What is the mass of one liter of H2O on the moon?

Question

If a liter of water has a mass of 1 kilogram on Earth, what would its mass be on the moon......our moon.

Ahhh, I see that my question did not fool anyone! So how do I get my 7th grade students to remember that mass always stays the same?

Answer 1

Try this: Get a spring loaded scale of some kind, and hang it up in your class room--and tie a cord to it (I'm going somewher with this, bear with me).

Then explain to them that gravity is a force--and it pulls them down toward the earth (pretty obvious). Now take a weight (say a 10 # weight) and demonstrate the scale shows that much weight. Now--to makeit clear what you mean by a force, take the weight out and have the student(s) pulll on the cord until it shows a "weight " of ten pounds.

At this point I'd explain it like this: On earth, gravity will pull down on theobject so the scale says 10 pounds. But the moon is a lot smaller than the earth, so its gravitational pull is less--just like if you didn't pull on the cord as hard. So on the moon, the scale will read only 1/sixth as much.
BUT--its the same object (in your example, a liter of water)--theees just as much water in the cup (or whatever)--even though the scale says something different on earth and on the moon. Mass is the actual amount of water. Weight is what we say when we are describing how much gravitational force a body like the earth or moon exerts on the water--and that amount of force is different even thought the amount (mass) of water stays the same.

PS At this point it should be easy to explain that, while people use the terms weight, pounds, kilograms, etc. casually in everyday speech, scientists (whenn talking about these hings) havepricise definitions: a kilogram is a measure of mass, while the actual scientific measure of weight (force) is the Newton. Or use English measures if you think it best. . . .Good Luck :)

Answer 2

It would still be 1 kilogram, because mass is a property of the matter itself and not a property of gravity. But that kilogram would weight 9.78 Newtons on Earth and only 1.62 Newtons on the Moon. This is because a kilogram is a unit of mass, not a unit of weight. This fact is totally lost on people without training in physics. Most people consider a kilogram to be the weight of a kilogram on the Earth. (By the way, a pound IS a unit of weight, so technically a kilogram does not equal 2.2 pounds. Technically, 9.78 Newtons equals 2.2 pounds. The unit of mass in the Imperial system is a slug. But nobody without physics training even knows that.)

Answer 3

Mass is mass is mass is mass, no matter where you are in the universe. Weight is dependant on the force of gravity. In Newtonian physics, weight = mass * gravitational constant. Therefore, something with a mass of 1 kg on Earth will also have a mass of 1 kg on the moon, even though it's WEIGHT is approximately 0.15 kg on the moon (about 1/6th).

Answer 4

It would be 1 kilogram. Don't confuse mass with weight. Its weight would be 1/6th of its weight on Earth.

Answer 5

Just remind them of carnival rides....usually they are based on motion and centrifugal force. The kids know from experience that they FEEL heavier, but they also (should) know that the carnival ride doesn't change the amount of material of which they are made (hence their mass doesn't change).

Answer 6

Mass never changes, only apparent weight.

Answer 7

A 32 pound slug makes a big impression on 7th graders.