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. M

    Coefficient of Kinetic Friction

    Homework Statement A crate of bananas has an initial speed of 4.2 m/s2. It slides across a floor, traveling 6.0m before coming to a stop. Find the coefficient of kinetic friction between the crate and the floor. Homework Equations Fnet=ma v2=v02+2aΔx The Attempt at a Solution...
  2. B

    Relationship Between Kinetic Energy and Temperature

    Hello everyone, I am currently reading the pages from a book called Thermal Physics, which was written by Daniel Schroeder; the pages to which I refer are 10-13. In these pages, he derives the relationship \bar{T} = kT. Here is one line that intrigues me, "So if this model is accurate...
  3. C

    Conservation of momentum and kinetic energy

    Homework Statement Two 0.3 Kg gliders collide elastically on a frictionless track. Prior to the collision , their total kinetic energy is 0.52 J and their total momentum is 0.12 Kg m/s [left] along the track. Calculate the final velocities of the gliders. Homework Equations m1vi1 + m2vi2...
  4. V

    Car acceleration, fuel usage and kinetic energy

    Someone said the following and it just doesn't sit right with me. How do I effectively tell them what's wrong here? " It applies to physics the same, as can be seen with the Kinetic Energy Formula Ke = 1/2MV^2...
  5. Y

    Why is kinetic energy conserved?

    For instance in an elastic collision of two indivisible particles... Why does m v^{2} stay constant? Why not m v^{3}? [That is, where other type of energy are unavailable (potential, electrical, mass, etc.)] In other words, why does the physical unit: kg \cdot m^{2} / s^{2} , or Joule...
  6. I

    Nonconservative work kinetic friction problem

    This isn't homework. I'm reviewing physics after many years of neglect. As with most of my posts, I made this problem up. Let object A have mass m_A and object B have mass m_B. One of A's surfaces is flat, as is one of B's. These flat surfaces are in contact and slide relative to each other...
  7. M

    Explaining the 2 in Δt = 2L/vx in Kinetic Theory

    http://en.wikipedia.org/wiki/Kinetic_theory#Pressure_and_kinetic_energy Based on the derivation for P in the above link, one of the early steps was: Δt = 2L/vx which is fine since it is analyzing one specific wall of the container, but then in the next step: F = Δp/Δt = mvx2/L And...
  8. A

    Relationship between Velocity, Kinetic Energy, and Heat

    Hi all, I'm new here. I am curious to know if I could simulate high velocity on an object by applying the forces (compression) to it that it would experience at those high velocities. Also, I'm curious to know if this high velocity (Kinetic Energy) would generate heat, and if so, what's the...
  9. C

    Kinetic energy of a pendulum in motion

    Homework Statement A trolley consists of an enclosed carriage of mass M supported by 4 wheels each of mass m/4. A simple pendulum consists of a bob of mass μ on a light rod of length l supported at a pivot attached to the roof; its motion is confined to a plane. a) Write down expressions for...
  10. H

    Kinetic Energy and Work Homework with Experiments and Calculations

    Homework Statement *The experiments drawing is attached. m=0.1kg Ek at point O (the moment the ball is being pushed) = 1.25 J D (0 ; 0) , C (0 ; 0.05), E (0.25 ; 0), F (0.50 ; 0), G (0.75 ; 0) Friction is neglected 1.Complete the chart by typing if the value of each variable is rising /...
  11. A

    Calculating Kinetic Energy of an Alpha Particle

    Homework Statement could you help me for the question b? Homework Equations E = mc^2 The Attempt at a Solution the mass of the alpha particle is 0.66466 x 10^-26 kg >> 4.022 u put it into the equation, so E = 4.022 x c x c = 3.615 x 10^17 eV >> 0.0579 joule my answer is that, but my...
  12. A

    Static and kinetic friction in a car halting

    In my textbook, there is a problem that made me think a bit more (this is not what the textbook was asking). In the problem, there is a 1500 kg car traveling at 30 m/s on a 10 degree sloped hill (the car is driving uphill). The driver slams on the brakes and skids to a halt. We have to...
  13. U

    What is the max kinetic energy of photoelectrons from monochromatic beam?

    Homework Statement The electric field associated with a monochromatic beam becomes zero 1.2*10^15 times per second. Find the max kinetic energy of the photoelectrons when this light falls on a metal surface whose work function is 2.0 eV. Homework Equations The Attempt at a Solution...
  14. B

    Calculate the kinetic energy of the mass

    Homework Statement A 5.0 kg mass is released from rest, at the top of a frictionless 30° inclined plane. Calculate the kinetic energy of the mass when it has slid 40cm down the incline starting from rest. (ANS = 9.80 J) Homework Equations KE=1/2(mv^2) a=v/t d=v1t + 1/2a(t^2) v2^2=v1^2...
  15. F

    Change in kinetic energy between reference frames

    I was trying to do some studying regarding the definitions of energy, and I've hit a road block. I know that since velocity is frame-dependent, an object's kinetic energy as observed in two different frames will yield two different results. My initial expectation was that changes in...
  16. C

    How Is Kinetic Energy Calculated from Force and Displacement?

    Homework Statement If the resultant force acting on a 2.0kg object is equal to (3i + 4j) N, what is the change in kinetic energy as the object moves from (7i -8j)m to (11i -5j)m Homework Equations w = Fd Ek = 1/2mv2 The Attempt at a Solution I assumed that Ek = w so I tried to...
  17. H

    Kinetic energy of a sled question

    Homework Statement A sled of mass m is coasting on the icy surface of a frozen river. While it is passing under a bridge, a package of equal mass m is dropped straight down and lands on the sled (without causing any damage). The sled plus the added load then continue along the original line of...
  18. S

    What is the potential and kinetic energy of a falling object?

    Homework Statement A physics book weighing 20 N is dropped from a position 2.0 meters above the floor. a) what is the potential energy of the book before it is dropped? b) what is the kinetic energy of the book when it is 1.0 meter from the floor Homework Equations Ek = 1/2mv^2 , Ep=...
  19. E

    What is its rotational kinetic energy?

    Homework Statement A 240g ball and a 570g ball are connected by a 48.0-cm-long massless, rigid rod. The structure rotates about its center of mass at 110 rpm. Homework Equations KE = Iω^2 I = 1/12(mr^2) The Attempt at a Solution Since it has two masses and two different radius...
  20. V

    Rotating ball and string Rotational Kinetic Energy?

    Homework Statement Hello, I am a bit confused on when rotational kinetic energy exists and when linear kinetic energy exists. For example when we spin a string with a ball attached to the end it has kinetic energy of 0.5mv^2 of the ball when we were solving for velocity of the ball at certain...
  21. H

    Work and Kinetic Energy of a Trebuchet

    Homework Statement This time you construct a trebuchet. A counterweight (M = 550kg) is at one end a distance ℓ2 = 1.5m away from the pivot. The child (m = 10kg) is a distance ℓ1 = 3m away from the pivot. The mass of the uniform rod connecting the two is mrod = 35kg. When released from the...
  22. R

    Kinetic Energy of a Rotating Hoop - Physics Forum

    Hey PF, this is just a quick question: If there's a hoop of mass M and radius a rotating around its vertical axis (see pic) and I want to write the kinetic energy for the Lagrangian is it just T = \frac{1}{2} M a^2 w^2 ? Considering w the angular velocity
  23. A

    Angular momentum and kinetic energy

    A boy sitting on a rotating chair (ignore friction) with his hands stretched outwards, pulls in his hands thus reducing the moment of inertia of the chair-boy system. The angular velocity will increase to conserve the angular momentum. The kinetic energy of the system will also increase (as...
  24. P

    Kinetic friction and acceleration

    Hello everyone, I would appreciate some help with this problem: The coefficient of kinetic friction between the 1.2 kg block in the figure below and the table is 0.38. What is the acceleration of the 1.2 kg block? Homework Equations F=M*A Fk = μk * N The Attempt at a Solution I created...
  25. W

    Expectation value of kinetic energy

    Homework Statement Given the following hypothetic wave function for a particle confined in a region -4≤X≤6: ψ(x)= A(4+x) for -4≤x≤1 A(6-x) for 1≤x≤6 0 otherwise Using the normalized wave function, calculate the...
  26. P

    Finding the angular velocity and rotational kinetic energy

    Homework Statement A solid cylinder is rolling along a horizontal plane and is friction less around its symmetric axis. The cylinder is pulled by a constant force, F and travels the distance, d. The cylinder does not glide and has a friction force, f on the ground. Known values: Mass: M...
  27. U

    How Does the Kinetic Theory of Gases Describe Particle Collisions?

    Hi guys, I'm studying the Kinetic Theory of gases from Pauli's book Vol. 3. Here he describes a section on the Mean free path, where the probability of two particles with speed v and v' colliding is described as:What is f(v,v') here? Is it the velocity distribution function? If so, isn't it...
  28. gauss44

    Can a reaction be exothermic due to increased kinetic energy?

    Homework Statement Examkrackers Lecture question - MCAT Prep book 116. A student holds a beaker of pure liquid A in one hand and pure liquid B in the other. Liquid A has a higher boiling point than liquid B. When the student pours a small amount of liquid B into liquid A, the temperature of...
  29. 0

    Archived Transformation of Gravitational to Kinetic Energy

    I am not following the template for the reason that this is a generic question. Consider that the change in kinetic energy is 1J. Suppose further you have two particles, both of equal mass that are gravitationally attracted to each other (and the change in energy comes from the fact that they...
  30. S

    Is the classical formula for kinetic energy wrong?

    Recently I have been troubled by several things which have been stated in my school physics classes, mainly concerning the classical kinetic energy formula and other formulae derived from it (as well as the 'Work Energy Theorem'). The classical formula for kinetic energy '(mv^2)/2' was first...
  31. M

    Kinetic energy, movement <-> energy ?

    Hi there, I always thought that kinetic energy was the energy associated to a movement, ie, I have a ball in movement so I had printed the energy of my body on it when I kicked the ball, right? So I gave energy to the ball that started to move it. energy -> movement. However, I was reading...
  32. G

    Rotational Kinetic Energy: Pole Balanced Vertically

    Homework Statement A 1.80m long pole is balanced vertically with its tip on the ground. It starts to fall and its lower end does not slip. What will be the speed of the upper end of the pole just before it hits the ground? [hint: Use conservation of energy] l=1.890m Homework...
  33. E

    Mixing Gases, temperature, and the Kinetic Model of Matter

    Homework Statement Consider the following statement: "The temperature of a gas is a measure of the speed of its particles (atoms and molecules). Now suppose that I mix two gases together, for example oxygen and carbon dioxide, which are initially at different temperatures; if I wait long...
  34. E

    How does rotational kinetic energy increase in rolling downhill?

    Homework Statement This is not a homework problem. Okay, it is -- but I have solved it correctly already, so the question is not there. I'm just not sure about a detail in one of many solutions. We have a hollow cylinder with a uniform mass distribution rolling down an incline with some...
  35. J

    What fraction of its total kinetic energy is rotational kinetic energy

    Homework Statement A spherical object with moment of inertia 0.57mr2 rolls without slipping down an incline. At the bottom of the incline. Homework Equations For a sphere (or cylinder) rolling without slipping ω = v/r. Rotational KE = ½Iω² The translational kinetic energy = ½mv²...
  36. S

    Can this method turn potential energy into kinetic energy?

    I am wondering if this method is in use already somewhere, or if its possible to be used. For example, using the equation of gravitational potential energy a 100kg weight that freefalls 100 meters has 98,000 J of energy, which equals 24,000 watts after approx 4 seconds of freefall. Assuming I...
  37. U

    Calculating Inertia and Kinetic Energy of Connected Shafts

    Homework Statement b) During the operation of a machine, two shafts rotate in opposite directions and are then connected by a clutch system. Shaft 1 has a rotational speed of 800 rev/min, a mass of 35 kg and a radius of gyration of 375 mm. Shaft 2 has a rotational speed of 1100 rev/min, a...
  38. I

    Potential energy and kinetic energy

    So today in class we were talking about gravity and my professor said that generally KE = -.5PE And so TME = .5PE I don't understand how that works.
  39. R

    How Much Kinetic Energy is Lost

    Homework Statement A 2.00-kg object traveling east at 20.0 m/s collides with a 3.00-kg object traveling west at 10.0 m/s. After the collision, the 2.00-kg object has a velocity 5.00 m/s to the west. How much kinetic energy was lost during the collision? (no image was given). Homework...
  40. R

    Rotational Kinetic Energy of Planet

    Homework Statement This question has two parts; the first part I understand but the second part I do not. Part 1: What is the rotational energy of a planet about its spin axis? Model the planet as a uniform sphere of radius 6420 km, and mass 5.59x10^24 kg. Assume it has a rotational period...
  41. S

    How much kinetic energy take the particles after the collision?

    Homework Statement What is the minimum amount of kinetic energy of proton that hits another still proton so reaction ##p+p\rightarrow p+n+\pi ^+## is possible? How big part of the initial kinetic energy is used for kinetic energy of new particles?Homework Equations The Attempt at a Solution...
  42. R

    Calculating Kinetic Energy with P=1/2Aρv3

    Hi Everyone, This is a real simple question, but I should make sure I have this right. In the following formula, is the value of P in watts? P = 1/2 Aρv3 I'm assuming that this is in watts, but you know what they say about assuming. Thanks in advance for your help! Cheers, Rick
  43. S

    Collision of electrons, kinetic energy?

    Homework Statement What is the minimum amount of kinetic energy of electron that hits another electron, so ectra electron-positron pair is produced. Homework Equations ##p^{\mu }=(E/c,\vec{p})## The Attempt at a Solution Before the collision: ##p^{\mu...
  44. M

    How Is Maximum Elongation Calculated in a Spring System?

    A 2-kg block is attached to a horizonal ideal spring with a spring constant of 200 N/m. When the spring has its equilibrium length the block is given a speed of 5 m/s. What is the maximum elongation of the spring? Is not the solution simply: 1/2kx2 = 1/2mv2 x = (mv2/k)0.5 x = 0.5 m...
  45. stripes

    Finding final kinetic energy of a system

    Homework Statement A uniform solid sphere of mass M and radius R has a light, thin metal spike driven through it, so it is free to turn about this spike. So imagine a bowling ball on a stick that is free to rotate about this stick. A bullet of mass m is fired at speed v into the ball at a...
  46. B

    Calculating Kinetic Energy of Hurricane Winds: A Simple Model Approach

    Homework Statement In a simple model of the wind speed associated with hurricane Emily, we assume there is calm eye 18.8 km in radius. The winds, which extend to a height of 4000 m, begin with a speed of 303.0 km/hr at the eye wall and decrease linearly with radial distance down to 0 km/hr at a...
  47. Crusader711

    Translational and rotational kinetic energy-Mass Unknown

    A hollow, thin-walled sphere (ICM = 2MR2/3) of mass 20 kg is completely filled with a liquid of unknown mass. The sphere is released at the top of a plane inclined at 30° to the horizontal, and it rolls 20 m to the bottom in 3.6 s. What is the mass of the liquid? 2.My approach is...
  48. J

    Conservation of kinetic energy/linear momentum problem

    Homework Statement An 8-kg sphere A is moving to the left with a velocity of 15 m/s when it strikes the 45° inclined surface of a 10-kg block B which is moving to the right a 5 m/s. The ball strikes the block 1 meter above the ground. The block is supported by rollers and impact is perfectly...
  49. E

    Rotational Kinetic Energy and Frictionless Surfaces

    What happens if a ball goes from rolling on a surface to a frictionless surface? I know friction is required for the ball to roll, so would the rotational energy be transformed into translational energy (the ball goes faster without rolling), or does something else happen? Thanks!
  50. O

    Given force, find the change in kinetic energy

    Homework Statement You are given Force as a function of distance (F vs x). Find the change in kinetic energy from $x=a $ to $x=b$. Homework Equations see attempt solution conservation of energy equations The Attempt at a Solution Since F = -dU/dx we get the integral of F = U...
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