“THERE IS NO GRAVITY”
• Cannot be the answer.
• Gravity is everywhere in the universe.
• You need very large objects to notice it, for example
• But it is inescapable…..
So what is going on?
• Understand that objects stay in orbit because of the pull
of gravity and the speed they are moving.
• Describe orbits
• Solve problems about orbits using Newton’s
Gravitational and Kepler’s Laws
Here is my thought
experiment. Imagine a
big cannon on a tall
The cannon ball
would travel very far
in a curved path
An even bigger
mountain and the
ball would travel
…but the Earth is
curved. So the
cannon ball actually
goes even further
cannon on a
What if the cannon
and mountain were
The cannon was
above the Earths
The cannon ball orbits
Continually falling under
the effect of the gravity
Never hitting the ground
because of it’s speed
and the curvature of the
Never slowing down
because there is no air
Things in orbit
is pulling them
Earth but they
too fast to hit it.
ROCKETS RATHER THAN MOUNTAINS
We put space craft and satellites in orbit around the Earth using rockets rather than
impossibly high mountains.
Once the rockets has carried the spacecraft above the atmosphere booster rockets
speed it up to it’s orbit speed.
WHAT IS THIS IDEA TO LAUNCH SATELLITES?
• In the 1950’s the USA
had the X-Plane project
it flew planes to the
edge of Earth’s
• Virgin Galactic launch
small spacecraft from
the back of aeroplanes.
SO WHY DO THINGS FLOAT AROUND INSIDE
SPACECRAFT IN ORBIT?
• Orbit is freefall
• The spacecraft and everything in it are falling under gravity
(around the Earth)
• Everything falls with same acceleration.
• Staying the same distance apart (floating)
DEMONSTRATE YOUR LEARNING
Explain using ideas about gravity and orbits what is happening in this video clip.
It was taken by the camera on an astronaut working on the International Space
1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of
3. The square of the orbital period of a planet is directly proportional to the cube of the
semi-major axis of its orbit.
We will treat
FIRST AND SECOND LAWS
NOT THAT RELEVANT FOR CIRCULAR ORBITS
• Circular orbits are types of ellipses (both foci at the same point)
• The speed doesn’t change in a circular orbit.
LET’S DERIVE THE THIRD LAW
• What is the only force acting
on a planet orbiting the
• What is the expression
for centripetal force?
• And if the planet is moving
at constant speed in a circle
it must have centripetal
force acting on it.
• For the gravitational
• Equating these
expressions, we have
KEPLER’S THIRD LAW
C = 2πr
• speed v can be calculated as distance
travelled in one orbit (2πr) divided by the
time taken, T:
• Plugging this into the previous equation,
and cancelling the m terms on both sides
GM/r2 = 4π2r/T2
• Rearranging again gives:
T2 = (4π2/GM) r3
The square of the orbital period of a planet is directly
proportional to the cube of the semi-major axis of its
• We know from Kepler’s third law that the further away a satellite is
from the body it is orbiting, the longer its orbital period.
• If an orbiting satellite had a period of 24 hours, and you saw it
overhead at, say 10.00 am, when would you next see it overhead?
(Because both the Earth would have completed one rotation in the
same time it took the satellite to complete one orbit, it would next be
overhead at 10.00 am the next day. Such a satellite is said to be
A difficult question – if you wanted the satellite to remain directly
overhead at all times (not just once per day) where on the Earth
would you have to be?
The only points on the Earth’s surface that orbit around the centre
of the Earth are those on the equator. Thus, you would have to be
on the equator.
If a satellite has a period of 24 hours and orbits above the equator
such that it always appears to be above one point on the equator,
it is known as a geostationary satellite, and its orbit is a
Geostationary satellites are predominantly used for
communications. Satellite TV companies use geostationary
satellites to cover a constant area on the Earth’s surface – hence
you point your satellite dish receiver in the direction of the
3 geostationary satellites placed into orbit 120 degrees apart
above the equator would be able to cover the entire Earth (except
for very near the poles).
Because geostationary satellites have to be launched so high
(other satellites orbit as low as a few hundred km), the energy and
costs required for launching a satellite into geostationary orbit are