Question

Weight means how strong gravity is pulling on an object right? So how does that work with people or objects? For a 80kg person, is his mass 80kg? or is the force of gravity 9.81m/s * 80kg? I'm confused :)

Answer 1

It does not necessarily mean that. An object in freefall has no weight, though it feels the full strength of gravity. It is more generally defined by the normal force exerted by a surface it lays on or by the tension of something it hangs off of. Depending on the motion of the object, the weight changes, though the mass does not. If a person is standing still on the ground, in your case the weight would be your latter suggestion.

Answer 2

So for a 80 kg person standing still on the ground, his weight would be the force of gravity against his mass?

Answer 3

It's rather the magnitude of the normal force of the ground, which happens to be of the same magnitude as gravity in this case since he is not accelerating. If he were in an elevator that was accelerating upward, for instance, the normal force would be greater than the force downwards due to gravity, and thus his weight would be greater. It is incorrect to think of weight as the magnitude of the gravitational force.

Answer 4

That's a really interesting answer, Yakey. I understand. Thank you very much! :)

Answer 5

Good to hear!

Answer 6

@JoshDavoll
The answer to your question depends on the depth of the concept you need.

Mass is just related to the amount of matter of a body or system, and is responsible of its inertia. It is an intrinsic property of the body.

Force of gravity is not an intrinsic property of a body. It depends on the gravitational field (direction and magnitude) at the point where the body lies.
That force is mass times gravitational field.

At first approach, weight seems just an easy name for gravitational pull near Earth's surface. There, gravitational field is about 9.83 N/kg directed to Earth's centre.

Now, equating gravitational pull and weight makes it impossible to understand certain aspects of weight.

An (almost) correct definition of weight was given by @yakeyglee:

"Weight is the opposite of the force needed to keep you at rest in the frame of reference you have chosen"
This can be the opposite of the tension of a string or spring if you hang a body to it, or the opposite of the normal force a scales exerts on you if you are standing still"

In an inertial frame of reference, there won't be any difference between weight and gravitational pull, but in non-inertial frames, such as a rotating Earth, an accelerating elevator or a space station in orbit, the difference can be such that gravitational pull is quite intense, whereas weight is zero or even directed upwards.

Answer 7

@Vincent-Lyon.Fr Thank you so much for your reply. I have a much better understanding of mass, weight and gravity now thanks to you and Yakee. It's quite fascinating! :)

Answer 8

Welcome :)