# Why do we make such

1. May 9, 2004

### JasonRox

Why do we make such a careful distinction between gravitational mass and inertial mass, rather that talking about one mass only, since they are equivalent?

2. May 9, 2004

Staff Emeritus
There is no apparent reason why they should be equal in classical physics. The fact that they are equal requires explanation.

3. May 9, 2004

### JasonRox

This is a question in the book. Practice question so it's not worth marks. I personally thought they are different, and that gravitational mass is a direct result from inertial mass.

Is that right?

Another question... I've been pondering this one.

Let's say we have they train cars. They say that the first couplings have more force exerted on them than the last one.

If they are moving at a constant velocity, and we ignore friction, would they have all equal forces of 0.

According to Newton's Laws, they will continue on forever. If one had to snap, which is impossible, it would be impossible to predict on top of that.

4. May 10, 2004

### pallidin

I could be wrong about this, but I am under the impression that acceleration does not increase gravitational mass of the object, only it's inertial mass.

5. May 11, 2004

### jcsd

Think about a collsion between two charged objects compared to a collison between two objects with no charge.

Pallidin in relativity the inertial mass and the gravitational mass are one in the same.

6. May 11, 2004

### pallidin

So, the gravitational field coming from a given object is increased by acceleration? Thus, would a finely balanced, perimeter weighted gyroscope, having a rest weight of "x" actually weigh slightly more when accelerated?

7. May 11, 2004

### robphy

The equivalence of inertial mass and gravitational mass is one form of Einstein's "Principle of Equivalence", which led to General Relativity.

Here's a nice discussion
http://www.pa.uky.edu/~cvj/as500_lec6/as500_lec6.html [Broken]
http://instruct1.cit.cornell.edu/courses/astro101/lec24.htm [Broken]
http://csep10.phys.utk.edu/astr162/lect/cosmology/equivalence.html

Here's a research group that tests the principle
http://www.npl.washington.edu/eotwash/index.html [Broken]

Last edited by a moderator: May 1, 2017