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Chapter 12
 Newton’s first law of motion - an object at rest
remains at rest and an object in motion maintains
its velocity unless it experiences an unbalanced
force.
 Objects tend to maintain their state of motion.
 Inertia - the tendency of an object to resist being
moved or, if the object is moving, to resist a
change in speed or direction until an outside
force acts on the object.
 Inertia is related to an object’s mass. Mass is a
measure of inertia. Causes all objects to fall with
the same acceleration regardless of mass
 Seat belts and car seats provide protection.
•Because of inertia, you slide toward the
side of a car when the driver makes a
sharp turn.
•When the car you are riding in comes to
a stop, your seat belt and the friction
between you and the seat stop your
forward motion.
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Forces and motion are connected
An object will have greater acceleration if a
greater force is applied to it
The mass of an object and the force applied to
it affect acceleration
Newton’s 2nd Law of Motion – states that the
unbalanced force acting on an object equals the
object’s mass times its acceleration. It connects
force, mass, and acceleration in the equation
a = f / m, acceleration = net force / mass,
same as f = ma
 Force is measured in newtons (N).
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1 N = 1 kg  1 m/s2
Newton’s Second Law
Zookeepers lift a stretcher that holds a sedated
lion. The total mass of the lion and stretcher is
175 kg, and the lion’s upward acceleration is
0.657 m/s2. What is the unbalanced force
necessary to produce this acceleration of the lion
and the stretcher?
1. List the given and unknown values.
Given: mass, m = 175 kg
acceleration, a = 0.657 m/s2
Unknown: force, F = ? N
2. Write the equation for Newton’s second law.
force = mass  acceleration
F = ma
3. Insert the known values into the equation, and
solve.
F = 175 kg  0.657 m/s2
F = 115 kg  m/s2 = 115 N
 Newton’s 2nd law can also be stated as:
The acceleration of an object is proportional to the net
force on the object and inversely proportional to the
object’s mass.
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Friction – force that opposes motion between
two surfaces that are touching each other
◦ Microwelds – areas where surface bumpers stick
together, are the source of friction
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Types of Friction:
◦ Static friction – friction b/n two surfaces that are not
moving past each other
◦ Sliding friction – force that opposes the motion of two
surfaces sliding past each other
◦ Rolling friction – friction b/n a rolling object and the
surface it roll on
◦ Fluid friction – force that opposes motion in a fluid
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Air resistance that opposes the force of gravity
The amount of air resistance depends on an
object’s shape, size, and speed
Terminal velocity – forces on a falling object are
balanced and the object falls with constant speed
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◦ air resistance is equal in magnitude and
opposite in direction to the force of
gravity
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Velocity is constant when air resistance balances
weight.
All matter is affected by gravity.
•Two objects, whether large or small,
always have a gravitational force
between them.
•When something is very large, like Earth,
the force is easy to detect.
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Gravitational force increases as mass increases.
Gravitational force decreases as distance
increases
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Free fall is the motion of a body when only the
force of gravity is acting on the body.
Free-fall acceleration near Earth’s surface is
constant.
•If we disregard air resistance, all objects
near Earth accelerate at 9.8 m/s2.
•Freefall acceleration is often abbreviated
as the letter g, so g = 9.8 m/s2.
Law of Gravitation – any two masses exert an
attractive force on each other, F = G (m1m2)/d2
G = 6.67 x 10-11 m3/kg.s2
Acceleration due to gravity on Earth is 9.8 m/s2
Mercury – 3.8 m/s2
Jupiter – 25.8 m/s2
 Gravity is one of the four basic forces that also
include the electromagnetic force, the strong
nuclear force, and the weak nuclear force
Gravity is a long-range force that gives the
universe its structure
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Weight is equal to mass times free-fall
acceleration.
weight = mass  free-fall acceleration
w = mg
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Weight is different from mass.
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Weight influences shape.
•Mass is a measure of the amount of
matter in an object.
•Weight is the gravitational force an
object experiences because of its mass.
• Gravitational force influences the shape of living things.
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Weight – gravitational force exerted on an object
Decreases as an object moves away from the
Earth
Weight results from a force; mass is a measure of
how much matter an object contains
Objects in the space shuttle float b/c they have
no force supporting them
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Orbiting objects are
in free fall.
The moon stays in
orbit around Earth
because Earth’s
gravitational force
provides a pull on
the moon.
Two motions
combine to cause
orbiting.
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Projectile motion is the curved path an object
follows when thrown, launched, or otherwise
projected near the surface of Earth.
Projectile motion applies to objects that are
moving in two dimensions under the influence of
gravity.
Projectile motion has two components—
horizontal and vertical. The two components are
independent.
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Projectiles have horizontal and vertical velocities
due to gravity, and follow a curved path
Centripetal acceleration – acceleration toward the
center of a curved path
Centripetal force – an unbalanced force, causes
centripetal acceleration
Newton’s 3rd Law of Motion – to every action
force there is an equal and opposite reaction
force
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Action-Reaction forces – act on different objects
and differ from balanced forces
Rocket propulsion is based on Newton’s 3rd law of
motion
Neptune was discovered based on predictions of
gravitational forces and Newton’s laws
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Momentum – related to how much force is needed
to change an object’s motion; p= mv,
momentum = mass x velocity
F = (mvf – mvi) / t - changing momentum
formula
Law of Conservation of Momentum – momentum
can be transferred between objects; momentum is
not lost or gained in the transfer