October 1, 2013.
Lecture 7 – Gravity and Kepler’s Laws
First law: the orbit of each planet around the Sun is an ellipse with the Sun at one focus
o “The Law of Inertia”
o In plain words: if a force does not disturb you, you keep on doing what you are
o Things in motion stay in motion, things at rest stay at rest
o If other forces are not involved, would a rocket (in space) in motion keep on
Yes, in any case, at constant speed
o An object moves at a constant velocity if there is no force acting upon it
Second law: force = mass x acceleration (or force = rate of change in momentum)
o If you want to accelerate a body, you need a force
o Do our bodies exert a force of gravity on Earth?
o Compared to the force of gravity the Earth exerts on you, the force of gravity you
exert on the Earth is:
The same; the force is always equal but the mass is not
o Why is the Earth not moving towards us?
Do we attract gravitationally Earth?
The difference is the mass!
Third law: for any force, there is always an equal and opposite reaction force
o Pushing each other on an ice rink
o When Canadian astronaut Julie Payette is on board the International Space
Station (ISS), why does she float?
In orbit around Earth, constant free-fall
If you drop a ball the gravitational pulls between Earth and the ball cause
it to drop
If I throw a ball, it will fall along a curved trajectory
Why doesn’t the moon, which is also affected by Earth’s gravitational
pull, crash into the Ea