# Centre of Gravity definition

1. Oct 17, 2016

### sassora

1. The problem statement, all variables and given/known data
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.

2. Oct 17, 2016

### PeroK

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.

3. Oct 17, 2016

### sassora

Thanks it's good to have that confirmation

4. Oct 17, 2016

### LemmeThink

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.

5. Oct 17, 2016

### jbriggs444

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?

6. Oct 17, 2016

### LemmeThink

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

7. Oct 17, 2016

### jbriggs444

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

8. Oct 17, 2016

### LemmeThink

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

9. Oct 17, 2016

### 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.

10. Oct 17, 2016

### haruspex

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.

11. Oct 18, 2016

### David Lewis

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