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Newton’s Laws
Unit 2
Contributors to the theory of
motion
Aristotle
Divided motion into two types:
natural and violent
Natural motion- either straight up or
straight down
Violent motion- the result of forces
that pushed or pulled
Copernicus
Reasoned that the Earth moved
around the sun
Galileo
Supported Copernicus
Force: any push or pull
Friction: force that acts between
materials that touch as the move
past each other
Argued that only when friction is
present is a force needed to keep an
object moving
Inertia: the property of a body to
resist change
Newton
Developed the famous laws of
motion
Newton’s First Law
Law of Inertia:
Every object continues in a
state of rest, or of motion in a
straight line at constant
speed, unless it is compelled
to change that state by forces
exerted upon it.
The more mass an object has, the greater
its inertia & the more force it takes to change
its state of motion
Mass is different than volume &
weight.
mass: a measure of the inertia an
object has
Volume: a measure of space
Weight: the force of gravity on an
object
weight= mass x gravity
The combination of all forces acting on
an object is called the net force.
It is the net force that changes an
object’s state of motion.
When an object is at rest, forces balance &
produce a net force of zero. One force is
gravity. The other force is the normal force.
This means that the object is in a state of
equilibrium.
Felicia, a ballet dancer, has a mass
of 45kg.
What is Felicia’s weight on Earth?
What is Felicia’s mass on Jupiter,
where the acceleration due to
gravity is 25 m/s2?
What is Felicia’s weight on Jupiter?
A 20g sparrow flying toward a
bird feeder mistakes the pane of
glass in a window for an
opening and slams into it with a
force of 2N. What is the bird’s
acceleration?
A 30g arrow is shot by William
Tell through an 8cm thick apple
sitting on top of his son’s head.
If the arrow enters the apple at
30m/s and emerges at 25m/s in
the same direction, with what
force has the apple resisted the
arrow?
Newton’s Second Law
Force = mass x acceleration
Force causes acceleration.
To increases the acceleration of an
object, you must increase the net
force acting on it.
The acceleration depends on the
mass.
Acceleration is inversely
proportional to the mass.
Force= mass x acceleration
Friction acts on materials that are in contact
with each other. It always acts in a direction
to oppose motion. The force of friction
between surfaces depends on the kinds of
materials in contact & how much the
surfaces are pressed together.
Friction also occurs in liquids & gases. Fluid
friction occurs as an object pushes aside the fluid it
is moving through. An example is air resistance.
When friction is present, an object may move with
a constant velocity even when an outside force is
applied to it. The friction force just balances the
applied force so the net force is zero and no
acceleration occurs.
Pressure= force/ area of application
The smaller the area supporting a
given force, the greater the pressure
on that surface.
Measured in pascals (Pa)
One Newton per square meter equals
one pascal
Galileo showed that objects in
free fall accelerate equally
regardless of their masses.
A 10kg cannonball and a 1kg
rock would fall with equal
accelerations- 10 m/s.
When air resistance is present, a falling object accelerates
only until it reaches its terminal speed. At terminal speed,
the force of air resistance balances the force of gravity &
the net force is zero
Acceleration= weight - resistance
Rose is sledding down an icecovered hill inclined at an angle
of 15 degrees to the horizontal.
If Rose and the sled have a
combined mass of 54kg, what is
the force pulling them down the
hill?
Brian is walking through the school
cafeteria but does not realize that
the person in front of him has just
spilled his glass of chocolate milk.
As Brian, who weighs 420N, steps
in the milk, the coefficient of sliding
friction between Brian and the floor
is suddenly reduced to 0.040. What
is the force of sliding friction
between Brian and the slippery
floor?
While redecorating her apartment,
Kitty slowly pushes an 82kg china
cabinet across the wooden dining
room floor, which resists the motion
with a force of friction of 320N.
What is the coefficient of sliding
friction between the china cabinet
and the floor?
At Sea World, a 900kg polar bear
slides down a wet slide inclined at
an angle of 25 degrees to the
horizontal. The coefficient of friction
between the bear and the slide is
0.0500. What frictional force
impedes the bear’s motion down
the slide?
Newton’s Third Law
Whenever one object exerts a force on a
second object, the second object exerts
an equal and opposite force on the first
object.
Newton realized that a force is not a
thing in itself but part of a mutual
action, an interaction, between one
thing & another.
In every interaction, the forces always
occur in pairs & cannot exist without
each other.
One force is called the action force.
The other is called the reaction force.
Action: Object A exerts a force on Object B
Reaction: Object B exerts a force on Object A
Remember that action-reaction force pairs
are equal but opposite in strength. We must
also take mass into account. A given force
exerted on a small mass produces a greater
acceleration than the same force exerted on
a large mass.
Newton’s Third law helps us to
explain how rockets are
propelled, how helicopters get
their lifting force, and how birds
and airplanes fly.
Since action and reaction forces
are equal but opposite, why
don’t they cancel to zero?
If action & reaction forces are internal to a system,
they cancel each other & produce no acceleration
of the system. A force external to the system is
needed to accelerate the system.
Flip, an exhausted gymnast,
hangs from a bar by both arms
in an effort to catch his breath. If
Flip has a mass of 65kg, what is
the tension in each of Flip’s
arms as he hangs in place?
At an art auction, Whitney has
acquired a painting that now
hangs from a nail on her wall. If
the painting has a mass of
12.6kg, what is the tension in
each side of the wire supporting
the painting?
Michelle likes to swing on a tire tied
to a tree branch in her yard. If
Michelle and the tire have a
combined mass of 82.5kg, and
Elwin pulls Michelle back far
enough for her to make an angle to
30 degrees with the vertical, what
is the tension in the rope
supporting Michelle and the tire?
After returning home from the
beach, Samantha hangs her
wet 0.20kg bathing suit in the
center of the 6m long
clothesline to dry. This causes
the clothesline to sag 4cm.
What is the tension in the
clothesline?
Brooke comes home from
school and puts her books down
on the kitchen table while she
goes to grab a snack. The
books have a combined weight
of 25N and the area of contact
is 0.19m by 0.24m. What
pressure do the books apply on
the table?
A full coffee mug has a mass of
0.6kg and an empty mug has a
mass of 0.3kg. If the full mug
applies a pressure of 1200Pa, what
is the area inside a circular ring of
coffee left on the table by the
bottom of the mug?
What is the radius of the ring of
coffee?