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Physics
Chapter 2: Force and Newton’s Laws
Section 1: Newton’s First Law
Vocab
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Force
Net force
Balanced forces
Unbalanced forces
Newton’s 1st Law
Friction
Static friction
Sliding friction
Rolling friction
Force
• Force: A push or a pull
– Force = mass x acceleration OR f = ma
– Units = Newtons (N)
f
m
a
Combining Forces
• More than one force can act on an object at the
same time
• When more than one force is acting on an object,
the net force determines the motion of an object
• Net force: combination of all forces acting on an
object
• If the forces are in the same direction  Add
forces together
• If the forces are in opposite directions  Subtract
the forces, the object will move in the direction of
the larger force
Balanced and Unbalanced Forces
• If two or more forces are exerted on an
object they can be balanced or unbalanced
forces
• Balanced forces: The effects of the two
forces cancel each other out
– The object will not move
– Net force = 0
• Unbalanced forces: The effects of the two
forces do NOT cancel each other out
– The object will move in the direction of the
greater force
– The net force is NOT equal to zero
What type of force is acting upon the door? Balanced
Will the door move? No
What is the net force? 0
What type of force is acting upon the door? Unbalanced
Will the door move? Yes
Will the door open or close? Close
Isaac Newton
• 1643 – 1727
• English physicist,
mathematician, astronomer,
natural philosopher, alchemist
and theologian
• In 1687 he published his
Philosophiæ Naturalis Principia
Mathematica
– It is said to be the greatest single
work in the history of science
Newton’s First Law of Motion
• Newton’s First Law of Motion: Objects at
rest remain at rest until a force acts upon it
Friction
• Friction: Force that acts to resist sliding
between two touching surfaces
• To keep an object constantly moving force
needs to be applied to overcome the
effects of friction
3 Types of Friction
1. Static friction: Type of friction that
prevents an object from moving when a
force is applied
– Ex. Trying to push a heavy refrigerator
– Caused by the attraction between the two
surfaces when they are in contact
– To move the object, you have to
exert a force large enough to
break the bonds holding the
surfaces together
2. Sliding friction: Type of friction that slows
down an object that slides
– Ex. Bicycle brakes sliding against a tire
– Friction between the two rough surfaces
causes the object to slow down
3. Rolling Friction: Type of friction needed to
make a wheel or tire turn
– Ex. Keeps a bicycle tire from slipping on the
ground, allows you to walk
– Friction exerts force on a tire/your foot that
pushes the bike/you forward
Physics
Chapter 2: Force and Newton’s Laws
Section 2: Newton’s Second Law
Vocab
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Newton’s 2nd Law
Gravity
Weight
Centripetal force
Air Resistance
Terminal Velocity
Force and Acceleration
• Newton’s 2nd Law connects force,
acceleration, and mass
• Newton’s 2nd Law of Motion: An object
acted upon by a force will accelerate in the
direction of the force
– Acceleration = net force
mass
Units of Force
• Force is measured in Newtons (N)
– 1 N = 1 kg  m/s2
• Because force deals with mass (kg) and
acceleration (m/s2 )
Gravity
• Gravity: A force that pulls objects toward
each other
– Exists between any two objects that have
mass
– Depends on the mass of the objects and the
distance between them
• More mass/less distance = stronger gravitational
force
Weight
• The force of gravity causes all objects near
Earth’s surface to fall with an acceleration
of 9.8 m/s2
• This gravitational force is also called weight
• Weight: Gravitational force between an
object and the Earth
Weight and Mass
• Weight and mass are different
• Weight is a force
– When you stand on a scale you are measuring
the pull of Earth’s gravity (a force)
• Mass is the amount of matter in an object
– NOT a force
Using Newton’s Second Law
• The second law tells you how to calculate
the acceleration of an object if its mass and
the forces acting on it are known
• Speeding Up
– An object will speed up if the force applied to it
is in the direction of motion
• Ex. Applying force (kick) to the ball in
the same direction it was previously
moving will cause it to speed up
The girl speeds up because the force is being
applied in the direction of motion
• Slowing Down
– An object will slow down if a force is applied in
the opposite direction of the object’s motion
• Ex. Sliding friction preventing a book from sliding
across the table after you stop applying force with
your hand
The boy slows down because the force is being
applied in the opposite direction of motion
• Turning
– Forces and motion are not always in a straight
line
– If a net force acts at an angle to the direction of
motion the object will follow a curved path
• Ex. If you shoot a basketball, the ball will NOT
continue to move in a straight line after it leaves
your hand
• Instead it will curve downward as the force of gravity
pulls the ball down
• The ball’s motion is a combination of its original
motion and downward motion due to gravity
Gravity exerts a force downward that makes it move
in a curved path
Circular Motion
• If you are traveling in a circular motion you
are constantly accelerating because you
are constantly changing direction
– According to Newton’s 2nd Law, if you are
constantly accelerating, there must be a force
acting on you the entire time
– The force that keeps the object moving in a
circle is called centripetal force
Satellite Motion
• Objects that orbit the Earth are
satellites of Earth
– Gravity is the centripetal force
• If gravity is pulling satellites towards the
Earth then why don’t the satellites fall
straight down?
– Satellites are being pulled vertically towards
the Earth’s surface.
– They are moving so fast horizontally that the
Earth’s surface curves away from the satellite
as fast as it falls downwards
Air Resistance
• If you are running, walking, or parachuting, air is
pushing against you
• Air Resistance: Form of friction that acts to slow
down any object moving in the air
– Air resistance gets larger as the object moves faster
– When an object falls it speeds up as gravity pulls it
downward
– At the same time air resistance pushes upward on the
object and increases as the object falls faster
– At one point the upward air resistance will equal the
downward force of gravity
– This is called terminal velocity
• When this happens the net force will be 0 (balanced)
Air Resistance and Shape
• Air resistance depends on the shape of an
object as well as the speed
• Moving at the same speed, the air
resistance on a pointed, narrow object is
less than a broad, flat object
Which person would experience the
most air resistance?
Because this person has
increased their surface area
they will experience more air
resistance and fall more slowly
Physics
Chapter 2: Force and Newton’s Laws
Section 3: Newton’s Third Law
Vocab
• Newton’s 3rd Law
• Action-reaction force pair
• Weightlessness
Action and Reaction
• Newton’s first two laws explain how the
motion of a single object changes
• Newton’s 3rd law explains what happens
when an object exerts a force on another
object
• Newton’s 3rd Law: For every action, there is
an equal but opposite reaction
The car jack is pushing up on the car with
the same amount of force with which the car
is pushing down on the jack
Action and Reaction Forces Don’t
Cancel
• Forces exerted by two objects on each
other are often called action-reaction force
pairs
– Either force could be considered the action or
the reaction force
• Action-reaction forces are opposite but they
do NOT cancel each other out
– This is because they act on different objects
When two bumper cars collide, each car pushes on the
other car. According to Newton’s 3rd law each car
pushes on the other with the same force but in opposite
directions. This force causes you to slow down.
When the child pushes against the wall,
the wall pushes against the child.
Large and Small Objects
• Sometimes it is difficult to notice actionreaction force pairs because one object is
so large it appears to remain motionless
– It has so much inertia that it appears to remain
still when a force acts upon it
– Ex. Walking
• When you walk you exert a force upon the ground
• The Earth pushes back on your shoe but the Earth
has so much mass compared to you that it appears
not to have moved at all
A Rocket Launch
• The launching of a rocket is a great example of
Newton’s 3rd Law
• 3 rocket engines supply the force, called thrust,
that lift the rocket
• When the rocket fuel is ignited, a hot gas is
produced
• The gas molecules collide against the walls, the
walls exert a force that pushes them out the
engine
– This downward push is the action force
• There is an upward push on the engine by the
gas molecules which propels the rocket forward
– This is the reaction force
Weightlessness
• Astronauts appear to float around inside of
a space shuttle as it orbits the Earth
• The astronauts are said to be weightless,
as if Earth’s gravity were no longer pulling
on them
• While this is happening the pull of Earth’s
gravity is still 90% as strong
Measuring Weight
• When you stand on a scale, your weight
pushes down on the scale causing the
springs to compress
• The scale pointer moves from zero and
points to your weight
• At the same time the scale pushes up on
you with a force equal to your own weight
(according to Newton’s 3rd Law)
– This force balances the downward pull of
gravity on you
Free Fall and Weightlessness
• Imagine that scale is now inside of an elevator
that is free falling
• An object is in free fall when the only force acting
on it is gravity
• Inside the free falling elevator both you and the
scale are in free fall
• Because the only force acting on you is gravity,
the scale is no longer pushing up on you
– The scale pointer would stay on zero and you would
appear to be weightless
• Weightlessness: The condition that occurs
in free fall when the weight of an object
seems to be zero
– You are not really weightless in free fall
because Earth is still pulling down on you
– With nothing to push up on you, you would
have no sensation of weight
• A: When you stand on a scale on Earth, the
reading on the scale is your weight
• B: If you were to stand on a scale in a
falling elevator, the scale would read zero
Weightlessness in Orbit
• To understand how objects move in the orbiting
space shuttle, imagine you were holding a ball in
the free falling elevator
• If you let go of the ball its position relative to you
and the elevator wouldn’t change because you
are all moving at the same speed
• If you push on the ball you are adding force to it
in addition to the downward pull of gravity so the
ball will drift around until it hits a surface