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.