Section 2.3
Gravity and Free Fall
Acceleration due to Gravity
► An
object is in free fall if it is accelerating
due to the force of gravity and no other
forces are acting on it.
► Objects
in free fall on Earth accelerate
downward at 9.8 m/s2, the acceleration
due to gravity.
Speed vs. Velocity
► Speed
only has a magnitude (60m/s)
► Velocity
has magnitude and direction
(60m/s downward)
We can use this to find speed
► Remember,
the acceleration due to gravity
(g) is 9.8 m/s2
Practice
A penny is dropped from a building. If it
takes 15 s for the penny to hit the ground,
what is the penny’s velocity when it hits?
http://www.youtube.com/w
atch?v=TKdtIKRFwoc
Practice
A penny is dropped from a building. If it
takes 15 s for the penny to hit the ground,
what is the penny’s velocity when it hits?
v = gt = 9.8m/s2 x 15s = 147 m/s
Finding the average speed
Practice: same problem as before
A penny is dropped from a building. It
takes 15 s for the penny to hit the ground.
We found that the final velocity was 147
m/s. What is the average velocity?
Practice: same problem as before
A penny is dropped from a building. It
takes 15 s for the penny to hit the ground.
We found that the final velocity was 147
m/s. What is the average velocity?
vavg = vf + vi = 147m/s – 0m/s = 147m/s = 73.5m/s
2
2
2
Find the distance traveled
Practice: same problem as before
A penny is dropped from a building. It
takes 15 s for the penny to hit the ground.
We found that the average velocity was
73.5 m/s. What is distance traveled by the
penny?
Practice: same problem as before
A penny is dropped from a building. It
takes 15 s for the penny to hit the ground.
We found that the average velocity was
73.5 m/s. What is distance traveled by the
penny?
D = vavgt = 73.5m/s x 15s = 1102.5 m
Weight and the
nd
2
Law
Weight is the force of gravity on an object
 Force is the force due to gravity (Fg)
 Acceleration is the acceleration due to gravity
(g)
Practice
Calculate the weight of a 60-kilogram
person (in newtons) on Earth and on Mars
(g = 3.7 m/sec2).
Practice
Calculate the weight of a 60-kilogram
person (in newtons) on Earth and on Mars
(g = 3.7 m/sec2).
Answer: 588 N, 222 N
So if acceleration due to gravity is the same
for everything, why do a feather and a
hammer fall at different speeds?
Air Resistance
Air applies a force upward that resists the
force of gravity
Shape and size of the object determines the
magnitude of the force
Terminal Velocity
When the force of air resistance equals the
force of gravity, the object stops
accelerating.
This is called terminal velocity
http://www.youtube.com/w
atch?v=5C5_dOEyAfk
2.3 Review
1. Describe the motion of a freely falling
object. Use the words speed, acceleration,
and distance in your answer.
2. What is the difference between mass and
weight?
3. If you drop a feather and a baseball in a
place where there is no air (a vacuum), how
will their motions compare? Why?