Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is momentum?

  1. Dec 31, 2011 #1
    I know momentum is mass times velocity, and that it is a conserved quantity, but I can't get the intuition of what momentum is, unlike mass and velocity.
    Mass relates to how heavy an object is, and velocity is how fast an object is moving.

    One idea I had is that momentum is how hard an object hits me, but I'm not entirely sure if that's an okay thing to say.
  2. jcsd
  3. Dec 31, 2011 #2


    User Avatar
    Gold Member

    Yeah, momentum is how hard something hits you ... that's not a bad way to think of it.

    If it's moving faster, it hits you harder. If it's heavier, it hits you harder. If it's both heavier and faster, you really should get out of the way.
  4. Jan 1, 2012 #3
    You could also think of it as "the quantity of motion" of an object.
  5. Jan 1, 2012 #4
    The easiest way for me to think of it is "mass in motion." Momentum is just how much "mass in motion."
  6. Jan 1, 2012 #5
    Momentum can also be thought of as Kinetic energy.

    KE = p2/2m

    p = SQRT(2m*KE)

    The higher the KE, the higher the momentum.
  7. Jan 2, 2012 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    True, for two objects of the same mass.

    Not true.
  8. Jan 3, 2012 #7
    Well they each have the same unit... the eV :p
  9. Jan 3, 2012 #8
    Not sure what you mean.The eV is a unit of energy,not momentum.
  10. Jan 26, 2012 #9
    momentum p is indeed mass m x velocity v
    p = mv

    mass..... is not actually the weight.... weight is mass x gravity..... so mass of something is its weight then deduct any gravity it was weighed in ie earth, moon etc

    velocity..... though is not actually speed..... it is a speed and a direction..... 50mph east
  11. Jan 27, 2012 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    2018 Award

    Kinetic energy scales at 1/2 mv^2. When you double your speed you quandruple your kinetic energy. This does not happen with momentum. It scales linearly at p=mv.
  12. Jan 27, 2012 #11
    If 2 objects made of the same material have the same momentum, but different KE, the little one will be harder to stop. If they both have the same KE but different momentum, the big one will be harder to stop.
  13. Jan 27, 2012 #12


    User Avatar
    Science Advisor
    Gold Member

    ??? Momentum has the Unit Ns (Newton seconds) but KE has the unit J (Joules). Not the same at all.
    Momentum is a Vector quantity - it has a direction associated with it. KE has no direction specified because it is a Scalar, not a Vector.

    Momentum is conserved in all collisions. Kinetic Energy is not.

    But, as an object speeds up, both its momentum and KE increase - so there is a kind of association between them.
  14. Jan 27, 2012 #13
  15. Jan 28, 2012 #14


    User Avatar
    Science Advisor
    Gold Member

    That article claims that "in high energy Physics, electron-volt is often used as a unit of momentum". I really doubt that, unless they are talking in some very isolated context. It sounds so wrong that I can't take that sentence seriously. Wiki can often be wrong and, more often, be written badly or with insufficient editing.
    Whatever the article says, it is important to realise that they are two distinct quantities. I can't think anyone would claim otherwise.
  16. Jan 28, 2012 #15
    I had the same reaction when I first read it but then the paragraph goes on to describe that momentum can be described by eV/c.I'm guessing that if there are people who use the eV as a unit of momentum then it is implied(though not stated) that c is included as above.If so it seems a bit sloppy to me.
  17. Jan 28, 2012 #16


    User Avatar
    Science Advisor
    Gold Member

    Sloppy, yes. But you get this sort of thing with terminology within specialised fields. Very confusing for the outsider. (Perhaps that's why it's used?)
  18. Jan 31, 2012 #17
    We used natural units in just about every physics class we had. Its pretty common.
  19. Jan 31, 2012 #18


    User Avatar
    Science Advisor
    Gold Member

    "Natural", meaning what?
  20. Jan 31, 2012 #19
    The system of natural units.


    ?? Im not sure where the confusion lies. Have you really not heard of natural units before? Your professors are doing you a disservice!
  21. Jan 31, 2012 #20
    Good King Hal established the only system of Natural Units of any importance and the Americans, God bless their cotton socks, are still using some of them.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook