Weight and Mass Quiz

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This was originally posted by a member on another forum I frequent. I thought it was a pretty good quiz. I'll post the answers at a later time, after people had a chance to mull over it a bit.

Original Post:

Here are a few questions to challenge those on their basic high school level scientific knowledge. You engineering types please try to let a few responses from the masses get in before you put your input in. Scenario: You're concerned about your health, and you're preoccupied with your weight, so you monitor it every chance you get.

1A. You step on a set of scales marked in U.S. customary units. What is your weight?

1B. What is your mass?

2A. You step on a set of scales marked in SI (metric) units. What is your weight?

2B. What is your mass?

3. You have been selected to crew on the moon station for a month. Your doctor lends you his balance scales, and you have a set of spring loaded bathroom scales as well. Mission control says you can take one scale but not both. Which one do you take, and why?

4. On the flight to Houston, you step on the bathroom scales in the lavatory of your 747 while it is on the ground. Your weight is 170 lbs. After you take off, the Captain informs you that the plane has attained a constant climb rate of 1932 feet per minute (that's 32.2 feet per second, or 9.8 meters per second, for you metric people. We're keeping the numbers simple here). You step on the scales again - what do they read?

5A. You get to the moon. You pull out your doctor's balance scale and weigh yourself. What is your weight?

5A. What is your mass?

(Hint - the moon's gravitational pull is approximately 1/6th of earth's.)
 
That's pretty basic grade school physics and I ain't telling you my mass until I lose some weight.
Joe
 
Areeda said:
That's pretty basic grade school physics and I ain't telling you my mass until I lose some weight.
Joe
Click to expand...

Climb a high mountain.... or go deep down into the earth...

or just ride the vomit comet.
 
1A. You step on a set of scales marked in U.S. customary units. What is your weight?
Something in slugs.

1B. What is your mass?
Something in pounds.

2A. You step on a set of scales marked in SI (metric) units. What is your weight?
Something in newtons

2B. What is your mass?
Something in grams

3. You have been selected to crew on the moon station for a month. Your doctor lends you his balance scales, and you have a set of spring loaded bathroom scales as well. Mission control says you can take one scale but not both. Which one do you take, and why?
Balance - duh.

4. On the flight to Houston, you step on the bathroom scales in the lavatory of your 747 while it is on the ground. Your weight is 170 lbs. After you take off, the Captain informs you that the plane has attained a constant climb rate of 1932 feet per minute (that's 32.2 feet per second, or 9.8 meters per second, for you metric people. We're keeping the numbers simple here). You step on the scales again - what do they read?
Trick question: 170lbs

5A. You get to the moon. You pull out your doctor's balance scale and weigh yourself. What is your weight?
Not knowing exactly how a doctors balance scale works, I can't say for sure, but with it being balance vs spring, I will say that if it is a true balance scale I will "weigh" whatever I do on earth.


5A. What is your mass?
Same as on earth.

(Hint - the moon's gravitational pull is approximately 1/6th of earth's.)
 
mantakos said:
Hmmm...
-harry
Click to expand...

Ah shoot, I probably flip flopped em...of course I haven't actually seen a scale with slugs on it.
Edit: Further research shows the pound being used for both mass and weight.
 
Last edited:
EdFred said:
Ah shoot, I probably flip flopped em...of course I haven't actually seen a scale with slugs on it.
Edit: Further research shows the pound being used for both mass and weight.
Click to expand...

Depends on the system. There are many. Common ones include:

Pounds force (lbf) (force) / pounds mass (lbm)(mass).

Pounds (force) / Slugs (mass)

Poundel(force) / Pound (mass)

And there is the official U.S. government definition of a "pound" in terms of the kilogram - which implies that the pound (as used in commerce) is a mass.

The question is: "Is 'weight' a force or a mass".

The answer is: "It depends." The concept of "weight" predates Newton's laws and the distinction between mass and force and tends to be used interchangably depending on the context. "You weigh 1/6 as much on the moon". You buy potatoes by the kg / pound. You calibrate your weight measuring device by using a reference mass.

So, when you take your scale to the moon and, since you moved it and it's been subjected to vibration and accelerations, you recalibrate it by placing a reference mass on it (as you should). Then you get the same results on the moon that you got on earth no matter what kind of device (balance or spring/force) you use. (and to answer the OP question - it doesn't matter which you take.)

Some people think that the so called metric system is better, but it is nearly as much as a mess with dynes, newtons, kg-force, and a few others that I can't think of off the top of my head in reasonably common use.

Then there are the units for pressure...
 
Speaking as an engineer, all the questions are extremely poorly worded since they expect the reader to have intimate knowledge of the topic yet lack sufficient detail to give precise responses.:D Re-write and resubmit.
 
EdFred said:
...of course I haven't actually seen a scale with slugs on it...
Click to expand...
Me neither, though if I leave one out in the garden, it will have slugs on it the next morning. And then I'll be able to see what they weigh.
-harry
 
EdFred said:
snip...

4. On the flight to Houston, you step on the bathroom scales in the lavatory of your 747 while it is on the ground. Your weight is 170 lbs. After you take off, the Captain informs you that the plane has attained a constant climb rate of 1932 feet per minute (that's 32.2 feet per second, or 9.8 meters per second, for you metric people. We're keeping the numbers simple here). You step on the scales again - what do they read?
Trick question: 170lbs

...snip
Click to expand...

Yes, trick question. He doesn't say how many minutes after take off (i.e. how high at the point of measurement), nor does he provide the, initial airport elevation/latitude, heading, latitude, or ground speed. Weight will decrease as one gets higher, and will vary depending on speed/direction/latitude. For example, if initial weight was at sea level at the equator, and if he was climbing through 30,000 ft, heading due East, at the Equator, with a ground speed of 500kts, then he would WEIGH about 169 lbs.
 
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skidoo said:
Yes, trick question. He doesn't say how many minutes after take off (i.e. how high at the point of measurement), nor does he provide the, initial airport elevation/latitude, heading, latitude, or ground speed. Weight will decrease as one gets higher, and will vary depending on speed/direction/latitude. For example, if initial weight was at sea level at the equator, and if he was climbing through 30,000 ft, heading due East, at the Equator, with a ground speed of 500kts, then he would WEIGH about 169 lbs.
Click to expand...

The deck angle also needs to be considered...side loading a scale would complicate things just a bit.
 
Of course the real question is why would anyone willing go to Houston?

(I did time there....7 years of it...fortunately, only spent 2 summers there)
 
Looks like everyone caught the trick question with no problem!

I agree the questions probably could have been worded better, but I didn't want to edit the original post since I planned on pasting in the original answers.

Looks like there is still some confusion though on the role of gravity and spring vs. balance scales though.
 
skidoo said:
Yes, trick question. He doesn't say how many minutes after take off (i.e. how high at the point of measurement), nor does he provide the, initial airport elevation/latitude, heading, latitude, or ground speed. Weight will decrease as one gets higher, and will vary depending on speed/direction/latitude. For example, if initial weight was at sea level at the equator, and if he was climbing through 30,000 ft, heading due East, at the Equator, with a ground speed of 500kts, then he would WEIGH about 169 lbs.
Click to expand...

Clark1961 said:
The deck angle also needs to be considered...side loading a scale would complicate things just a bit.
Click to expand...


Both good points although the intention wasn't to take altitude and other minor corrections in to factor.
 
Okay, I guess I'll just post the answers, for those interested. This was the original poster's response but I concur with the answers.

>1A. You step on a set of scales marked in U.S. customary units. What is your weight?

I weigh 190 lbs.

>1B. What is your mass?

My mass is 5.9 slugs, expressed in primary units. My mass could also correctly be reported as 190 lbm, but for reasons that will be discussed later, 190 lbm is not exactly equal to the weight unit of 190 lbf.

>2A. You step on a set of scales marked in SI (metric) units. What is your weight?

8.8 Newtons. But I had to calculate that. Metric scales are expressed in Kilograms, which is a mass.

>2B. What is your mass?

86 Kilograms.

>3. You have been selected to crew on the moon station for a month. Your doctor lends you his balance scales, and you have a set of spring loaded bathroom scales as well. Mission control says you can take one scale but not both. Which one do you take, and why?

Good question. A doctor's balance scale (the thing with the sliding weights) measures MASS ONLY, and not weight. So on the moon, any other planet, or even in a vertical acceleration, the scale will read the same. Now, on the other hand, a spring loaded bathroom scale is opposite, and it measures WEIGHT ONLY, and not mass. That scale, when used on the moon, will correctly report your weight as 1/6th of what it would be on the earth.

As a side note, U.S. customary balance scales that read pounds are actually calibrated to read pound-mass (lbm), which by definition is 1 slug/32.2. Since the actual gravitational acceleration is 31.some odd and variable number, your reported mass in lbm will be slightly different than your weight in lbf, although in reality the difference will be less than the accuracy of the scales so we usually don't worry about it.

>4. On the flight to Houston, you step on the bathroom scales in the lavatory of your 747 while it is on the ground. Your weight is 170 lbs. After you take off, the Captain informs you that the plane has attained a constant climb rate of 1932 feet per minute (that's 32.2 feet per second, or 9.8 meters per second, for you metric people. We're keeping the numbers simple here). You step on the scales again - what do they read?

Okay, okay. That question was intended to confuse the "C" students. A "C" student would have assumed that 32.2 and/or 9.8 was 1 G and that the weight would be 340 lbs (170 x 2 total gravities.) The "B" student would have assumed that I screwed up my units, and meant to report "feet/sec/sec" (an acceleration) and not "feet/sec" (a speed) and would have said 340 lbs. The "A" student would have recognized by the phrase "constant climb rate" that there is in fact no acceleration, and no corresponding change in weight. That's the correct answer.

>5A. You get to the moon. You pull out your doctor's balance scale and weigh yourself. What is your weight?

As discussed before, your doctor's balance scale will report the same "weight" that it would on the earth, because a balance scale can only measure a mass. You would have to calculate your actual weight if you wanted it.

>5A. What is your mass?

It's whatever the balance scale reported it as. Again, if you used a spring bathroom scale, your weight would be reported as 1/6th of the value it would read on earth and you would have to calculate your mass.
 
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