CHAPTER 3 SECTION 1: GRAVITY, FRICTION, AND PRESSURE

Big Idea: Newton’s laws apply to all forces.

Key Concept: Gravity is a force exerted by masses.

Now we will learn:

· How mass and distance affect gravity.

· What keeps objects in orbit.

I. Gravity

· The force that objects exert on each other because of their masses.

· Gravity is not only present on Earth.

· Considered a universal force.

II. The Force of Gravity

· If there is a force between all masses, why are you not pulled toward your desk by the desk’s gravity? (See page 78 of book)

· Strength of gravity depends on:

1. Mass

· The more mass two objects have, the greater the force of gravity.

2. Distance

· As distance increases, gravity between objects decreases.

III. Gravity on Earth

· Acceleration due to Earth’s gravity is called g.

· 9.8 m/s/s

· Calculate the force of gravity: F=ma or F=mg

IV. Weight and Mass

· Weight is the force of gravity on an object.

· You have the same mass no matter where you are.

· Weight can change depending on where you are.

· How much would you weigh on the moon? Multiply your weight by .17 to find out!

V. Gravity keeps objects in orbit

· An orbit is an elliptical path one body follows around another body due to the influence of gravity.

Read bottom of page 80

VI. Spacecrafts and people in orbit

· In order to send an object into orbit it needs to accelerate to 8000 m/s to match the curve of the earth’s surface.

· Escape velocity: 11000 m/s

· How do you feel when you are riding down in an elevator? Read page 80.