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

Inertia and Momentum.

  1. Apr 11, 2006 #1
    Are momentum and inertia kind of the same thing? I don't understand the big difference between them. The only difference I can see is that for instance my chair isn't moving but it has inertia since I would have to apply such a force to get it moving. However, I don't think I can say it has momentum right now. If a ball is rolling towards me then I feel as if I can say it has momentum and it has inertia, and in my mind I can't see the difference between them.

    Please help. Thanks
  2. jcsd
  3. Apr 11, 2006 #2


    User Avatar

    Inertia is a property of matter, but matter at rest has what momentum?
  4. Apr 11, 2006 #3
    What do you mean by at rest? Constant velocity or zero velocity.
    Last edited: Apr 12, 2006
  5. Apr 12, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Only one of those two choices accurately describes a state of being "at rest." I'll give you two guesses which one...:rolleyes:
  6. Apr 12, 2006 #5


    User Avatar
    Homework Helper
    Gold Member

    Inertia cannot be tranferred to another body while momentum can.

    If you push something and get it moving it does not loose inertia, and you do not gain any inertia.

    Where as when you push something forward, the object does gain postive momentum. You will also move the opposite way and gain negative momentum. And both object gain momentum and still adhere to the conservation of momentum.
  7. Apr 13, 2006 #6
    Inertia is that quantity which resists the changes in momentum of an object. A particle sitting still willl still resist changes in momentum.

  8. Apr 15, 2006 #7
    Inertia is the resistance of a massive object to changes in its acceleration. It is just a word used to say that it is difficult to make an object go faster, even though we don't really understand why.

    Momentum, unlike inertia, is actually a vector quantity that gives measurement to the total amount of motion in a direction. An object twice as massive but moving in the same speed and direction has twice as much motion in that direction. An object moving twice as fast as another in the same direction, but have the same mass, has twice as much motion in that particular direction.

    So in short, inertia is a property of massive objects, momentum is the measurement of a seperate property (motion) of massive objects.
  9. Dec 4, 2008 #8
    i am still wondering the same thing because i have this weird homework paper thats hard
  10. Dec 4, 2008 #9

    thanks that really helps
  11. Dec 4, 2008 #10
    thanks go1 what you said about momentum and inertia helped me!! :) !!!!
  12. Dec 5, 2008 #11
    what school do youguys go 2
  13. Aug 8, 2011 #12
    Um Ok

    But if you push a ball it initially has some inertia to over come to get it moving, but once it is rolling, because it is now spinning around, doesn’t gain inertia (which it must give up/transfer to something when it comes to a rest)? Or am I way off here?

    The three types of inertia are

    (1) The general concept of inertia, according to newton's first laws, concerning objects' masses when it comes to resistance.
    (2) Rotational inertia
    (3) Gyroscopic inertia

    SO a ball rolling down a has gained inertia and momentum?

  14. Aug 8, 2011 #13


    User Avatar

    Staff: Mentor

    This thread is very old and we've had more recent discussions, but..
    Objects do not gain inertia unless they gain mass.
    No, it has only gained momentum.
  15. Aug 9, 2011 #14


    User Avatar
    Science Advisor

    "Inertia" is mass.

    "Momentum" is mass times velocity.
  16. Aug 9, 2011 #15
    Inertia and mass are nearly the same thing (although not exactly). And as you know, momentum is mass times speed.

    That's even so in relativity if you use the relativistic mass definition: then "inertia" is simply the inertial property of mass when you try to change an object's velocity - it's the resistance to that attempt.
    If you use the rest mass definition of mass, then "inertia" is the inertial property of mass times the relativistic gamma factor.

    I now see that it was a very old thread, here's a quick comment on the new post:
    No, in classical physics it doesn't gain inertia. But see my comments here above.

    Last edited: Aug 9, 2011
  17. Aug 17, 2011 #16
    If an object has inertia of 10 kg*m^2, what does this physically mean? For example, if an object travels at 10 m/s, then in my head I can see that the object travels 10 m every sec. I guess I'm having problem understanding what kg*m^2 really mean.
  18. Aug 19, 2011 #17
    The higher Inertia an object has the more force you need to apply to alter it's velocity / placement in space.
    Simply put.
  19. Aug 19, 2011 #18


    User Avatar
    Staff Emeritus
    Science Advisor


    EDIT: Peterbo I didn't realise that you had posted on an old thread when I replied. Looking back you can see that this thread is many years old. In future if you want to ask something feel free (in fact it is preferred) to start a new thread rather than resurrecting an old one.
  20. Aug 19, 2011 #19


    User Avatar

    Staff: Mentor

    That is moment of inertia, which is resistance to angular acceleration. It has nothing to do with linear speed.
  21. Aug 19, 2011 #20
    I did notice it was started a long time ago.
    How ever others have been replying to it recently :)
    Im new in the forums as you can see, my first post. Just wanted to post a reply to this rather simple question, since some posters in it couldnt see the awnser when told in it's mathmatical form.

    Thank you
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook