Question about mass vs. weight..........

[paigevz]

.....in our conference today, the presenter was talking about teaching children how to measure mass. She used a balance scale as an example of comparing masses. She then said you could take the scale to the moon, and even though each thing would weigh differently, the mass comparison would be the same. No problem, right?

THEN she said that that's because when you use a balance scale you are removing gravity from the equation entirely. She said you can take that balance scale anywhere and all you will be doing is measuring true mass. You can even take it OUT IN SPACE she said. This doesn't sound right to me..........if there is zero gravity, how would the balance scale be able to balance?

 

[Cool-Beans]

If you weigh a guy here, he weighs 100 pounds. Put him in a shuttle and zip him up to the moon, he weighs maybe 20 (I don't remember exactly). However, his mass hasn't changed. Still the same guy, body is still the same...unless he had a Ho-Ho or something on the way up.

 

[CEDmom]

At the Smithsonian Air & Space Museum in DC they have a scale which shows what you weigh on earth and the moon. You're weight is much less on the moon because of the change in gravity but you have the same body mass.

 

[kirbsam]

in zero gravity, mass would be the same, but weight would be zero. hence, the balance scale would be accurate.

 

[paigevz]

Yeah, I know about the weight differences due to changes in gravity...........I just didn't think it would work in zero gravity.

Would the weight of both sides be zero? If one is greater mass than the other, then you can't compare them on a balance scale if both will be zero?

But in what way is that actually measuring true mass?

I get it that the toy truck is greater mass than the pencil here, there and everywhere...........but isn't that only comparing its mass to the pencil?

Measuring mass is formulaic, right?

 

[Free4Life11]

How do you measure mass? Water displacement?

 

[paigevz]

That's what I always heard.

I am just wanting to make sure I really understand this.........can't teach it if I don't understand it!

 

[Bbgrizzle]

Paige what you are saying makes sense to me. I don't get her theory either. Gravity is what pulls the balance scale downward on the heavier side, right?

 

[paigevz]

Exactly.........that's why it didn't sound right to me. How can a balance scale even work without gravity?

 

[salmoneous]

A normal scale works by measuring the "tug" of gravity on an object. You basically have an equation with three variables - the mass of the object, the power of the tug, and the strength of the gravitational field. On the earth, the gravitational field has a strength of 9.81 meters per second squared (m/s/s). On the moon, gravitational field has a strength of 1.62 m/s/s.

When your presenter talked about "removing gravity" from the equation, I believe she was talking about removing the factor representing the strength of the gravitational field (the 9.81 or 1.62 m/s/s). This factor is usually represented by "g" as is often simply called gravity.

I do not believe she was saying to remove the universal force called gravity from the equation. You are absolutely right that a balance scale needs some gravity to work. It doesn't matter how much you have, but if there were some place with *no* gravity whatsoever then it wouldn't work. But we don't have to worry about that as gravity is everywhere.

Hope this helps,
Sal

 

[cardaway]

A real smarty pants would ask how they are going to get things to stay on the scale without changing the parameters.

 

[eeyore45]

All I know is I took DD to the Chicago Planetarium and they have a great demo hands on for this, she loved it.... went back to school and when they did the study of mass and weight - she came home so upset - the teacher told her to "keep quiet - you know too much"

She tried to explain it to the teacher like she learned from the hands on demo - teacher just kept throwing gravity into the equation... dd had to sit in the office the rest of the week during science class!!

So I appreciate you wanting to get it right before taking it before the class!!!

 

[golfgal]

In zero gravity the balance scale would be moot but for "demonstration" purposes the instructor said it would be the same because mass and weight are confusing. In zero gravity, both sides of a balance scale would be at the same height because there is no gravity pulling one side down so the "illusion" of a balanced scale would happen in zero gravity.

Cardaway has the only REAL point, the items wouldn't stay on the scale anyway so it really doesn't matter. It is just to give students a visual.

 

[paigevz]

Thanks guys. So I DID understand it. I was afraid I was dead wrong on the mass/weight issue.

I had thought about that with the stuff staying on the scale, but I realized she was talking hypothetically.........I just didn't leap into the hypothetical speak extending to the scale itself.

I thought too much and confused myself!

 

[cardaway]

IMo bringing something like the "space" aspect could just confuse people because it wouldn't work and doesn't make sense because they know everything is weightless.

Howeever the planet/moon thing is a good exmaple IMO. If scale was balanced, the same scale (with the same items) will always be balanced no matter which planet you visit. Only the weights will change, and they will change the same for both items.

Personally my favorite is proving that a feather and a bowling ball will hit the ground at the same time if dropped from the same height in a vaccuum.

 

[paigevz]

Yeah, I'm good with the Earth/Moon same mass thing.........but that feather and bowling ball do blow my mind..........................thankfully, my second graders aren't ready for that yet!