Mass and weight (yr 11)

[krappleby]

HI all, 

OK, I am here to question some science that my son brought home the other night, He wasn't sure how to do it so i sat down with him and went through it, and he got all his answers Wrong.. but as far as i'm concerned they are not wrong, and from what i've read on the internet Science is wrong.. 

Let me explain

The homework, was a list of planets, with their gravitational pulls, he had to mark down his weight (4 stone 6) and then in the first two columns put his mass, and weight on each planet.

Now. he got it wrong because apparently according to Science, his mass is the same on all planets (this is also what his teacher said), But i;m afraid to say it is Not.

Let me explain.

To work out the mass of an object, you take its density and multiply it by its volume, this gives you the mass.

now, to work out your density you take your weight and divide by your volume

now since your weight is effected by gravitational pull, your mass on earth and mars is completely different. since your weight is different.

so my question is, How can your mass be identical no matter what planet your on, when your mass is effected by your weight, and your weight is effected by the gravitational pull of the planet?

 

please if i'm missing something here, then feel free to enlighten me, but as far as i'm concerned, the teacher and science is wrong.

 

 

 

[Sensei]

Weight is force.

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

[math]W=m*a[/math]

and on Earth it's

[math]W=m*g[/math]

 

Different planets have different a (acceleration), thus different weight.

(actually, to complicate it even more, a is not constant but depends on distance to center of mass, which is close to center of planet)

Edited March 16, 2018 by Sensei

[beecee]

26 minutes ago, krappleby said:

HI all, 

OK, I am here to question some science that my son brought home the other night, He wasn't sure how to do it so i sat down with him and went through it, and he got all his answers Wrong.. but as far as i'm concerned they are not wrong, and from what i've read on the internet Science is wrong.. 

Let me explain

The homework, was a list of planets, with their gravitational pulls, he had to mark down his weight (4 stone 6) and then in the first two columns put his mass, and weight on each planet.

Now. he got it wrong because apparently according to Science, his mass is the same on all planets (this is also what his teacher said), But i;m afraid to say it is Not.

Let me explain.

To work out the mass of an object, you take its density and multiply it by its volume, this gives you the mass.

now, to work out your density you take your weight and divide by your volume

now since your weight is effected by gravitational pull, your mass on earth and mars is completely different. since your weight is different.

so my question is, How can your mass be identical no matter what planet your on, when your mass is effected by your weight, and your weight is effected by the gravitational pull of the planet?

 

please if i'm missing something here, then feel free to enlighten me, but as far as i'm concerned, the teacher and science is wrong.

it is you who are wrong...really!

http://hyperphysics.phy-astr.gsu.edu/hbase/mass.html

[Endy0816]

55 minutes ago, krappleby said:

to work out your density you take your weight and divide by your volume

 

This isn't accurate.

If

Weight = mass * g

then Density can't both equal:

mass * g / volume (inaccurate)

and

mass / volume (accurate)

[koti]

1 hour ago, krappleby said:

HI all, 

OK, I am here to question some science that my son brought home the other night, He wasn't sure how to do it so i sat down with him and went through it, and he got all his answers Wrong.. but as far as i'm concerned they are not wrong, and from what i've read on the internet Science is wrong.. 

Let me explain

The homework, was a list of planets, with their gravitational pulls, he had to mark down his weight (4 stone 6) and then in the first two columns put his mass, and weight on each planet.

Now. he got it wrong because apparently according to Science, his mass is the same on all planets (this is also what his teacher said), But i;m afraid to say it is Not.

Let me explain.

To work out the mass of an object, you take its density and multiply it by its volume, this gives you the mass.

now, to work out your density you take your weight and divide by your volume

now since your weight is effected by gravitational pull, your mass on earth and mars is completely different. since your weight is different.

so my question is, How can your mass be identical no matter what planet your on, when your mass is effected by your weight, and your weight is effected by the gravitational pull of the planet?

 

please if i'm missing something here, then feel free to enlighten me, but as far as i'm concerned, the teacher and science is wrong.

 

 

 

Mass is the same on all the planets, its the weight that changes due to gravity. Mass and weight are two very different things.

1 hour ago, Sensei said:

Weight is force.

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

W=m∗a

and on Earth it's

W=m∗g

 

Different planets have different a (acceleration), thus different weight.

(actually, to complicate it even more, a is not constant but depends on distance to center of mass, which is close to center of planet)

Sensei for fs keep it simple for the guy  

I’d sure wouldnt want you to explain a complex issue to me that I don’t understand  

[Sensei]

3 hours ago, krappleby said:

To work out the mass of an object, you take its density and multiply it by its volume, this gives you the mass.

@koti

[math]F(r) = m * a(r)[/math]

[math]a(r) = \frac{GM}{r^2}[/math]

g=~ 9.81 m/s^2 when r = ~ 6370 km = ~ 6370000 m

so mass of planet can be calculated by reversing equation:

[math]9.81 = \frac{6.67*10^{-11} * M_e }{6370000^2}[/math]

[math]M_e=\frac{9.81 * 6370000^2}{6.67*10^{-11}}[/math]

[math]M_e= 5.97*10^{24}[/math] [kg]

Me = mass of the Earth.

 

To calculate mass of other planet, or other cosmic object, spaceship has to have accelerometer and measure distance to surface of object using photons (t=2d/c).

It'll also reveal what is below ground of planet, if accelerometer will inform about anomalies in acceleration while flying above certain terrain. Such as Iron ore (from meteorite)

If you're more interested in gravity anomalies read this article:

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

 

 

Edited March 17, 2018 by Sensei

[swansont]

12 hours ago, krappleby said:

 To work out the mass of an object, you take its density and multiply it by its volume, this gives you the mass.

now, to work out your density you take your weight and divide by your volume

You are using two different equations here, and they can't both be right. If m = pV (p is density), then W/V cannot be the density. The former is correct, the latter is not.

[pzkpfw]

21 hours ago, krappleby said:

...

so my question is, How can your mass be identical no matter what planet your on, when your mass is effected by your weight, and your weight is effected by the gravitational pull of the planet?

...

Just to have a go at putting this into words:

You can think of mass as a measure of the "stuff" in something.

Say you make a ball of lead that you weigh as 6 pounds on your bathroom scales. There's a certain number of Pb atoms in that ball.

If you take that ball - and your bathroom scales - to the Moon: with its 1/6 surface gravity compared to Earth you'll now measure it to be 1 pound. But all the Pb atoms are still in the ball. None of them vanished.

The mass of the ball is what stays the same, it's the weight that varies, based on the strength of gravity where the measurement takes place.

To take it to the extreme, consider dropping the ball and scales off in space. The scales might measure zero ... the ball is "weightless" - but again, all the lead atoms are still in the ball. It doesn't have zero mass. e.g. It would still take force to push the ball, to accelerate it to some speed.

[Strange]

12 hours ago, pzkpfw said:

It would still take force to push the ball, to accelerate it to some speed.

This might be a key concept: inertia is mass. So on the International Space Station, everything appears weightless but it takes more effort to push a fellow astronaut across the room than it does an apple.

[StringJunky]

5 hours ago, Strange said:

This might be a key concept: inertia is mass. So on the International Space Station, everything appears weightless but it takes more effort to push a fellow astronaut across the room than it does an apple.

I thought that's what mass is: the force required to move something. That's why balances can measure mass but spring scales can't.

[Area54]

18 hours ago, pzkpfw said:

Just to have a go at putting this into words:

You can think of mass as a measure of the "stuff" in something.

Say you make a ball of lead that you weigh as 6 pounds on your bathroom scales. There's a certain number of Pb atoms in that ball.

If you take that ball - and your bathroom scales - to the Moon: with its 1/6 surface gravity compared to Earth you'll now measure it to be 1 pound. But all the Pb atoms are still in the ball. None of them vanished.

The mass of the ball is what stays the same, it's the weight that varies, based on the strength of gravity where the measurement takes place.

To take it to the extreme, consider dropping the ball and scales off in space. The scales might measure zero ... the ball is "weightless" - but again, all the lead atoms are still in the ball. It doesn't have zero mass. e.g. It would still take force to push the ball, to accelerate it to some speed.

I suspect that this is the only reply that will have a chance of being understood by the Opening Poster. Thank you for getting it right. +1