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Laws of Motion
Chapter 11
Newton’s First Law
an object at rest remains at rest and
 an object in motion maintains its velocity
(stays in motion) unless it experiences an
unbalanced force (outside force).
 Aka Law of Inertia

INERTIA
tendency of an object to resist being
moved or
 if object is moving, to resist a change in
speed or direction until an outside force
acts on object.
 All objects resist changes in motion
 related to an object’s mass.
 Objects with small mass have less inertia
than objects with large mass

Chapter 11
INERTIA EXAMPLE

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.
Newton’s Second Law



unbalanced force acting on an object equals
object’s mass times its acceleration.
Force = mass  acceleration
F = ma
SI Unit: newtons (N)

1 N = 1 kg  1 m/s2
F
m a
Chapter 11
Newton’s Second Law
FORCE
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
GIVEN:
WORK:
lion and the stretcher?
F=?
m= 175 kg
a = 0.657 m/s2
F
ma
F = ma
F = (175 kg) (0.657 m/s2)
F = 115 N
FORCE
What is the net force necessary for a 1.6 x 103
kg automobile to accelerate forward at 2.0
m/s2?
GIVEN:
WORK:
F=?
m= 1.6 x 103 kg
a = 2.0 m/s2
F = ma
F
ma
F = (1.6 x 103 kg) (2.0 m/s2)
F = 3.2 x 103 N
FORCE
A baseball accelerates downward at 9.8 m/s2.
If the gravitational force is the only force acting
on the baseball and is 1.4 N, what is the
baseball’s mass?
GIVEN:
WORK:
F = 1.4 N
m= ?
a = 9.8 m/s2
m = F/a
F
ma
m = 1.4 N / 9.8 m/s2)
m = 0.14 kg
FORCE
A sailboat and its crew have a combined mass
of 655 kg. Ignoring frictional forces, if the
sailboat experiences a net force of 895 N
pushing it forward, what is the sailboat’s
acceleration?
GIVEN:
WORK:
F = 895 N
m= 655 kg
a=?
F
ma
a = F/m
a = 895 N / 655 kg
F = 1.37 m/s2
GRAVITY
Chapter 11
Law of Universal Gravitation



All objects in universe attract each other through
gravitational force
Sir Isaac Newton (1642–1727)
Universal Gravitation Equation
F G
•
•
•
m1m2
d2
m1 and m2 are masses of two objects
d distance between two objects
G constant
Law of Universal Gravitation
•
All matter is affected by gravity.
Two objects, whether large or small,
always have a gravitational force between
them.

Gravitational force increases as mass
increases.

Gravitational force decreases as distance
increases.

Free Fall Acceleration
motion of a body when only force of
gravity is acting on body.
 constant near Earth’s surface.
 g =9.8 m/s2

Weight

equal to mass times free-fall acceleration.

weight = mass  free-fall acceleration
w = mg

Weight is different from mass.
Mass
• measure of amount of matter in an object.
Weight
• gravitational force an object experiences
because of its mass.
• influences shape
Terminal Velocity

constant velocity of a falling object when force of
air resistance is equal in magnitude and opposite
in direction to force of gravity
Two Motions Cause Orbiting
Chapter 11
Projectile Motion and Gravity

curved path an object follows when thrown,
launched, or otherwise projected near
surface of Earth.

applies to objects that are moving in two
dimensions under the influence of gravity.

has two components
horizontal and vertical.
two components are independent.


Chapter 11
Projectile Motion
Chapter 11
Newton’s Third Law

for every action force, there is an equal and
opposite reaction force.

Forces always occur in action-reaction pairs.
Action-reaction force pairs are
 equal in size and
 opposite in direction.
Chapter 11
Action and Reaction Forces

Force pairs do not act on the same object.

Equal forces don’t always have equal effects.
example, the action force of Earth
pulling on an object and causing it to
fall is much more obvious than the
equal and opposite reaction force of
the falling object pulling on Earth.
Chapter 11
Momentum

quantity defined as product of the mass and velocity of an
object.
momentum = mass  velocity
p = mv

Moving objects have momentum.
• the more mass an object has, the greater its momentum
is.
• faster an object is moving, the greater its momentum is.
• When you force an object to change its motion, you
force it to change its momentum.
Momentum
Calculate the momentum of a 6.00 kg
bowling ball moving at 10.0 m/s down the
alley toward the pins.
GIVEN:
WORK:
p=?
m= 6.00 kg
v = 10 m/s
p = mv
p = (6.00 kg) (10 m/s)
p
mv
p = 60 kg x m/s down the
alley
Momentum
A 75 kg speed skater moving forward at 16
m/s. Calculate the momentum.
GIVEN:
WORK:
p=?
m= 75 kg
v = 16 m/s
p = mv
p = (75 kg) (16 m/s)
p = 1200 kg x m/s forward
p
mv
Momentum
A 135 kg ostrich running north at 16.2 m/s.
Calculate the momentum.
GIVEN:
WORK:
p=?
m= 135 kg
v = 16.2 m/s
p = mv
p = (135 kg) (16.2 m/s)
p
mv
p = 2190 kg x m/s north
Momentum
Calculate the velocity of a 0.8 kg kitten with
a momentum of 5 kg x m/s forward.
GIVEN:
WORK:
p = 5 kg x m/s
forward
m= 0.8 kg
v=?
p
v = p/m
v = 5 kg x m/s / 0.8 kg
mv
v = 6.3 m/s forward
Chapter 11
Law of Conservation of Momentum
•
total amount of momentum in an isolated system
is conserved.
•
•
•
•
•
When a moving object hits a second object, some
or all of the momentum of first object is transferred
to second object.
Momentum can be transferred in collisions, but
total momentum before and after a collision is
same.
rocket propulsion.
Cars collided
Billiards