Newton’s First and
Second Laws
Section 10.3
Page 389
Objectives for 10.3
State Newton’s first law of motion.
Give examples of how Newton’s first law is
acting around you.
State Newton’s second law of motion.
Know how to calculate force or acceleration,
from Newton’s second law.
From Newton’s second law, be able to describe
the relationship between the mass of an object
and the amount of force needed to change its
velocity.
Newton’s First Law of Motion
• An object at rest will stay at rest, an object in
motion will stay in motion unless acted upon by
an unbalanced force. (Law of Inertia)
Newton’s First Law of Motion
• Inertia is the tendency of an object to resist
change in its motion. (vid 1)
– Example: In a car that stops quickly, inertia causes
you to continue moving forward. (You have inertia!)
– What is the force that causes your motion to change
in the above example? (Your seatbelt) What if you
are not wearing a seatbelt?
Newton’s First Law of Motion
• Intertia means that unless an object is acted
upon by an unbalanced force, the object’s
velocity does not change.
• What do we call a change in velocity?
• So … what would an unbalanced force cause
an object to do? (vid 2)
Mass
• Mass the amount of matter in an object.
– A jar of nails has more mass than a jar of cotton
balls.
– The SI unit for mass is kilograms.
– Large objects like a car or yourself would be
measured in kilograms. How many grams are in a
kilogram?
– Small objects are measured in grams. A nickel may
have a mass of 5 grams.
Mass and its effect on inertia
• The amount of inertia an object has
depends on its mass.
– The greater the mass, the greater the inertia.
• Mass can therefore also be defined as a
measure of the inertia of an object.
Newton’s Second Law of Motion
• Newton’s 2nd Law:
– Acceleration depends on the net force acting on
the object and on the object’s mass
Acceleration = Net Force
mass
-or-
– The net force of an object is equal to the product
of its acceleration and its mass.
–
Force= mass x acceleration
Newton’s Second Law of Motion
F=mxA
• Acceleration is measured in m/s2
• Mass is measure in kg
• Force is measured in kg-m/s2
• One Newton equals the force required to
accelerate one kg of mass at 1 meter per
second per second.
1N= 1 kg x 1 m/s2
Changes in force and mass
• To increase acceleration, increase the
force.
F= m x A
• An increase in mass causes a decrease in
acceleration.
A= F
m
(Vid 3)
Calculating Force
• A speedboat pulls a 55-kg water skier. The
skier accelerates at 2.0 m/s2. What is the
net force that causes the acceleration?
Force = mass X acceleration
mass = 55-kg.
acceleration = 2.0 m/s2
Force = 55-kg X 2.0 m/s2 = 110 kg- m/s2 = 110 N.
Calculating Force
• A girl has a mass of 50-kg. The
acceleration due to gravity is 9.8 m/s2.
What is the girl’s weight?
Force = mass X acceleration
mass = 50-kg.
acceleration = 9.8 m/s2
Force = 50-kg X 9.8 m/s2 = 490 kg- m/s2 = 490 N.
Do Cornell notes for
Chapter 10, Section 3