1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding the concept of Kinetic Energy

  1. May 24, 2013 #1
    Hi, I'm new to physics and have some trouble understanding the concept of Kinetic Energy.
    It's not that I don't understand the definition given to me, I do, but I don't feel the intuition behind the concept. Here's what I understood of the concept : Kinetic energy comes from moving objects (If it's not right, please correct me.) Here's the problem : If it comes from the moving object, does it have any purpose ? Or is it only energy for energy, doing nothing ? Does the object move because of that energy? Or am I complicating things for nothing ? Thanks for your patience and answer ! (An answer in depth, adjusted for a beginner, would be great!)
  2. jcsd
  3. May 24, 2013 #2


    Staff: Mentor

    I am not sure what your question is getting at. Energy never has any purpose, it doesn't have goals or desires or hopes or dreams or aspirations or any other human desire.

    Energy means that something can do work (work is a force applied over a distance). A moving object has kinetic energy, meaning that it can collide with some other object, exert a force on the other object and move it, and thereby do work on it.
  4. May 24, 2013 #3


    User Avatar
    Science Advisor

    I sense a new Pixar movie in the making!
  5. May 24, 2013 #4


    User Avatar
    Gold Member
    2017 Award

    I'm with DaleSpam, but I'd like to try to formulate it in this way:

    A massive object that moves in a direction has a kinetic energy; the formula is

    [itex]E = \frac{1}{2} m v^2[/itex]

    (E = Energy, m = mass, v = velocity, v<<c). The larger the mass and/or the velocity is, the larger the kinetic energy is. If the object does not move (velocity v=0), the kinetic energy is zero, i.e. the object has no kinetic energy. The kinetic energy of an object can be transferred to other objects by e.g. collision and/or transformed to other types of energies, i.e. potential energy.

    So, in short, the kinetic energy does not make the object move; rather, a moving object has kinetic energy.

    (Note: A rotating massive object also has energy - rotational kinetic energy)

    See also: Kinetic energy, Rotational kinetic energy (HyperPhysics)
    Last edited: May 24, 2013
  6. May 24, 2013 #5


    User Avatar
    Science Advisor

    I don't know whether this portrayal will capture your intuition.

    If you push a car up a hill, it takes energy. The higher the hill, the more energy it takes. That's a direct proportion. Twice as far up the hill uses twice as much energy.

    It's also proportional to the weight of the car. Twice as much weight requires twice as much energy.

    Now, suppose that the car got a running start and coasted up the hill. How far up the hill would it go? Well if it goes twice as fast then it would get twice as far up the hill in a fixed amount of time. But it would also take twice as long to slow down. So twice as fast means four times as far up the hill.

    If you equate the energy in a moving car with its ability to coast up a hill it follows that the "kinetic energy" in a moving car is proportional to its mass and also proportional to the square of its speed.

    If you take away the hand-waving, and use a coherent set of units so that f=ma, it ends up being a deducible consequence that e = mgh = 1/2 mv2
  7. May 24, 2013 #6
    I doubt many people 'feel the intuition' behind KE when first exposed to it...I know I didn't. ....
    or many other physical entities, for that matter. A lot of physics is not intuitive until you have some additional perspective on a subject.

    You may want to read about momentum [mv] and see how it differs from KE, [1/2 mv2 .

    I suspect the motivation was usefulness....helpful in describing observed physical situations.
    If you do read a bit about momentum, note the differences from the perspective of the center of mass and also energy conservation. Also note momentum is a vector quantity, KE a scalar.

    It's best to remember carefully, memorize if you wish, definitions of each physical entity you encounter to distinguish the way scientists have found it useful to designate each from others.

    I always found it useful to remember that KE is defined as the WORK needed to accelerate a body of a given mass from rest to its stated velocity. This is the flip side of Dalespam's description.
  8. May 24, 2013 #7
    I guess my problems comes from trying to understand too hard, but I think it kinds of makes sense now. I don't know why, but I thought that this energy HAD to do something, like to make the object move, my mind was only fixed on that idea. My lack of knowledge or experience in physics was the cause of this problem, I think, knowing that I ONLY begun to study it. Anyway, thanks for your answers !
  9. May 25, 2013 #8
    nothing really wrong with that statement, but why not use a standard type definition for KE....along the lines of "the energy of motion that results from the work done on an object", for example...that way others will know for sure what you have in mind.

    When you do 'work' on an object, something does happen. If it is W = F x d, then an object is displaced, but if it is,say, sliding a book across a flat table, no change in energy is imparted to the book. [You have done work against friction] if you compress something, especially a gas, you impart heat energy which results from the work done on it....If you press as hard as you can against,say, a wall, you might compress it a smidgen, but not much 'work' is done [according to the definitions of physics] because there is no movement.....and the 'energy' you exert is simply burned up calories used by your muscles. But you'll sure get tired.
    All this type of energy expenditure does is warm you up a bit that you can feel.

    You can also try to explain to yourself how potential energy and kinetic energy are similar and how they are different. Both can do work, for example.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook