non-constant force is pushing an object horizontally, ignoring friction. But, I'd like to understand how Drag Force...
  47. Richardparker800

    Need help with collision and conservation of momentum problem

    Homework Statement Two blocks move along a horizontal frictionless surface and collide head-on with each other. The mass of the first block, moving to the right with speed v is m, and the mass of the second block moving towards the left with speed 3v is 2m. After the collision, the block of...
  48. B

    Kinetic energy and potential energy look very dissimilar

    The more I think about it the less clear the respective natures of kinetic energy and potential energy (and of their sum, the so-called total energy) become. The thing is I have the impression that once you try to go a bit further than the usual description of "scalar values assigned to systems...
  49. Asmaa Mohammad

    The momentum and the kinetic energy

    Homework Statement Multiple choice question: The momentum of a body has increased by 25%, then its kenitic energy will roughly increase by... 1. 25% 2. 5% 3. 38% 4. 56% 5. 65% Homework Equations Pl = mv (Pl is the momentum, m is mass and v is velocity) K.E = 0.5 mv^2 The...
  50. R

    Mechanical energy to kinetic energy storage....?

    Can there be a system that utilizes mechanical energy to produce kinetic energy, but "storing" the mechanical energy to be used at a different time for kinetic energy? Like a bike where you pedal but don't move; that mechanical energy is waiting to be "released" by some mechanism *** exclude...
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