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Equivalence principle to argue mass and weight are the same?

  1. Aug 20, 2014 #1
    Hi pf, I have recently watched a YouTube physics video from SixtySymbols channel which is a channel of short physics videos presented by professors of physics from Nottingham University. The video (pasted below) argues that because of inertial mass and gravitational mass being equivalent this provides an argument that mass and weight are the same thing? He ends the video by saying that if some smarty pants scientist tells you that mass and weight are different because weight is a force then you can tell them that fundamentally they are the same due to the equivalence principle. I really don't understand the connection. I have come across mass equivalence before and I understand what it is and understand his explanation of it in the video but what I don't understand is how this can be used to argue that mass and weight are the same? I was always taught by all my physics teachers that mass and weight are strictly different. For example, if you have a mass sitting far out from any gravitational field then it has no weight since the product of its mass and acceleration due to gravity is zero. However, it still definitely has mass so how can these two things ever be the same?

    Link to video:


    Many thanks for any help!!
  2. jcsd
  3. Aug 20, 2014 #2


    Staff: Mentor

    Congratulations, you answered your own question correctly.
  4. Aug 20, 2014 #3
    But if I'm right then why does a professor of physics argue otherwise?
  5. Aug 20, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    His conclusion at the end of the video makes no sense to me either. For example: the mass of a bag of sugar is the same on the Earth and on the Moon, but the weight is different.

    I don't follow his argument that the equivalence of inertial and gravitational mass leads to the equivalence of mass and weight.
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