The unit question has been well answered, but I thought the following was interesting in regards to an actual understanding of the difference between mass and weight.
Perhaps a good thought experiment is this:
You are on the ISS. Being in orbit, you (and everything around you) is experiencing no gravity*. There is a large object, say an anvil, floating on one side of the room. You would like to move it to the other side of the room. Will this object be easy to move?
Or another way to think about it. The anvil is floating toward you. Will it be easy to stop?
Quote:
Originally Posted by arizonaguide
Oh yeah, for what it's worth I'm talking about small arms at sea level on the surface of the earth.
I'm not too worried about the difference in LB (weight/force) if it's on the moon, and all my projectiles are small arms so I don't have to adjust my trajectory for the gravitational pull of the moon either.  We can assume sea level small arms calculations. |
So in regards to the above, the projectile will have the same energy on the moon as it does on the earth. That is the point being made in distinguishing mass from weight. If we use metric units (because I think they are generally more clear and easy to work with), one unit of energy (Joule) is kg*m/s^2, as mentioned earlier. The unit of mass, kg, does not change (mass is conserved). The acceleration obtained from the chemical propellent (m/s^2) is also the same.
A way to think about the difference between the two is this. Think of mass as some quantity that every object has. Think of gravity (or any other accelerations) as an arrow (vector) from the object in the proper direction. Weight is a combination of the quantity mass, and the length or size of the arrow.
Taking this further, if the size of the arrow is zero (like in our thought experiment), the object still has mass. So in trying to push the object you are in effect making the arrow point towards yourself. So the object has weight relative to you as long as you push on it.
This representation can be very powerful for looking at mechanics problems with clarity. Drawing it out is called a Free Body Diagram, which I'm sure is explained better on the internet with pictures than it is by me.
Cheers.
*Not strictly true, since objects in orbit are actually in a state of freefall inside the earth's gravitational field.