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. 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...
  2. O

    Why Negative in Lagrangian: Exploring the Physics Behind T-V

    I know this has been asked before: "Why is there a negative in the Lagrangian: L = T - V" I have read the answers and am not happy with them so I tried to formulate my own justification and now ask if anyone could comment on it? First, I am not happy with those who say "Because it works and...
  3. 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...
  4. 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...
  5. 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...
  6. 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?
  7. J

    Does the kinetic theory work for solids?

    Hello all, i have been asked a question about a lump of iron being warmed, and to calculate the difference between enthalpy and internal energy. I did some algebra and found that the change in the product pV , in the case of a liquid would suffice. But, since the volume doesn't change (Assumed...
  8. 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...
  9. 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...
  10. 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)...
  11. 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...
  12. ognik

    Trying to verify my program's output

    Hi - I have written a Fortran 'beginner' program to find the kinetic energy matrix for a simplistic non-relatavistic, 1-D, radial Schrodinger system - using the Lagrange - Laguerre Mesh method. Firstly, I have verified that my roots for the Laguerre poly ##L^{\alpha}_{N}(x_{i}) = 0## are...
  13. 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...
  14. 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...
  15. 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...
  16. 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...
  17. P

    WHy is kinetic PE assigned K, instead of U_k?

    Im confused about the different letters assigned to energy (in intro physics). SO far I've learned about gravitational potential energy (U) Elastic potential energy (U) electric potential energy (U) and kinetic energy (K) How do we differentiate between all the different trypes of energy? Why...
  18. J

    Direction of Kinetic Friction and Static Friction

    Can someone please explain me the direction of static friction? I know kinetic friction is always going against the motion of an object but for static friction it depends.
  19. T

    Average potential and kinetic energies for H ground state

    Homework Statement Calculate the average potential and kinetic energies for the electron in the ground state of hydrogen. Homework EquationsThe Attempt at a Solution I know that KE = E - <U(r)>. I know that E = (-KZ^2e^2)/(2a0) and I know that U(r) = -KZe^2/r but I can't figure out how to...
  20. S

    Need Help with Solving Point Kinetic Equation for One group

    Does anybody can help solve point kinetic equation for one group of delayed neutrons in steps. I am looking forward to solve it by analytical methods. dn(t)/dt=ρ-β/l n(t)+ λC(t) dC(t)/dt= βi* n(t)/l- λC I would really appreciate your help as i am have to submit to clear this paper next week
  21. N

    Thermodynamics question thanks so much for the help

    Homework Statement a small lump of ice at 0 degrees celsius falls to the Earth's surface. when the hailstone hist the surface, all of the kinetic energy of the hailsotne is transferred to thermal energy in the ice. calculate the minimum speed of the hailstone so that it just melts when it hits...
  22. 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...
  23. 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 =...
  24. 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
  25. L

    Finding the Force for Constant Velocity with Kinetic Friction

    Homework Statement in the question you're having to solve for this. A force is applied at a angle of 30 degrees on a object that weighs 12kg that is also moving at a constant velocity. kinetic friction is equal to .70 i am confused on how to find the Force applied to achieve a constant...
  26. T

    What about potential energy in the Work Kinetic Energy Theorem?

    W_{total} = \delta K What about lifting a block upward? If you lift a 10kg block vertically and bring it to a rest, you are doing work on it but the velocity in the beg and the end is 0, thus the equation says the work done on it is 0. But isn't there potential energy? Does the equation not...
  27. Sace Ver

    Calculate the coefficient of kinetic friction for a sliding box

    Homework Statement An electric motor is used to pull a 125 kg box across a floor using a long cable. The tension in the cable is 350N and the box accelerates at 1.2m/s2 (forward) for 5.0s. The cable breaks and the box slows down and stops. Calculating the coefficient of kinetic force? Given...
  28. A

    Kinetic energy of 2 cart system

    Homework Statement On cart with a mass of 371 kg and another cart with a mass of 495 kg are rolling toward each other. Just before they collide, the 495 kg cart has a speed of 2.09 m/s, and the 371 kg cart has a speed of 1.90 m/s. We will consider the friction between the cart wheels and track...
  29. H

    Calculate how much force you would have to apply to crate?

    Homework Statement Calculate how much force you would have to apply to the crate to get it to go at constant speed up a 30 degree ramp. Homework Equations Using kinetic friction for equation. The Attempt at a Solution I couldn't find the formula for kinetic friction which also includes...
  30. 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)...
  31. K

    Can Friction Cause Torque & Work on Rolling Objects?

    can kinetic friction cause a torque? (can it result in rotation) Or is it that only static friction can cause a torque? Can static friction do work on a rolling object? I think it can? because the rotational kinetic energy is increasing? is this right? HELP!
  32. 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...
  33. N

    Solving for Skid: Exploring Kinetic Friction on a Circular Track

    Homework Statement This isn't a problem from a textbook or homework but just a general question. Say there's a car traveling in a circle and that the track has some coefficient of kinetic friction µ. What would make the car skid? Homework EquationsThe Attempt at a Solution I'm thinking that...
  34. C

    Setting up the double integral

    A sheet of metal in the shape of a triangle massing 10 kg per square meter is to be spun at an angular velocity of 4 radians per second about some axis perpendicular to the plane of the sheet. The triangle is a right triangle with both short sides of length 1 meter. (a) The axis of rotation is...
  35. 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...
  36. 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...
  37. U

    Kinetic equation -- 2 masses attracted in space and collision....

    Hi, I like to ponder basic physics puzzles to pass time in boring meetings on metro etc...but I have one that I don't understand and I am obsessing over it (to detriment of working out etc.) it should be easy but I don't understand. Problem: Imagine you have two blocks (Mass of Block 1 is...
  38. K

    Finding the coefficient of Kinetic force with an angle

    Homework Statement A 1000N crate is being pushes across a level floor at a constant speed by a force (F) of 300N at an angle of 20 degrees below the horizontal, what is the coefficient of kinetic force? Homework Equations Fk= uk*N uk= Fk/N[/B] Fk=F2 The Attempt at a Solution in the back of...
  39. 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)
  40. I

    Does the Kinetic Theory of Matter Always Apply Across Different Substances?

    This came up whilst helping my kid with her GCSE physics so ought to be pretty straightforward. Here goes: At his level the kinetic theory of matter is taught in a simple way and one "key point" which is stated time and time again is "the particles in a gas have more energy than the particles...
  41. B

    Coefficient of Kinetic Friction

    Homework Statement A horizontal force of 150 N is used to push a 50.0 kg packing crate a distance of 5.00 m on a rough horizontal surface. The acceleration of gravity is 9.81 m/s2 . If the crate moves with constant velocity, calculate: a) the work done by the force. Answer in units of J. b)...
  42. 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...
  43. 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 =...
  44. R

    Calculating Kinetic Force for Sci-Fi Story: Help Needed

    Can you help me work something out, please? I'm writing a science fiction story, and want to work out how much impact energy would be produced by railgun. As a starting point, though I'm sure it should be way too high, if I accelerated a 300kg shell to 290,000 m/s, the kinetic energy should be...
  45. S

    Coefficient of friction on an inclined plane?

    Homework Statement A lab I've performed was measured using forces. As the mass increases, the force does as well. The inclined plane is at an angle of 30 degrees. Now, finding the coefficient, should the kinetic and static friction be decreasing on a plane? I've measured the frictions on a flat...
  46. 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...
  47. S

    Why wouldn't this space propulsion work?

    Hi! I'm pretty sure someone would have already made this if it did work, but I don't know why it wouldn't. Can someone explain why this spacecraft couldn't move forward?
  48. 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...
  49. 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...
  50. 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...
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