Simple question that needs an answer (1 Viewer)

[Viper]

After reading everyones posts on this, i realised that only 1 person (well done *underthesun*) spoke about Forces being measured in Newtons, and Mass being measured in Kg.

This is of major importance for this question. I just did the question myself, i i got the answer of 240kg. Although i don't particularly like the wording of the question because it gives weight in kg (when in fact it should be measured in Newtons).

Out of interest, ill ask my teacher what he thinks the answer is, and ill post here for you guys. Despite all the trouble we have all gone to in order to try and find the correct answer, i dont really think that we will get a question like this in the HSC as all questions are very carefully planned and questions like these are weeded out.

Cheers

 

[Viper]

Here's the Proof

Ok, i have changed my mind now. I agree that it is 40kg. I have typed up my proof in Microsoft Word Equation Editor, and attached it to this post. I encourage everyone to look at it and tell me whether you agree.

P.S. Id like to take this oppotunity again to express my concern about the wording of this question. It should not refer to a weight force in kg.

 

[underthesun]

At your proof, you used W=mg for moon. Of course, this would be true on a "moon kilogram", but us humans use "earth kilogram", and hence the g value would be the earth surface value.

Hence, Fmoon=mg/6

and F[size=1=]earth[/size]=mg

It should be like above instead of using Fmoon=mg

Now, Fmoon is displayed by the sanyo weight scale. Note that the weight scale is designed for earth.

Had you used the sanyo weight scale on earth, it would convert the Newton weight force it experiences to a kilogram reading, or else it won't be used in households.

On earth, if you have actual mass 240kg, if you weigh yourself the sanyo scale would experience a force of 240 x 9.8 N. This is from W=mg using g ( gravitational acceleration) value of the earth surface.

From a weight force of 240 x 9.8 N, the sanyo weight scale displays 240 kg. It is clear that the sanyo weight scale automatically divides the value of N by 9.8 to obtain the real kilogram amount.

Now, if you go to the moon and weigh yourself, the scale would experience a force of magnitude Fmoon :

Fmoon = m(g/6) <-- because moon has less gravitational acceleration, 6 times less on the surface than the earth's surface
Fmoon = 240 x 9.8 / 6 N

But then the sanyo scale automatically divides the amount of N it experiences by 9.8 to obtain the kilogram amount. It doesn't divide it by (9.8 / 6) because it is designed for earth, calibrated for earth.

Display = Fmoon/9.8 = 240/6 = 40 kg

There you go . I think i might challenge the teachers..

edit : edited for clarity

 

[underthesun]

Originally posted by Rahul
show him this working and ask what is wrong?

Also, tell the teacher to read the question carefully: The scale gave a reading of 50 kg. therefore the weight is 50 kg, not the mass.

the confusing thing is the fact that mass has units, kg. and we use that same unit for weight, practically(not scientifically).

Screw him. Im positive i know more physics than my teachers. You ask any question from the syllabus i could answer, he has to look from the textbook. You'd know this from the way he marks papers..

 

[Viper]

Could everyone please stop referring to weight in kg??? We all know that its Newtons, and its bad practice to continually refer to it incorrectly.

 

[Rahul]

i think the fact that the measurements from scales on different places than they were calibrated for causes the confusion.

like viper mentioned, i dont think hsc questions will so ridiculous. simialr hsc questions are much easier to interpret.

2002(q3, multiple choice): a person has a weight of 550 N on the surface of earth.
what would be the persons weight on the surface of mercury.
given: g_earth=9.8 and g_mercury=3.8

it will be okay for the hsc.

 

[L!MP]

ur teacher is right lol
40kg.
w=mg
m=40

mass is constant (ignore einstein for a sec), and even though the question is worded crappily, given the question, the answer must be 40kg.

 

[wogboy]

The correct answer is 240 Kg, not 40 Kg. Here's the thing in Viper's proof that I consider to be the fundamental flaw.

We are told that the force on the moon is 40kg. So:

40*6g = F(earth)*g

The force on the moon is not 40 Kg, and in fact Kg are the wrong units to start off with to describe a force. Force is measured in Newtons. So you cannot simply sub in F(moon) = 40 Kg. In fact you don't know what F(moon) is (you can work it out though, not that you really need to).

With regards to the argument about the "constancy" of mass, of course this is always true (one kilogram on the moon is the same as one kilogram on earth), however it is not applicable to this question. Why not? Because the scale reading of "40 Kg" on the moon is bullshit . It is untrue. It clearly says in the question that the scale was designed for Earth, not the moon. Take the "40 Kg" reading with a grain of salt.

How does a scale work? It has a spring in it which extends as a force is applied to it (in accordance with Hooke's Law). The amount the spring is stretched is measured (electronically nowadays, mechanically back in the old days), and this is directly proportional to the weight force acting on it. Now how does it convert weight force -> mass, to read it out on the digital display? m=F/g. The scales aren't smart enough to measure g at any location and then calculate the mass. g is preprogrammed into the scales as a constant. Even on the moon, you're scales are still foolish enough to think g = 9.8 m/s^2, not 1.6 m/s^2, hence giving a wrong reading.

On the moon, this spring will not stretch as much as it would on the Earth (given the same mass on it) because g on the moon is six times less. Because the spring streches six times less than it should, you need to multiply the "false" 40 Kg reading by six, to get the true mass of the body which is 240 Kg.

 

[rx72c]

Although your mass never changes. what the scale does is measures the weight and divides by 9.8 to give you a mass. Thats when its designed for earth.
But when they weigh themselves on the moon, the scale is not made for it. so what happens it measures the weight and divides by 9.8, now of course the weight is not as great as the gravity on the moon is 1.6m/s. So you get a much smaller mass.
So the answer will be 240 kg.
Im 100% about this according to the wording of the question specified.
The wording of the question may have been different, but this question was slightly different to the question in CSSA.
in the CSSA question the mass was read on earth.so then it is the same everywere else.

 

[underthesun]

Originally posted by L!MP
ur teacher is right lol
40kg.
w=mg
m=40

mass is constant (ignore einstein for a sec), and even though the question is worded crappily, given the question, the answer must be 40kg.

but it never said mass is 40

p.s are you L!MP the starcraft boy?