# Mass in deep space

I get so confused how mass works in space. I know these may sound like stupid questions but I've never had anyone answer them before. First if you're flying a spacecraft around in space. What role does mass play if you're weightles?
For example why couldn't you hit the gas and indefinitely accelerate to however fast you wanted to go?
I guess another question is in space what forces act on mass if there is no friction wind resistance or gravity? Is there a good link to a website that explores these questions?

## Answers and Replies

Mech_Engineer
Science Advisor
Gold Member
Regardless of whether a spacecraft (or any object) is in a gravity field, it is subject to inertia which is in turn a primary manifestation of mass. It is inertia which prevents infinite acceleration, not gravity. See here:

https://en.wikipedia.org/wiki/Inertia

Wikipedia.org said:
Inertia is the resistance of any physical object to any change in its state of motion (this includes changes to its speed, direction or state of rest). It is the tendency of objects to keep moving in a straight line at constant velocity.
...
Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems. Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica...

PeroK
Science Advisor
Homework Helper
Gold Member
2020 Award
Mass is the amount of something. Mass is the same wherever something is. Weight is the force exerted on an object by a gravitational field. In everyday speech we tend to mix these up: I weigh 90kgs. That really means my mass is about 90kgs and the force exerted on me by the Earth is about 873N. If I were on the moon, my mass would be the same, but my weight would be about 1/6th of that on Earth. And, if I were in space, distant from any gravitational source, the my mass would still be 90kgs, but I would be weightless.

Newton's second law states that ##F = ma## (force = mass x acceleration). This applies everywhere, including in space. The acceleration depends on the mass: the more massive something is, the more force is required to accelerate it at a given rate.

But, out in space, there is no obvious means of causing motion. Hitting the gas in a car or opening the throttle on a jet engine would do nothing, because there is nothing to get a grip. Science fiction in this respect is very fictional: it's very difficult to engineer anything that could accelerate itself through space the way a plane can accelerate itself along the ground and in the air.

You could try googling for "spacecraft propulsion" and see what that turns up.