PHYSICS
Lecture 3
Free Fall
Southern Boone County High School
Bill Palmer
Physics-Free Fall
 All
freely falling objects fall at the same
rate-Galileo 1600s
 Neglecting the effects of air
 True in air or water
 Galileo “Diluted” gravity by rolling things
down ramps (sound familiar?)
 Leaning Tower of Pisa
Physics-Free Fall

g=Acceleration due to gravity
 Slight differences at various spots on earth
due to due to uneven distribution of mass
over earth’s surface
 Think of g over a iron mine vs. g over ocean
 g=9.80 m/s2
 For each second an object falls the
downward velocity increases by 9.80 m/s
Physics-Free Fall
 What
if a ball is thrown up?
 The ball slows down so a is negative.
 a= -g = -9.80 m/s2
Physics-Free Fall
 Throw
a ball upward with a velocity of
20 m/s.
 Gravity slows the velocity at a rate of
9.80 m/s
 So, after 1 sec the ball has slowed from
20 m/s – 9.80 m/s = 10.2 m/s
 After 2 sec the ball has slowed another
9.80 m/s or 10.2 m/s – 9.8 m/s = .4 m/s
Physics-Free Fall
 What
happens after three seconds?
 .4 m/s – 9.80 m/s = - 9.4 m/s
 Now the ball is going down
 At 4 seconds the ball is moving down at
–9.4 m/s plus another 9.80 m/s = 19.2
m/s
 At about 2 sec, the balls velocity is 0
Physics-Free Fall
 The
Demon Drop is a ride at an
amusement park where a car falls freely
for 1.5 sec after starting from rest.
 What
is its velocity at the end of 1.5 s?
 How far did it fall?
Physics-Free Fall
 A brick
 What
is dropped from a high scaffold.
is its velocity after 4.0 s?
 How far does the brick fall in this time?
Physics-Free Fall
 A brick
is dropped from a high scaffold.
 What
is its velocity after 4.0 s?
 How far does the brick fall in this time?
 Answer:
- 39 m/s
 Answer: -78 m (downward)
Physics-Free Fall
 A tennis
ball is thrown straight up with
an initial speed of 22.5 m/s. It is caught
at the same distance above ground.
 How
high did the ball rise?
 How long does the ball stay in the air?
(hint) time to rise is same as time to fall
Physics-Free Fall
 A tennis
ball is thrown straight up with
an initial speed of 22.5 m/s. It is caught
at the same distance above ground.
 How
 How
high did the ball rise? A=25.8 m
long does the ball stay in the air?
(hint) time to rise is same as time to fall
2.3s going up so 2.3s + 2.3s = 4.6 s total
Physics-Free Fall
 A spaceship
far from any star or plant
accelerates uniformly from 65.0 m/s to
162 m/s in 10.0 sec. How far does it
move?
Physics-Free Fall
 A spaceship
far from any star or plant
accelerates uniformly from 65.0 m/s to
162 m/s in 10.0 sec. How far does it
move?
 Answer:
1.14 x 103 m
Physics-Free Fall
 In
English system, g equals 32 ft/s2
 Try
these problems:
Physics-Free Fall
 Mr.
Palmer takes some students rock
climbing.
 Ditzy Nicole is watching from the top of
a cliff. She slips and falls of the cliff and
her body crashes into the ground
directly below 4.6 s later.
 How
high was the cliff?
 How fast was she going when she hit the
ground? (f/s and m/h)
Physics-Free Fall
 Mr.
Palmer takes some students to a pit
cave. They have a 600 f rope. To find
out if their rope is long enough they
throw a rock into the cave and watch it
hit the bottom (why don’t they just
listen?) The rock takes 3.5 seconds to
hit the bottom. Is the rope long enough?
Physics
 Complete
the following 10 problems
page 107-115: 33, 38, 44, 47, 54, 57,
62, 66, 70, 76.
 Critical thinking problems>>>work in
teams of two. Problems 77, 78, 79
 Review these on Monday
 Test on Wednesday