GRAVITYIt’s what keeps us down.

The diagram shows a man holding a rock while standing in four different locations on the Earth.

Answer the following questions in your journal.

•Do you think the diagram is drawn to scale? Why or why not?•Draw a diagram to show what will happen to each rock when the man drops it. Use arrows to show the path the rock will take at each of the four locations.

The diagram shows a man holding a ball at the North Pole of the Earth. Pretend that a tunnel is dug inside the Earth starting at the North Pole & exiting at the South Pole.

Answer the following questions in your journal.

• Do you think the diagram is drawn to scale? Why or why not?

•Draw a diagram to show what will happen to the ball when the man drops it into the opening at the North Pole. Draw an arrow to show the path the ball will take.

Gravity Planet Earth has a pulling force called gravity. Gravity

pulls everything toward the center of the planet and gives meaning to the words “up” and “down”. Up means away from the Earth’s center; down means toward the Earth’s center. When we measure the force of gravity pulling things downward, we are measuring a physical property called weight.

Earth is not the only object in the Solar System that has a gravitational force. Actually all objects, even you, exert pulling forces on each other. How strongly objects pull on each other depends upon two factors: How much matter the objects have (mass) How far apart the objects are (distance)

Mass An object with more mass will have a

stronger gravitational force than objects with less mass. For example, the most massive object in our solar system is the Sun. The Sun also has the strongest gravitational pull in our Solar System. In fact, the Sun’s gravitational force is strong enough to keep all the other objects in our Solar System traveling around it.

Distance The farther you are from an object, the less its gravity

pulls on you. For example, Pluto is much farther from the Sun than Mercury. Therefore, the gravitational pull between the Sun and Pluto is much less than it is between the Sun and Mercury. Here on Earth, the farther you go from the Earth’s center, the weaker the Earth’s gravitational pull becomes. When the space shuttle is in orbit around the Earth, the pull of Earth’s gravity is much, much less than when the shuttle is parked on the Earth’s surface.

Gravity is the force that keeps the Moon traveling in a path around the Earth, the planets traveling in a path around the Sun, and you from floating off into space.

Read the article and answer the questions in your journal. 1. Which one does not belong in a group with the others?

a. weight b. spring scale c. triple-beam balanced. newtons 2. All of these are true except –

a. an object has weight no matter where it is located b. an object has gravity no matter where it is located c. an object has volume no matter where it is located d. an object has mass no matter where it is located

3. An object weighs 86 N on Uranus but 107 on Neptune. This means that – A. Neptune is closer to the Sun C. Neptune has more mass than Uranus B. Uranus is closer to the Sun D. Uranus has more mass than Neptune

4. Venus has less mass than Earth. Which of these will happen if a 50g object is taken from Earth to Venus? A. the object’s mass will be more than 50g. B. the object’s mass will be less than 50g. C. the object’s weight will be more. D. the object’s weight will be less.

5. Mars is closer to the Sun than Jupiter. This means that – A. an object will weigh less when it is on Mars than when it is on Jupiter B. the Sun’s gravity pulls harder on Mars than it does on Jupiter. C. an object will weigh more when it is on Mars than when it is on Jupiter D. the Sun’s gravity pulls less on Mars than it does on Jupiter.

Gravity: A Pulling Force People often use the terms weight and mass to mean the same thing. Even

though they are related, weight and mass are not the same. Weight is the pull of gravity on an object while mass is the amount of matter that makes up the object.

To help understand the difference between weight and mass, think about placing a 120g mass in your pocket. There are 120g of matter that actually make up that object. If you place the object on a spring, what will its weight be? The spring scale will read approximately 1.2N.

Now suppose you travel to the Moon with the 120g mass still in your pocket. On the moon, how much matter makes up the object? The object is still made of 120g of matter. The object’s mass did not change. However, if you place the object on a spring scale while you are on the Moon, you may be surprised to find that the object’s weight has changed. Why? Remember, weight is a measure of the pull of gravity on the object. The 120g mass has not changed, but the pull of gravity has. Since the moon is made out of much less matter than the Earth, the Moon has less gravitational force to pull on your 120g mass.

An important point to remember is that weight changes as the pull of gravity changes; mass remains the same.

What other places have gravitational force that is different from the Earth’s?

Weight : A changing physical property

Location in our Solar System

Gravity at Surface(when Earth = 1)

Weight at Given Location

Sun 27.9

Mercury 0.37

Venus 0.88

Earth 1.00 100N

Moon 0.17

Mars 0.38

Jupiter 2.13

Saturn 0.74

Uranus 0.86

Neptune 1.07

Pluto 0.08

Copy the table below into your journal & complete it.

Answer the following questions about the data on the table you just completed.

1. Based on the table, at which location in the Solar System will an object weigh the most?

2. Based on the table, at which location will the same object weigh the least?

3. How does changing the object’s location affect the object’s mass?

4. How does changing the force of gravity pulling on an object affect its mass?

DO YOU THINK IT'S POSSIBLE TO THROW A

BASEBALL SO FAST THAT IT NEVER FALLS

TO THE EARTH?

Newton’s CannonNewton's Thought Experiment

"Imagine a huge cannon located on the surface of the earth, firing a cannonball at greater and greater speeds. Ignore the effects of air resistance. As the speed increases, the cannon ball will fall at greater and great distances from the cannon (A, B, C). At some point (D), the speed will be so great that by the time the cannonball "falls" to the earth, the surface of the earth will have curved away from it! Instead of

striking the earth, the ball swings

all the way around the earth. Now

the cannonball is in orbit! If you

continue to give the cannon ball

more and more speed (D, E, F), its

path will carry it farther and farther

away from the earth as it travels

around it."

Web links

http://spaceplace.nasa.gov/en/kids/orbits1.shtml

http://www.smaphysics.ca/phys40s/field40s/newtmtn.html

http://www.youtube.com/user/GravityCanyon1?ob=5

http://www.gameshot.org/?id=83