# Conceptual Physics Worksheet Gravity, Weight, and Mass

**Physics Worksheet – Gravity, Weight, and Mass**

1. Pretend that you have a mass of 70 kilograms. Use the Law of Universal Gravitation to find your weight. That’s the pulling force that the earth exerts on you in Newtons (if you are metric.) The law refers to any two things attracting each other. Don’t question it right now. All pieces of mass attract all other pieces of mass. That’s what gravity actually is. Even if you don’t believe it, just use it. The two items attracting each other in this problem are you of mass m and the earth of mass M. Look up the mass and radius of the earth.

2. You should have gotten something in the last exercise that rounds to about 700 Newtons. That is the feeling of about 700 fist bumps. Convert the answer from #1 to pounds. Realize that there are about 2 pounds in every 10 fist bumps. Forces expressed in pounds are not metric so don’t use the answer in later exercises. Your answer to #2 should seem reasonable for any 70 kg people you might know on earth.

3. Under the assumption that you contain 70 kg of matter, find the ratio between your Earth-weight W (in Newtons) to your mass m. In other words, find the ratio W/m. The ratio W/m should obviously not be in Newtons. Lack of units = wrong.

4. A) If you considered a person of half the mass of 70 kg, what would the answer for weight be, as calculated in #1?

B) Again calculate the ratio W/m, now with the lesser mass.

5. A huge hamster has a mass of 1 kg. Calculate the hamster’s weight in Newtons. Calculate the ratio W/m for the hamster. That’s two answers for this question.

6. How does the ratio W/m change for any object located on the surface of the earth?

7. What shortcut can you learn from this for calculating the weight of any object of mass m located on the surface of the earth?

8. Now take your 70 kg mass, and use Newton’s Universal Gravitation to calculate your weight on the Moon. (Look up the Moon’s statistics; Moon mass, radius, etc.)

9. Now find the ratio W/m on the moon? Now use the W = mg shortcut to calculate the weight of the 1 kg hamster if it were on the moon? How about a 3 kg hamster?

If you read carefully, you saw that g came out of nowhere on this worksheet. Where did it come from?

By the way, the word gravity by definition is the force that a source body exerts. It should be seen as catastrophic for a person to refer to g as gravity. Understand exactly what I mean by this to avoid a world of frustration. Physics class is not designed to be difficult, yet through not learning how to communicate it is possible for a non-communicating student to have an impossible time. That can always be avoided. I’ve illustrated this clearly and repeatedly. Some pay attention; some don’t.

**Some (but not all) Conclusions:** *GMm/r2 is gravity and mg is gravity; g is not gravity. And g* is only sometimes equal to 9.8 N/kg