What is the Difference Between Gravitational Force and Weight?

[sassora]

Homework Statement

Centre of gravity - the point at which:
1) gravity acts on a body or 2) weight of a body may be considered to act.

The answer is 2) and I understand why - because gravity acts all over but it is easier to calculate a single point, an average point of where the mass is located.

My question is, gravitational force and weight seem to be the same thing, so why does the question use different terminology? Is there actually a difference they are trying to express? Not that I think it's relevant to answering the question but does make me wonder why they are doing that.

 

[PeroK]

Homework Statement

Centre of gravity - the point at which:
1) gravity acts on a body or 2) weight of a body may be considered to act.

The answer is 2) and I understand why - because gravity acts all over but it is easier to calculate a single point, an average point of where the mass is located.

My question is, gravitational force and weight seem to be the same thing, so why does the question use different terminology? Is there actually a difference they are trying to express? Not that I think it's relevant to answering the question but does make me wonder why they are doing that.

I would say it's a poor question. Option 2) should be something like 2) all the gravitational force on the body may be considered to act.

As you suggest, changing the wording from "gravity" to "weight" suggests there's more to it.

 

[sassora]

Thanks it's good to have that confirmation

 

[LemmeThink]

Hi!

I would say it's a poor question. Option 2) should be something like 2) all the gravitational force on the body may be considered to act.

Are you sure? I think the difference is that part (2), weight, refers to the normal reaction on the body as opposed to simply the force of gravity acting on the body.

 

[jbriggs444]

Hi!

Are you sure? I think the difference is that part (2), weight, refers to the normal reaction on the body as opposed to simply the force of gravity acting on the body.

If I throw a baseball into the air, is the "normal reaction force" of my hand on the baseball during the throw equal to the baseball's weight?

 

[LemmeThink]

jbriggs444, I mentioned the normal reaction earlier. What if I measure weight using a spring balance?

 

[jbriggs444]

jbriggs444, I mentioned the normal reaction earlier. What if I measure weight using a spring balance?

If you are throwing the spring balance when you weigh something, the reading will not reflect the object's weight.

 

[LemmeThink]

Right. So, to surmise, weight is independent of the value measured using an instrument. Thanks;)

 

[David Lewis]

It depends on how you define weight. It can be either gravitational force, or the reading of a scale. They don't give the same value in all situations, however, so you have to clarify which one you're using.

 

[haruspex]

It depends on how you define weight. It can be either gravitational force, or the reading of a scale. They don't give the same value in all situations, however, so you have to clarify which one you're using.

I agree with PeroK and jbriggs here. The weight is the force exerted by gravity. The apparent weight indicated by a scale may be something else.

 

[David Lewis]

Correct. For example, an astronaut in orbit is weightless only under the apparent weight definition.