AP Physics and Orbits
When you leave here today

You will be able to relate GRAVITATION
to ENERGY now.

You can understand how orbits work and
relate to energy.
HW

Answers online,

How can I help you out with this?
What fraction of the Force does the front
block have for each?
 What about the back block?
 How does the force of the front block on
the string compare to the back block on
the string?

Gravitational Potential Energy

Ug = mobjectgh
for on a planet with
gravity g

But what if we were far away, so g is
different?




Ug = mobjectgh
G∗Mplanet
Ug = mobject
𝑟2
G∗Mplanet
Ug = mobject
𝑟2
G∗Mplanet
Ug = mobject
𝑟
h where r and h
are both the
distance
r
but we make a
change



G∗Mplanet
Ug = mobject
GMm
𝑟
Ug =
but as height from Earth
𝑟
increases…U should increase
Ug =
−GMm
𝑟
Summary of the Energy

Remember height is arbitrary… so lets
call super far away ZERO energy.

Can you use the potential energy to figure
out the velocity it is going at?

Student A: No, knowing the potential does
not let you find the KE unless you know the
mechanical energy at a point.

Student B: Wrong, you can set the potential
energy equal to the KE since at max
potential the KE is zero.

How fast would the satellite be going if it
started from rest at that altitude and hit
Earth?
Now to Orbits:

Look here
 Idea: At
slow speeds, the ball will
simply fall back and hit Earth.
 Idea: At the ORBITAL VELOCITY at
that height, the ball will have a
CIRCULAR orbit
 Idea: At a velocity HIGHER than the
ORBITAL VELOCITY but less than
the ESCAPE VELOCITY, the ball has
an ELLIPTICAL orbit
 Idea: At higher velocities, it escapes
Earths ‘Gravity Well’
Here a satellite is orbiting in an ellipse
and is measured from the Earth’s center.
 We want to find the velocity at the ? in
terms of the variables given AND the
earths mass Me.


Please create a table of values at R1 and R2

Is the work here + - or 0

What is the work done on the satellite
here 0, + or -