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For a mass on a string to travel in a circle, a force
must act along the string to overcome inertia.
Without that force, inertia makes the mass move in
a straight line.
Newton's First Law of Motion
“The Law of Laziness”
Newton's first law of motion is often stated as:
• An object at rest tends to stay at rest and an object in
motion tends to stay in motion with the same speed and
in the same direction unless acted upon by an
unbalanced force.
Newton's First Law of Motion
There are two parts to
this statement – one
which predicts the
behavior of stationary
objects and the other
which predicts the
behavior of moving
objects. These two
parts are summarized
in the following
Pass the Water exercise
• the container is at rest and you attempt to move it
• the container is in motion and you attempt to stop it
• the container is moving in one direction and you attempt
to change its direction.
The Car and The Wall
The Motorcyclist
Newton's First Law of Motion
• blood rushes from your head to your feet when riding on a
descending elevator which suddenly stops.
• the head of a hammer can be tightened onto the wooden handle by
banging the bottom of the handle against a hard surface.
• a brick is painlessly broken over the hand of a physics teacher by
slamming the brick with a hammer. (CAUTION: Do not attempt this
at home!)
• to dislodge ketchup from the bottom of a ketchup bottle, the bottle is
often turned upside down, thrust downward at a high speed and
then abruptly halted.
• headrests are placed in cars to prevent whiplash injuries during rearend collisions.
• while riding a skateboard (or wagon or bicycle), you fly forward off
the board when hitting a curb, a rock or another object which
abruptly halts the motion of the skateboard.
Newton's First Law of Motion
Now it’s your turn!
Inertia and Mass:
Things in motion stay in motion – forever!
• Inertia is the resistance an object has to a change in
its state of motion.
Friction slows things down…
Isaac Newton built on
Galileo's thoughts
about motion.
Newton's first law of
motion declares that a
force is not needed to
keep an object in
motion. Slide a book
across a table and
watch it slide to a stop.
Due to the large mass of the books, the force
of the hammer is sufficiently resisted (inertia).
Check Your Understanding
• Imagine a place in the cosmos far from all gravitational
and frictional influences. Suppose an astronaut in that
place throws a rock. The rock will …
• An 2-kg object is moving horizontally with an
acceleration of 4 m/s2. What is the net force acting on
the object?
• If you were in a weightless environment in space, would
it require a force to set an object in motion?
Newton's First Law of Motion
State of Motion
Inertia is the tendency of an object to resist changes
in its velocity.
The Truck and Ladder
Inertia is the tendency of an object to resist accelerations.
Mass – A measure of inertia
• Mass is not Volume
• Mass is not weight
1 kg = 9.8 Newtons
• Weight is the force of gravity on
something that has mass:
– Weight (N) = mass x a (gravity)
= m (9.8) kg-m/s2
• Mr. Wegley spends most Sunday
afternoons at rest on the sofa,
watching pro football games and
consuming large quantities of food.
What effect (if any) does this practice
have upon his inertia?
Newton's First Law of Motion
Balanced and Unbalanced Forces
Newton's First Law of Motion
Balanced and Unbalanced Forces
Types of Forces
All forces (interactions) between
objects can be placed into two
broad categories:
– contact forces, and
– Long-Range forces resulting
from action-at-a-distance
Force is a quantity which is
measured using a standard
metric unit known as the
Newton. One Newton is the
amount of force required to
give a 1-kg mass an
acceleration of 1 m/s2.
Frictional Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance
Applied Force
Spring Force
For each situation, determine the net force
acting upon the object.
Newton's Second Law of Motion
Check Your Understanding
1. What acceleration will result when a 12-N net force is
applied to a 3-kg object? A 6-kg object?
2. A net force of 16 N causes a mass to accelerate at the
rate of 5 m/s2. Determine the mass.
3. An object is accelerating at 2 m/s2. If the net force is
tripled and the mass of the object is doubled, what is
the new acceleration?
4. An object is accelerating at 2 m/s2. If the net force is
tripled and the mass of the object is halved, what is the
new acceleration?
Free Fall with no Air Resistance
Free Fall with Air Resistance
• As an object falls through air, it
usually encounters some
degree of air resistance.
• Air resistance is the result of
collisions of the object's
leading surface with air
• The actual amount of air
resistance encountered by an
object depends upon a variety
of factors.
Free Fall with Air Resistance
• The elephant encounters a smaller force of air
resistance than the feather and therefore falls
• The elephant has a greater acceleration of
gravity than the feather and therefore falls
• Both elephant and feather have the same
force of gravity, yet the acceleration of gravity is
greatest for the elephant.
• Both elephant and feather have the same
force of gravity, yet the feather experiences a
greater air resistance.
Free Fall with Air Resistance
The amount of air resistance is dependent upon
two variables:
•the speed of the skydiver, and
•the cross-sectional area of the skydiver.
Newton’s Third Law: Action-Reaction
A force is a push or a
pull upon an object
which results from
its interaction with
another object.
Forces result from
You can’t touch without
being touched…
Newton’s Third Law: Action-Reaction
"For every action, there is an equal and opposite reaction.“
Forces always come in pairs – equal and opposite actionreaction force pairs.
Newton’s Third Law: Action-Reaction
In the top picture, a physics student is pulling upon a rope
which is attached to a wall. In the bottom picture, the
physics student is pulling upon a rope which is held by the
Strongman. In each case, the force scale reads 500
Newton’s Third Law: Action-Reaction
According to Newton's third law, for every action force there
is an equal (in size) and opposite (in direction) reaction
force. Forces always come in pairs — known as "actionreaction force pairs."
Newton’s Third Law: Action-Reaction
Identify at least five pairs of action-reaction forces in the
following diagram.
Friction Force
• Friction acts in opposite direction of applied force.
• Friction force increases as applied force increases.
• Friction caused by molecular interaction between two
rough surfaces.
Ffriction = μ x Fnorm
• μk = coefficient of kinetic friction
• μs = coefficient of static friction
Static friction/Kinetic friction
• Static friction- Force required to get an object moving
• Kinetic friction-Force necessary to keep an object
• Static friction is greater than kinetic friction
• Videos- phone book friction, why bart and lisa need
Friction Force
A shoe (mass of 1.5kg) is pulled at constant speed across
concrete by its laces (µ= .65). The shoes has a rubber
sole. What force is needed?
Draw free body diagram
List “knowns”
Find the correct (μ) if needed.
Solve by plug and chug.
Double check for direction and magnitude.
Friction practice
• A boy exerts a 36-N horizontal force as he pulls a 52 N
sled across a cement sidewalk at constant speed. What
is the coefficient of kinetic friction between the sidewalk
and the metal sled runners?
• Suppose the sled runs on packed snow. The coefficient
of friction is now only 0.12. If a person weighing 650N
sits on the sled, what force is needed to pull the sled
across the snow at constant speed?
Now you try
• What force is needed to start moving a wooden packing
crate on a wooden jetty? The crate has a mass of 225
kg(μs =.50 μk = .20)
• What force is needed to keep the packing crate moving
at a constant speed on the wooden jetty? The crate has
a mass of 225 kg.
Check your understanding…
• When you walk along a floor, what pushes you along?
• When you jump up, what is the action reaction pair?
• How do rockets travel in space when there is not “air to
push on”?
• Why can’t you hit a feather in midair with a force of 200
• What would a convict’s last wish before a firing squad?
Think and explain….
• An aircraft gains speed during takeoff due to the
constant thrust of its engines. When is the acceleration
during takeoff greatest – at the beginning of the run
along the runway or just before liftoff?
• Why does a sharp knife cut better than a dull knife?
• Why is it easier to walk on a carpeted floor than on a
smooth, polished floor?