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Physics
Newton’s Laws of Motion
Newton’s First Law of Motion
• Every object continues in its state of rest, or a
uniform speed in a straight line, unless acted
on by a nonzero force
– Restating Galileo’s concept of Inertia
When you pull out a tablecloth, the dishes
are left in their initial state of rest.
When a driver slams on the breaks, the
car stops faster than the driver’s body,
which causes the body to lurch forward
causing whiplash.
Newton’s Second Law of Motion
• The acceleration produced by a net force on
an object is directly proportional to the net
force, is in the same direction as the net force,
and is inversely proportional to the mass of
the object.
– Units
• Mass: kg
• Force: N
• Acceleration: m/s2
Newton’s Second Law of Motion
If the mass increases, to produce the same
amount of acceleration, what must you do
to the force?
If the force on an object doubles and the mass
does not change, what happens to the
acceleration?
Newton’s Second Law of Motion
• If a force acts opposite the direction of motion,
then the object will slow down
– deceleration
– ex: using the breaks in a car
• If a force acts in the same direction as the
direction of motion, then the object will speed
up
– ex: putting your foot on the gas
When acceleration is g
Free Fall
• Free fall: When the force of gravity is the only
force acting on an object (air resistance is
negligible)
a = g = 9.8m/s2
• Objects of different masses fall at the same rate
The elephant has a greater gravitational
attraction than the person. BIGGER “F”.
BUT, the elephant also has a bigger
mass proportional to that BIGGER F.
BIGGER “m”.
That means that the BIGGER “F”
cancels the BIGGER “m” and the objects
fall at the same acceleration.
When acceleration is less than g
Non-Free Fall
• Most of the time, air resistance is NOT
negligible.
– Air resistance
• Force of friction acting between object and air
• Depends on SPEED and SURFACE AREA
Increase Speed,
increase air
resistance….
Increase Surface Area, increase air resistance
Gliding
Increase Surface Area in Nature
“Flying” squirrels have
large flaps of skin
“Flying dragons” (lizards in
genus Draco) have long ribs
that support gliding
membranes
“Flying” frogs have very
large toes with webbing
between them
Increase Surface Area, Increase Air Resistance, Slow Your Fall!
Non-Free Fall
• Air resistance decreases the acceleration due
to gravity
– Terminal speed: Object is no longer accelerating
downward – falling at a constant rate
• Human skydiver: 150 – 200km/hr
• Parachute increases surface area, which slows
terminal speed to 15-25km/hr
Removing Air Resistance
• Air Resistance:
– Coin falls faster than feather
because the feather has less
mass and greater surface area
• Remove Air Resistance
(Vacuum)
– Coin and feather fall at the same
rate
Forces and Interactions
• A force is an interaction between one object
and another
When you push on a wall, the
wall pushes back on you!
Newton’s Third Law of Motion
• Whenever one object exerts a force on a
second object, the second object exerts an
equal an opposite force on the first
– Action and Reaction Forces
• Equal in strength, opposite in direction
When a boxer hits a bag, he
exerts a force on the bag. The
bag exerts an equal force on the
boxer opposite the original force
Boxer = Action Force
Bag = Reaction Force
Newton’s Third Law of Motion
You swing an ax and
get it stuck in a
stump…
What was the Action Force?
What was the Reaction Force?
Action and Reaction on
Different Masses
• Falling objects pull upward on Earth with as
much force as the Earth pulls downward on it!
Because the Earth has such a great
mass, we can’t see the acceleration
Action and Reaction on
Different Masses
• When you fire a cannon, the cannonball
exerts and equal an opposite force on the
cannon.
Why does the cannon
move very little, but the
cannonball flies very far?
The cannon has a large mass,
the cannonball has a small
mass.
Defining Your System
• Since action and reaction forces are equal
and opposite, why don’t they cancel to zero?
– When we have action and reaction systems, they
are isolated from other forces. These other
forces can cause acceleration!
Scalars and Vectors
• Scalar quantity: Quantity that describes a
magnitude
– Ex: mass, volume speed
• Vector quantity: Quantity that is described by
BOTH magnitude AND direction
– Ex: velocity, force, acceleration
– Can be shown by drawing arrows
The Size of the arrow
corresponds to the
magnitude
Bigger quantities =
Bigger Arrows
Adding Vectors
• Resultant: Sum of two or more vectors
– Same direction, add
– Opposite direction, subtract
Adding Vectors
When Newton’s Laws are not valid
• Objects moving near the
speed of light
• Objects that are very
small – on the scale of an
atom
• Objects under the
influence of very strong
gravitation forces