__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.