In The Feynman Lectures on Physics Vol-1, it is written

How hard it is to get a body going is one thing, and how much it weighs is something else. Weight and inertia are proportional, and on earth's surface are often taken to be numerically equal, which causes a certain confusion to the student. On Mars, weights would be different but the amount of force needed to overcome inertia would be the same

I've been thinking about the last sentence for a while now but just couldn't figure out what it means.

If two bodies have different weights and the gravitational acceleration is same for both since they are on the same planet, then clearly one body is more massive than the other.

Then how is it possible that the amount of force needed to overcome inertia be the same?

Isn't inertia an intrinsic property of a body which is proportional to its mass?

  • 2
    $\begingroup$ This seems more of an English question than a physics question: "the same" is supposed to mean that the amount of force needed to overcome inertia is the same for the same body on different planets, not the same for two different bodies. $\endgroup$
    – ACuriousMind
    Nov 7, 2016 at 13:17
  • $\begingroup$ I think they mean "On Mars, weights would be different from their respective values on Earth…", rather "…different from each other". $\endgroup$
    – pela
    Nov 7, 2016 at 13:18

1 Answer 1


I believe he's sayings that the gravitational relationship between objects varies based on the masses at play, but the inertia for a particular mass is invariant regardless.

Like ACuriousMind said i believe the wording is what's confusing you. He means weights on Mars would be different (from weights on Earth), but the inertia of a given object would be the same in both places.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.