What can you tell me about gravity?

• Gravity is usually assumed to be a uniform vertical downward force...

...it is actually radial

g = 9.81ms-2 is not constant!

Watch video clip

Do now: copy the notes, answer the questions, and... May the Force (of Gravity) Be With You!

1) G = 6.67 x 10-11 - what are the units? 2) Calculate the force of gravitational attraction between you

and another person in the room.3) How does this compare to your weight force?

Newton’s Law of Gravitation:

Nm2/kg2

(their centres)Board notes!

1) G = 6.67 x 10-11 - what are the units? 2) Calculate the force of gravitational attraction between you

and another person in the room.3) How does this compare to your weight force?

Newton’s Law of Gravitation:

Nm2/kg2

(their centres)

Learning Outcome:Use Newtons Law of Gravitation to calculate gravitational force and satellite speed

g cF F

2

2

Mm vG mr r

v GM r

• Geostationary satellites: T = 24 hours• Satellite velocity does not depend on its mass

- so all geostationary satellites orbit at the same radius!

Geostationary orbit : radius = 35,786 km , velocity = 3.07 km/s

A 110kg satellite is placed in orbit around the Earth at a height of 35,900km.

Use mE = 5.98 x 1024kg and rE = 6370km to calculate:a)The force of gravity on the satelliteb)The orbital speedc)The orbital period (in hours)

a) 25N b) 3100ms-1 c) 24 hours

Example:

Trivia: how many satellites are there in space?• 24,500 (Space Surveillance Network actively tracks 8,000 of these, 560 of which are currently operational). • The Global Positioning System (GPS) is a US-owned global navigation satellite system that makes use of 24 to 32 satellites.

Video clips

Activity 8C