# Does mass as a measure of inertia have a meaning in zero gravity? [duplicate]

Mass apart from the amount of material, is also a measure of the inertia of an object, ie. the resistance to change its motion. In zero gravity, does still mass count as the amount of inertia? In other words, the resistance to move a rock is the same in zero gravity and on earth, if there is no friction etc?

## marked as duplicate by Qmechanic♦Apr 30 '16 at 10:07

• Why wouldn't it be? Your mass doesn't change if you are on Earth, on the Moon or in a spaceship on the way. Inertia is as such not at all about gravity. Acceleration by any force will depend on the "resistance" against the acceleration, which is the inertia/mass – Steeven Mar 18 '16 at 13:20
• Imagine sliding a smooth pebble across an iced pond to collide with another smooth pebble. The reaction that you get is precisely the type of reaction that you could expect in weightless space. The primary difference is that on earth, there is a minimal amount of friction being applied. – Neil Mar 18 '16 at 13:26

## Yes, mass has meaning in zero gravity

You are confusing mass with weight. Mass is the "amount of stuff" that is constant in whatever gravitational field the object is in -- even in 0g.

Picture this: mass is used as a measure of inertia, as you say. In math, that's:

$$a = \frac{F}{m}$$

So imagine that if you and something of $m = 1kg$ were on Earth. The object is lying on a flat, frictionless surface. You exert a force of $F = 1 N$ on that mass. Therefore, acceleration you produce is $a = 1 ^m/s^2$.

The important thing to notice is, this is also true in zero gravity. That is, if you wanted to accelerate a $1 kg$ object by $1 ^m/s^2$, whether you are on Earth or in space, you have to apply a $1 N$ force. It's the same everywhere. It resists motion the same way, whether it's on Earth or in space.

It will be much easier to lift and move the mass left/right, up/down in the outer space (zero g, to say). However that should not give an impression that mass does not have inertia in zero g. Inertia is there with the mass, not with the gravity.

The reason it is easier to move stuff in zero g is because you do not have to overcome the continuous pull of gravity. However you would realize the presence of inertia when you try to stop a moving body, or try to accelerate it, or try to spin it around you by attaching it to a string.