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Forces
Newton’s First and Second Laws
• Newton’s 1st law of motion deals with inertia
• An object at rest remains at rest, an object in
motion maintains its velocity, unless acted
upon by an outside force
• Objects change their state of motion only
when a net force is applied to the object
• Inertia: the tendency of an object to maintain
its state of motion.
Inertia
• The tendency of an object to maintain its state
of motion
• Inertia is related to the mass of an object
– An object with more mass has more inertia
– An object with less mass has less inertia
• Which is harder to push? A couch or a car?
– Which has the most inertia? Why?
Inertia
• Which is harder to stop? A train or a car
traveling at 40 mph?
– Why?
• Use the concept of inertia to explain why seat
belts are useful.
• A pickup truck is at rest. A basketball is in the
bed of the truck, near the cab.
– What happens to the ball when the truck starts to
move?
– Why?
Newton’s 2nd Law of Motion
• Unbalanced forces cause an object to
accelerate
• Unbalanced forces produce a net force
• Fnet = ma
• Greater net force causes greater acceleration
• Units of force
– SI unit is the Newton
– English unit is the pound
Newton’s 2nd Law and Acceleration
• Acceleration depends on mass and net force
Fnet
a
m
• Acceleration increases with more force
• Acceleration decreases with more mass
Gravity (12.2)
• Weight and mass are not the same thing
• Mass is a measure of matter
• Weight is the force of gravity acting on a mass
Weight
• When an object falls, it experiences
acceleration due to gravity, this is called free
fall acceleration (g)
• Weight = mass x free-fall acceleration
• This is an application of Newton’s 2nd Law…
F = ma
or
W = mg
• Since weight is a force, units of weight are the
same as units of force.
Newtons (N) or pounds (lb)
Weight and Mass
• Weight of an object depends on the
acceleration due to gravity
• On Earth, W = mg Earth
• WEarth = 66 kg x 9.8 m/s2 = 650 N ≈ 150 lb
• On the Moon, W = mg Moon
• W moon = 66 kg x 1.6 m/s2 =110 N ≈ 24 lb
Gravity of Earth, Sun, Moon
Law of Universal Gravitation
• Gravity is a field force
• It is always an attractive force between two
masses
• The force of gravity between two objects
depends upon…
• How big the masses are
• How far apart they are
Newton’s Law of Universal
Gravitation
• If the masses are bigger, the force of gravity
between them is bigger
• If the distance between the masses is larger,
the force of gravity is smaller
m1m2
F G
2
r
Newton’s Law of Universal
Gravitation
Free Fall
• When the only force acting on an object is
gravity
• In the absence of air resistance, all objects fall
at the same rate…
• Regardless of their masses
Terminal Velocity
• When air resistance acts on an object, it slows
its rate of fall
• Eventually the object will stop accelerating
• Terminal velocity is the velocity of a falling
object when the force of air resistance is equal
to but opposite direction of force of gravity
• In other words, the force of air resistance
balances the force of gravity
Are Astronauts Weightless?
• No
• Astronauts are in free fall
• But the earth is curving away from them as
they fall
• So they don’t fall to Earth
Projectile Motion
• A projectile is a moving object that is under
the influence of gravity only
• Examples
– Baseball, bullet, a hopping toad, a long jumper
– NOT: powered rocket, airplane, etc.
• Projectiles follow a parabolic path (trajectory)
• Projectile motion has both vertical and
horizontal components (parts)
Projectile Motion has Horizontal
and Vertical Components
Projectile Motion