Transcript File

Gravity and Freefall
The Law of Gravitation
1. You exert an attractive force on
everything around you and everything
is exerting an attractive force on you.
2. This attractive force is called gravity.
3. Anything that has mass is attracted by the
force of gravity.
4. The Law of Gravitation states that any two
masses exert an attractive force on each
other.
The Law of Gravitation
5. Gravitational force depends on two things:
– The mass of the two objects
– The distance between the two objects
6. Why do you suppose the Earth exerts a force
on you that you can feel, but you can’t feel
the force the desk is exerting on you?
The Mathematical Relationship
7. The greater the mass of either
object, the greater the attraction (Fg).
8. The greater the distance between the
objects, the smaller the attraction (Fg).
or
The Inverse square law:
9. Since the distance (d) is
in the denominator of this
relationship, it can be said
that the force of gravity is
inversely related to the
distance.
10. And since the distance is raised to the second
power, it can be said that the force of gravity is
inversely related to the square of the distance.
Gravitational Acceleration
11. When objects fall, the
gravitational pull is
9.8 m/s2
(sometimes rounded to 10 m/s2)
12. When a falling object is only
affected by gravity it is said to
be in free fall.
Gravitational Acceleration
• If you drop a a feather and a coin, which will
hit the ground first?
• How about if there was no air?
13. Okay, Which has more gravitational
A bowling ball?
FORCE?
Or a marble?
14. Force of gravity is greater on the
bowling ball because of its larger mass.
Remember, Fg= m x g
15. The larger mass means it has a larger
inertia so more force is needed to
change its velocity.
16. Gravitational force on the marble is
smaller because it has a smaller mass.
17. The inertia on the marble is less and
less force is needed to change its
velocity.
18. Therefore, all objects fall with the
same acceleration!
Air Resistance
• What two forces are
acting on an object
when it falls?
19a. Gravity
19b. Air resistance
Air Resistance
• Imagine dropping two
pieces of paper. One is
crumpled and the other is
flat.
• Which one will reach the
ground faster and why?
20. The crumpled one
because it has less surface
area.
Air Resistance
21. When something falls,
air resistance acts in the
opposite direction to the
force of gravity.
22. Air resistance acts in the
opposite direction of the
object’s motion.
Air Resistance
• The amount of air
resistance an object
experiences depends
on two things:
23 a. Speed
23 b. Surface Area
Air Resistance
• Why do leaves, papers, and feathers fall
at different speeds than acorns, pens,
and glasses?
• Because of Air Resistance…
24. Air resistance, not mass, is responsible
for the differences in the speeds of
falling objects.
Terminal Velocity
25. As an object falls, it
accelerates and its
speed increases.
26. The force of air
resistance increases with
speed.
Terminal Velocity
27. The force of air resistance
increases until it becomes
large enough to cancel the
force of gravity.
• When the forces cancel each
other out the object no longer
accelerates. It is in
EQUILIBRIUM!
28. The object then falls at a
constant speed - called
terminal velocity.
Terminal Velocity
29. Terminal Velocity is the
highest velocity that a
falling object will reach.
30. A low terminal velocity
allows a skydiver to land
safely.
Terminal Velocity
30. Why would a skydiver
want to lay out flat versus
falling standing up?
• Think about our paper
example… crumpled vs flat
Point Break Clip!
• https://www.youtube.com/wat
ch?v=0yeqtZrjhcA