I’m attracted to you

Gravity

Mass and Weight

• Mass is a measure of a body’s INERTIA

• Units are kg, scalar

• Weight is the gravitational force acting on an object

• Units are N, vector• Gravitational field strength, g, is the

weight per unit mass, Nkg-1

Gravitational field strengths

• Earth’s moon, 1.6 N kg-1

• Earth 9.8 N kg-1

• On the surface of the sun 273.7 N kg-1

• Black holes , so big that even light can’t escape

Newton• The bigger the mass the bigger the force

of attraction• The further away from the planet / moon /

star the smaller the force of attraction

2

21

r

mmGF

F = force of attraction

G = Universal Gravitational constant

= 6.67x10-11Nm2kg-2

m1 = mass m2 = mass 2

r = distance between two masses ( for planets and so on we assume this is the distance from centre to centre )

Newton’s Law of Gravity

• Mass of earth = 6.0x1024 kg• Radius of earth = 6.4x106 m• Calculate the force acting on you ( assume all the

mass is concentrated at the centre of earth )

2

21

r

mmGF

N

x

xxxx

1.762

104.6

78100.61067.626

2411

Try using W = mg , can you account for the difference ?

Mass of Earth

• Equate F = mg with

2

21

r

mmGF

Allows the mass of the earth to estimated

Try it

gr

Gm

mgr

mGmF

21

221

kgx

x

xxm

G

gr

24

11

262

1002.6

1067.6

8.9104.6

Mass/ distance /radius of other planets

• Equate the gravitational force with the centripetal force ( AH Physics ) to find mass

• Use triangulation to find distance

Radius of planetsIf you know how far away a planet is from you, you can determine its linear diameter, D.

D = 2Π × (distance to the planet) × (the planet's angular size in degrees)/360° )

This technique is used to find the actual diameters of other objects as well, like moons, star clusters, and even entire galaxies.

Slingshot Effect

Think of the comet gaining some momentum as it approaches the planet which of course loses some momentum( it’s orbit speed will decrease slightly )

Problems

What is the problem with Newton’s Theory ?

2

21

r

mmGF

Newton : Static Universe and

intervenes from time to time to maintain the equilibrium