P199: Weight vs. MassLecturer Stacy McCormackPhysics Department, Indiana University

Aren’t they the same thing?

They both appear to be objects with a fixed “heaviness” that we can measure on a scale.

But mass and weight are two COMPLETELY DIFFERENT QUANTITIES OR CONCEPTS that are quite often confused.

1 lb = 454 grams = 0.454 kilograms = 4.45 N

MassDefinition: The measure of the amount of matter an object contains.

matter=“stuff”

Unit: grams or kilograms (1 kilogram = 1000 grams)

We call these “masses” because we have measured the precise amount of matter in grams that each one contains.

The mass of an object depends only on what is INSIDE the object.

WeightDefinition: The measure of how strongly gravity pulls on an object.

Units: pounds (US), Newtons (metric). 1 lb = 4.45 N

Weight depends on two factors:•Mass: the greater the mass, the greater the weight.•Gravity: the greater the gravity, the greater the weight.

These “weights” are measured in pounds.

So whereas mass is determined by something inside the object, weight is determined by something OUTSIDE the object…gravity!

Gravitational Factor

Weight in Newtons = (Gravitational factor) * (Mass in kilograms)

Location Gravitational Factor (N/kg)Moon 1.6

Mercury 3.7

Venus 8.8

Earth 9.8

Mars 3.7

Jupiter 26.4

Saturn 11.7

Uranus 9.1

Neptune 2.0

Pluto 0.5

Your weight on other worlds http://www.exploratorium.edu/ronh/weight/

Example #1-Mass on Earth

1 lb = 454 grams = 0.454 kilograms

To do this, we can use the conversion we learned a few slides ago…

Since the weight given was ON EARTH and the mass we were finding was ON EARTH, there was no need to use a gravitational factor.

The gravity didn’t change.

Example #2-Jupiter or bust!

TRICK QUESTION…WATCH FOR THIS!!!

MASS NEVER CHANGES…ONLY WEIGHT!!!

61.29kg .

Example #3-WEIGHT on JupiterLocation Gravitational

Factor (N/kg)Moon 1.6

Mercury 3.7Venus 8.8Earth 9.8Mars 3.7

Jupiter 26.4Saturn 11.7Uranus 9.1Neptune 2.0

Pluto 0.5

Compare and predict…will your weight on Jupiter be MORE or LESS?

Because Jupiter has a stronger gravity than the Earth, we expect that your weight has increased.

How did I know…

…to put the 26.4 on the top and the 9.8 on the bottom?If I know that my weight will increase (higher gravitational factor

on Jupiter than on Earth-gravitational factor increases), then I needed to put the larger number in the numerator.

However…if in the future I know that the weight will decrease (gravitational factor decreases), then I need to put the larger number in the denominator.

Example #4-To the moon!A box on Earth has a weight of 20 lbs. What is the weight on the moon?

Location Gravitational Factor (N/kg)

Moon 1.6Mercury 3.7Venus 8.8

Earth 9.8Mars 3.7

Jupiter 26.4Saturn 11.7Uranus 9.1Neptune 2.0

Pluto 0.5

QUICK CHECK:

Gravitational factor DECREASED.

Weight DECREASED.

Looks great!

Motion on the Moon-1969

Why does walking look different? Because of GRAVITY. Less gravitational factor on the moon, less weight.

Less weight, motion appears and feels different.

Example #5-Motion on Moon

Check…how did I know that the 9.8 went in the denominator???

Weight vs. Mass ReviewLocation Gravitational

Factor (N/kg)Moon 1.6

Mercury 3.7Venus 8.8Earth 9.8Mars 3.7

Jupiter 26.4Saturn 11.7Uranus 9.1

Neptune 2.0Pluto 0.5

1 lb = 454 grams = 0.454 kilograms = 4.45 N

Weight in Newtons = (Gravitational factor) * (Mass in kilograms)

MASS NEVER CHANGES…ONLY WEIGHT!!!

If gravitational factor increases, then weight should increase…so the larger gravitational factor goes in the NUMERATOR.

If gravitational factor decreases, then weight should decrease…so the larger gravitational factor goes in the DENOMINATOR.

If gravitational factor increases, then time and distance should decrease…so the larger gravitational factor goes in the DENOMINATOR.

If gravitational factor decreases, then time and distance should increase…so the larger gravitational factor goes in the NUMERATOR.

The “Vomit Comet”