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

Homework Help: Why is mass special?

  1. Nov 24, 2008 #1
    I'm trying to better understand these concepts:


    One major thing I noticed was that every single one of those quantities is directly proportional to mass. Length and time have various positive and negative exponents, but mass never, ever gets touched.

    Why don't we play with the dimensions of mass the way we play with the dimensions of everything else?
  2. jcsd
  3. Nov 24, 2008 #2

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    First of all, it is a little misleading to say that force is proportional to mass. The force of gravity (weight) is proportional to mass, that is true, but that is due to the fact the acceleration due to gravity (in free fall) is the same for everything.

    Other forces can be applied to a mass that can be of any quantity at all. Newton's 2nd law specifically says that the acceleration is directly proportional to net force and inversely proportional to mass.

    Mass is not a vector quantity, so there is no inherent direction to a mass, and therefore no "opposite" direction. Furthermore, mass is defined as the total quantity of matter in an object. Once the amount of matter goes to zero, it doesn't get less. Even "anti-matter" has positive mass.
  4. Nov 24, 2008 #3
    Before I go off the deep end here, let me say that I only have exposure to classical mechanics, so if I say something ridiculous, just respond that it gets explained at some higher level and I'll shut up:

    ...But force is proportional to mass and to the acceleration. 10 times larger mass with unchanged acceleration produces/yields/implies 10 times more force. Is mass not just some measure of the magnitude of inertia?

    Or is this a better definition: force is the cause of acceleration, and what that acceleration is going to look like depends on the mass that force is acting upon... ? (chicken makes eggs, eggs don't make chickens)

    Maybe I'm looking at this wrong...
  5. Nov 24, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There are many examples of physical quantities that are always non-negative, for example:
    1. Speed
    2. Kinetic Energy
    3. Frequency
    4. Wavelength
    5. Distance
    6. Absolute Temperature
    7. ...
    mass isn't special in this respect. However, you are quite right that mass is an interesting concept. For example, we still have no explanation for why the inertial mass (the mass is Newton's second law) should be equal to the gravitational mass of an object.
    What do you mean, "play with the dimensions"?
  6. Nov 25, 2008 #5

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

  7. Nov 26, 2008 #6
  8. Nov 26, 2008 #7
    A 3.00-kg object undergoes an acceleration given by a = (2.00ˆi + 5.00ˆj)ms2. Find the
    resultant force acting on it and the magnitude of the resultant force.

    That's what I mean ><. The very first question given in a chapter introducing students to force implies that acceleration results in force. That's just disgusting. I won't name any names, but it rhymes with Bhomson Crookes/Bole. No wonder physics is so hard.
  9. Nov 26, 2008 #8
    The second law is clasically written F=m*a, despite the fact that it really should be F/m=a but that's physics for you.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook