Who was Isaac Newton?
Sir Isaac Newton (1643-1727), an
English scientist and mathematician
famous for his discovery of the law of
gravity, also discovered the three
laws of motion. He published them in
his book Mathematic Principles of
Natural Philosophy in 1687. Today
these laws are known as Newton’s
Laws of Motion and describe the
motion of all objects we experience in
our everyday lives.
Newton’s Second Law States:
“The acceleration produced by the force on
an object is directly proportional to the
magnitude of the net force, is in the same
direction as the net force and is inversely
proportional to the mass of the object.”
Let’s put this law into simpler terms…
In Other Words…
The acceleration of an object is directly
proportional to the net force and inversely
proportional to its mass.
What is Force?
Force is any push or pull, and they can be
balanced or unbalanced. Balanced results
in no change in speed or direction, while
unbalanced forces result in a change in
speed and/or direction.
What is Mass?
Mass is the measure of the amount of
matter in an object; It is measured in
Kilograms.
Is Mass the same as Weight?
• No! Weight measures the pull of gravity on
an object’s mass. Weight can change
according to where the object is. For
instance, on the moon, you would weigh
1/6 of what you weigh here on Earth
because its force of gravity is much less.
• Mass, however, only measures the
amount of matter in an object and it never
changes!
What is Acceleration?
Acceleration is a change in velocity. You can
accelerate by:
– Speeding up
• Positive acceleration
– Slowing down
• Negative acceleration (Deceleration)
– Changing direction
Calculating Acceleration?
• To calculate acceleration, subtract the
initial velocity from the final velocity, and
divide it by the time it took.
a = Vf – Vi
____________________________
time
Calculating Acceleration?
Barry Boredalot is really bored one day and
goes outside to watch ants. He sees an
ant carrying a leaf traveling at .1 m/s. The
ant, seeing Barry approach, speeds up
and runs for safety. In just 2 seconds it
accelerates to .6 m/s. Find the ant’s
acceleration.
V f- V i
_____________
Time
=
.6
m/s - .1 m/s
__________________________
2 sec
.25 m/s2
.5 m/s
= _________________
2 sec
=
Calculating Acceleration?
A semi-truck is stopped at a red light. When
the light turns green, the driver pushes on
the gas pedal. After 10 seconds, he is
moving at 25 m/s. What is his
acceleration?
V f- V i
_____________
Time
25 m/s - 0 m/s
=
__________________________
10 sec
2.5 m/s2
25 m/s
= _________________
10 sec
=
What is Acceleration?
• Acceleration is DIRECTLY related to
the size of the force and the direction
of the force.
• Acceleration is INVERSELY related
to the mass of the object.
In other words….
• Using the same amount of mass…
– Large Force = Large Acceleration
– Small Force = Small Acceleration
• Using the same amount of force…
– Large Mass = Small Acceleration
– Small Mass = Large Acceleration
Force Causes Acceleration
• Acceleration is directly proportional to
force.
• Example: A golf ball is still until hit by a
club.
• If you swing the club with
a lot of force, the ball will
travel farther than if you
hit it with a little bit of force.
Mass Resists Acceleration
• Acceleration is inversely proportional to
mass.
• Example: A bus vs. a mouse
• The greater the mass the more force it
takes to accelerate the object
Force = Mass x Acceleration
• The units used for force are Newtons (N)
• The units used for mass are kilograms (kg)
• The acceleration units are meters per
second squared (m/sec2).
Problem Solving
• One Newton = the force needed to give
a mass of one kilogram an acceleration
of one meter per second per second.
Problem Solving
How much force must a 20,000-kg plane
develop to achieve an acceleration of 1.5
m/s2
F=ma
F = (20,000 kg) (1.5 m/s2)
F = 30,000 kg m/s2
F = 30,000 N
Problem Solving
If we know two quantities, we can solve
for the third:
Problem Solving
What acceleration is produced by a force of
200 N applied to a 100-kg car?
a=F/m
a = 200 N/ 100 kg
a = 200 kg m/s2 /100 kg
a = 2 m/s2
Free Falling Objects
• What about acceleration caused
by gravity? Do all objects fall at
the same rate?
• Galileo did his famous
experiment off the leaning tower
of Pisa. He dropped a 10 kg
cannon ball and a 1 kg stone at
same time. Both objects landed
at the same time, meaning their
accelerations were equal…..but
why?
Acceleration Due to Gravity
• Newton’s second law provides the
explanation:
– A falling object accelerates toward the
Earth because of the gravitational force
between the object and the Earth.
– The acceleration due to gravity is a
constant.
Acceleration Due to Gravity
• F/m = a
9.8 N / 1 kg rock = 9.8 m/s2
• F/m = a
98 N / 10 kg cannon ball = 9.8 m/s2
The acceleration due to gravity is a
constant on Earth, 9.8 m/s2.
Acceleration Due to Gravity
• Why don’t a feather and a bowling ball fall
at the same rate if I drop them from a
window? Air resistance!
• Air resistance is the force of air exerted on
a falling object. The air pushes up as
gravity pulls down. It is dependent upon
the shape and surface area of the object.
• In the absence of air resistance, all objects
will fall at the same rate due to gravity.
Acceleration Due to Gravity
• If you were on the
moon an dropped a
hammer and a
feather at the same
time, would they
strike the surface of
the moon at the same
time? YES!
Comprehension Check
1. What acceleration will result when a 15 N
net force applied to a 3 kg object?
15 N = 3 kg x 5 m/s2
2. A net force of 1250 N causes a mass to
accelerate at a rate of 5 m/s2. Determine
the mass.
1250 N = 250 kg x 5 m/s/s
Comprehension Check
3. How much force is needed to accelerate a
60 kg skier 10 m/sec2?
600 N = 60 kg x 10 m/sec2
4. What is the force on a 1000 kg elevator
that is falling freely at 9.8 m/sec2?
9800 N = 1000 kg x 9.8 m/sec2