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Motion and Forces
Motion
 Many of the objects we encounter in everyday life
are in motion or have parts that are in
motion. Motion is the rule, not the
exception. The physical laws that govern the
motion of these objects are universal, i.e. all the
objects move according to the same rules.
Newton’s Laws of Motion
• 1st Law – Law of Inertia
• 2nd Law – Force = Mass x Acceleration
• 3rd Law – For every Action, there is a
Reaction
Everything to do with motion, forces,
gravity, speed, or acceleration is governed
by these laws.
Newton’s First Law of Motion
“Law of Inertia”
• An object will stay at rest
unless acted on by an
unbalanced force
• An object in motion will
stay in motion unless acted
on by an unbalanced force
What happens when a car
hits a stationary object?
What happens to you
riding in that car?
Inertia
Bike Race
Crash
Car crashes and
seatbelts:
Physics of
car crashes
Why wear
Seatbelts?
Inertia
• Inertia – the tendency of an
object to resist change in
motion (whether at rest or
moving)
• Table Cloth Inertia
• Egg Drop Inertia
What does this mean?
• An object will “keep doing what it was doing”
unless acted on by an unbalanced force
• If the object was sitting still, it will remain
stationary (still)
• If it was moving at a constant velocity, it will
keep moving
• It takes force to change the motion of an
object
Mass and Inertia
 The amount of inertia an
object has depends on its
mass
 The greater the mass the
greater the inertia
 Mass is measured in
grams or kilograms.
 Eureka video
Inertia at work
• Objects with a
greater mass have
greater inertia
• Example: It is more
difficult to change
the motion of
bowling ball than
the motion of a golf
ball
Why?
• The bowling ball is much heavier (it has more mass)
than the golf ball.
• It takes more force to move the bowling ball and also
to stop it
What about changing it’s direction?
• Again, the bowling ball has more mass and it took
more force to get it going. However, now that it has
accelerated, changing the speed or direction is
difficult!
Friction and Gravity
• Newton’s First Law of
Motion states that an
object in motion will
stay in motion unless
acted on by an outside
force.
• Two forces that act on
objects are friction and
gravity.
Friction
 Friction acts in a direction
opposite to the object’s
direction of motion
 Without friction, the
object would continue to
move at a constant speed
forever
 The strength of the force of
friction depends on two
factors:
1. the types of surfaces
2. how hard the surfaces
push together.
Racing on ice
Starting a fire
Types of Friction
1. Sliding Friction
– solid surfaces
slide over each other.
2. Rolling Friction – when an
object rolls over a surface.
• Force of friction is less in
rolling friction than with
sliding friction.
3. Fluid Friction – when an object
moves through a fluid.
• Force of friction is less in
fluid friction than with
sliding friction.
Gravity
• Gravity – is the force that pulls objects toward
each other. The larger object has more gravity.
Air Resistance
 Objects falling through air
experience a type of friction
called air resistance
 As surface area increases, air
resistance increases.
 Eventually, air resistance
equals gravity
 The greatest velocity an object
reaches is called terminal
velocity
 In a vacuum, all objects fall at
the same rate
Force, Mass, and Acceleration
Weight vs. Mass
• Weight – a measure of the force of gravity on an
object. (weight is a force)
• Your weight on the moon is 1/6 of your weight on
Earth – the moon is 1/6 the size of Earth
• Mass – a measure of the amount of matter in that
object.
Weight = Mass x Acceleration due to gravity
• Units of Measure:
• Weight – measured in Newtons
• Mass – measured in Kg
• Acceleration – measured in m/s2
Newton’s Second Law of Motion
•When a net force acts on an object the object will
accelerate in the direction of the net force.
Unit of measurement for force: A “Newton” - is the
force needed to accelerate one kg of mass at 1
meter/sec 2 (1 N = 1kg x 1m/s2)
Newton’s 2nd Law
Inversely Proportional
• Mass and acceleration are
Inversely proportional:
• As the mass increases,
the acceleration
decreases
Directly Proportional
• Force and acceleration are
Directly proportional:
• As the force increases,
the acceleration
increases
Push the shopping cart
Shopping Cart Example:
Graphing Newton’s 2nd Law
• The graphs show the
relationship between
force, mass and
acceleration
(F = ma)
• For the same force:
• more mass causes
less acceleration
• less mass causes
more acceleration
Graphing Newton’s
• For the same mass:
• more force
causes more
acceleration
• less force causes
less acceleration
nd
2
Law
What does F = ma say?
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force.
Something very small changing speed
very slowly will have a very weak force.
A Little Practice
• How much force will you need to accelerate a 200kg
wagon at 5m/s2?
 1000
N
• How much force will you need to accelerate a
10,000kg train at 5m/s2
 50,000
N
Newton’s Second Law Summary
Newton’ s
rd
3
Law of Motion
• For every action there
is an equal and
opposite reaction
• All forces occur in pairs,
and these two forces
are equal in strength
and opposite in
direction.
What does this mean?
•Right now, gravity is
pulling you down in your
seat, but Newton’s Third
Law says your seat is
pushing up against you
with equal force.
•This is why you are not
moving. There is a
balanced force acting on
you– gravity pulling down,
your seat pushing up.
Equal but Opposite
• Ice Skaters (push off of each
other with equal force in
opposite directions):
The speed at which the two
skaters move depend on their
masses. (the greater the
mass the slower the speed)
But there are always 2 forces!!
Think about it
What happens if you are standing on a
skateboard and push against a wall? You
slide in the opposite direction (away from the
wall), because you pushed on the wall but the
wall pushed back on you with equal and
opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe
against it, the more force the rock exerts
back on your toe (and the more your toe
hurts).
Check Your Understanding
• While driving down the road,
an unfortunate bug strikes the
windshield of a bus.
• The bug hits the windshield
and the windshield hits the
bug.
• Which of the two forces is
greater:
• the force on the bug or
• the force on the bus?
Answer
•Trick Question!
• Each force is the same size.
• For every action, there is an equal ... (equal!).
• The fact that the bug splatters only means that with
its smaller mass, it is less able to withstand the
larger acceleration resulting from the interaction.
Check Your Understanding: Sports
• The racer’s foot is pushing on the starting block, and the starting
block is pushing back
• When the runner pushes off, the motion is the result of actionreaction force pairs.
• The runner accelerates slowly at first and then faster and faster
as the mass is carried along
More explanation:
• The force on the foot equals the force on the
block. (Newton’s 3rd Law)
• Yet, acceleration depends on both force and
mass. (Newton’s 2nd Law)
• Remember: acceleration and mass are inversely
proportional. (Newton’s 2nd Law)
Check Your Understanding: Cars
• According to Newton's First Law of Motion, an object at rest tends
to stay at rest and an object in motion tends to stay in motion,
unless acted upon by an unbalanced force.
• A car will tend to either keep standing still or keep moving in a
straight line at the same speed unless some unbalanced force
makes it do otherwise.
• Examples of unbalanced forces are air resistance or the friction
between tires and the road
Check Your Understanding: Cars
• Newton’s 2nd Law is F = ma
• If F = ma, then a = F/m. The acceleration of a car equals the sum of
the external forces applied to the car, divided by the car's mass.
• For a car of any given mass, the more unopposed force is applied,
the faster the car accelerates
• If the driver applies the brakes and produces enough friction to
stop the car, then F = 0. In this case, acceleration also equals zero,
because zero divided by anything equals zero.
Check Your Understanding: Cars
• Newton’s 3rd Law states: for action there is an equal and opposite
reaction
• The force exerted by the tires on the road is matched by an equal
and opposite force exerted by the road on the tires. If that force is
enough to overcome the car's inertia, the car accelerates forward.
Review
• Newton’s First Law (Inertia):
• Objects in motion tend to stay in motion and objects at rest
tend to stay at rest unless acted upon by an unbalanced force.
• Newton’s Second Law (F=ma):
• Force equals mass times acceleration
(F = ma).
• Newton’s Third Law:
• For every action there is an equal and opposite reaction.
Physics Essay
Let’s look at
the project
Final due date
is: March 7-8.
Time to Explore!
Find your Lab page
Let’s try out Newton’s work for
ourselves!
• Find
this lab
page:
Wacky Washers!
• Make sure leave each
station set up and
ready for the next
group!
• Read the instructions
and answer the
questions in
complete sentences!
Drop it!
• Try to get the washer in the cup!
Rocket Balloon
• Follow the directions on this one!! Fill out the
data table and then answer the questions.
Speed racer:
• You will need to set up
two ramps and send the
cars down the ramp
• Answer the questions in
relation to what you
have learned about
Newton’s Laws