Check Prior Knowledge:
• Summarize the contributions made by
Galileo and Newton
• Distinguish between speed, velocity, and
• What are the three “laws of motion”
• Heavy objects fall faster than light ones.
True? False? Depends?
--branch of science that deals with motion of
It was not until the 17th century that our
modern understanding of motion began to
Galileo Galilei (1564-1642)
• Math professor
• Forerunner to modern
• What‟s he famous for?
Father of Experimental Science
At the surface of the
earth, all objects
speed up at the
same rate as they
Analyzing falling objects
• 2 variables: distance and time
• Galileo devised an inclined plane to “slow
motion down” for study.
• To understand Galileo‟s results, you have to
distinguish speed, velocity, & acceleration.
• Speed is the distance an object travels
divided by the time it takes to travel that
• Speed = distance / time
• Velocity is same numerical value as
speed, but velocity always includes
direction of travel.
What is the Speed of a race horse that runs
1500m in 2 minutes?
• S = 1500m / 2 min
S = 750 m/min
• S = 1500m / 120 sec
S = 12.5 m/sec
Colonel John Stapp
acceleration in rocket
sled experiments. The
severe contortion of
soft facial tissues was
recorded by a movie
Courtesy U.S. Air Force
• Acceleration is the amount of change in
velocity divided by the time it takes the
change to occur.
• Acceleration (m/s2) =
[final velocity – initial velocity (m/s)] / time (s)
• A = (vf - vi) / t
A car traveling at a rate of 10 m/s
accelerates to 90 m/s in 12 seconds.
Calculate its acceleration?
• A = (vf - vi) / t
• A = 90 m/s – 10 m/s / 12 s
= 80 m/s / 12 s
= 6.67 m/s/s
or 6.67 m/s2
3 devices in your car make it accelerate:
Whenever an object changes speed or
direction it accelerates.
Galileo’s falling-ball apparatus with a table of
measurements and a graph of distance versus time.
Galileo found the following:
• a ball rolling down a ramp moves with
• a ball attains a greater acceleration from
• regardless of weight, when air resistance
is negligible, all objects fall with the same
• The velocity of a falling object is
proportional to the length of time it has
• Velocity (m/s) = constant g (m/s2) x time (s)
• Galileo found g = 9.8 m/s2
Acceleration due to Gravity
• Suppose a falling rock is equipped with a
• In each succeeding second of fall, the rock‟s speed
increases by the same amount: 10 m/s
• Time of Fall (s) Instantaneous Speed (m/s)
• Suppose a falling rock is equipped with
• The readings would indicate that the
distance fallen increases with time
according to the relationship d = ½ gt2
• Time of Fall (s)
Distance of Fall (m)
Isaac Newton and
the Universal Laws of Motion
• English scientist
• Synthesized the work
of Galileo and others
• 3 laws describe all
First Law: Inertia
(matter resists change)
• A moving object will continue moving in a
straight line at a constant speed, and a
stationary object will remain at rest, unless
acted upon by an unbalanced force.
Second Law: F = m x a
• The acceleration produced by a force on an
object is proportional to the magnitude of
the force, and inversely proportional to the
mass of the object.
Free Fall and Air Resistance
• In free-fall, force of air
resistance counters force
• As skydiver falls, air
resistance increases „til it
approaches the magnitude
of the force of gravity.
Once the force of air
resistance is as large as the
force of gravity, skydiver
is said to have reached a
Third Law: action / reaction
• For every action there is an equal and
• See some examples
• Quantity of matter in
• The measurement of
• Brick = 1kg
• The gravitational force
exerted on an object
by the nearest, most
massive body (Earth)
• Brick = 2.2 pounds
The Newton (metric unit)
• In the metric system, the unit of weight, or any
other force, is the newton, which is equal to a
little less than a quarter pound.
• Newton = force needed to accelerate 1 kg 1 m/s2
• 1 kg brick weighs about 10 N
• Or a baseball = 1 N
• Example Problem, page 41 will help
calculate the force needed to produce a
given acceleration on a given mass
(F = ma)
• A 20 kg mass has an acceleration of 3 m/s2.
Calculate the force acting on the mass.
• F = (20 kg) (3 m/s2)
• F = 60 kg m/s2 = 60 N
What force is needed to accelerate a 75 kg
sprinter from rest to a speed of 10 meters
per second in half a second?
• First find acceleration.
Accel = final vel – initial vel (m/s) / time (s)
= 10 m/s – 0 m/s / .5 s = 20 m/s/s
• Force (N) = mass (kg) x accel (m/s2)
F = 75 kg x 20 m/s2
F = 1500 N
Newton’s Law of Universal
• Between any two objects in the universe there
is an attractive force proportional to the masses
of the objects and inversely proportional to the
square of the distance between them.
• F = (G x m1 x m2) / d2
• The more massive 2 objects are, the greater the
force between them.
• The farther apart 2 objects are, the less the
force between them.
An apple falling, a ball being thrown, a space shuttle
orbiting the Earth, and the orbiting Moon, all display the
influence of the force of gravity.
Study Guide: The Sciences, Ch 2
• Read pp 24-46
• Discussion Questions 1-10
• Problems 1-7