Lesson 14 notes

Surface Gravity

Background:

1. Gravitation refers to a force of attraction that draws particles or bodies together – the attraction of matter to other matter.

2. Gravity, specifically surface gravity, refers to the gravitational attraction exerted by the mass of a planet (or heavenly body) on objects at or near its surface.

Mass and Weight:

1. Weight of an object is proportional to its mass.

2. Mass is often used as a measure of weight.

3. Mass is a measure of inertia (the resistance to any change in a body’s state of motion) and is related to the amount of matter present in a body, regardless of its volume.

4. Weight is the measure of the force of gravity on an object, and the force of gravity may change from planetary body to planetary body.

5. Force is a push or pull on or by an object.

Measuring Mass and Weight:

1. Mass is measured in kilograms and grams in the metric system and weight is measured in newtons.

a. On the surface of Earth, a mass of 1 kilogram weights 9.8 newtons (N).

2. A spring scale measures the strength of the gravitational pull on the mass of an object, or weight in newtons.

3. To find the mass of an object we use a beam balance.

Mass and Weight on the Nine Planets:

1. Objects with mass attract any other object with mass.

2. The amount of attraction depends on the mass and radius of each object and the distance between the objects. (see magnets)

3. Weight measures the pull of gravity. This force of gravity depends on the object’s mass and the mass of the planet.

a. If the mass of a planet is doubled, gravity pulls on the object twice as hard.

b. The force of gravity also depends on the distance of an object from the center of the planet.

c. The farther an object is from the planet’s center, the weaker the pull between the planet and object.

d. If the distance between the planet’s center and the object is doubled, the force is one-fourth. If the distance is tripled, the force drops by one-ninth.

e. The force drops off with the square of distance between the center of the planet and the object.

i. F = G x (M1 x M2)/D2

ii. Where M1 and M2 are the masses of two objects.

iii. D is the distance (called Gravitational Constant)

iv. F is the force of gravity that results between the two objects.

4. Surface gravity varies from planet to planet and from place to place.

5. Weight varies depending on the location of an object.

6. surface gravity varies from planet to planet and from place to place; therefore, weight varies depending on the location of an object. A person has a certain weight on Earth’s surface because Earth attracts or pulls on the person. That same person has less weight standing on the Moon’s surface because the Moon exerts less gravitational force on its surface than Earth.

7. An object must have a large amount of mass before it can exert a measurable gravitational pull. Earth has a large amount of mass, so its gravitational pull is easily observed.

8. Jupiter is 318 times more massive than Earth, so one might assume that an object would weigh 318 times more on Jupiter than on Earth. This would be true if Jupiter were the same size as Earth, but the diameter of Jupiter is more than 10 times the diameter of Earth. Surface gravity depends on both the mass and the radius of a planet.

9. The distance from the surface of the center of the planet Jupiter reduces its gravitational pull to only 2.36 times the gravitational pull on Earth.

10. Gravity factor is used to find out how much an object would weigh one each planet. The gravity factor is the number of times that each planet’s surface gravity is stronger or weaker than Earth’s, given that planet’s assumed diameter. Earth’s gravity is 1 and Jupiter’s gravity is 2.36.

11. By multiplying an object’s Earth weight by the gravity factor, you can determine the weight of an object on each of the nine planets.