Download Ch. 13 Notes

Chapter 13
Forces and Motion
Section 1
Gravity
• Gravity is the force of
attraction between
two objects
– Depends on size and
distance
– All matter is affected
by gravity
Isaac Newton
• Law of Universal Gravity
– All objects in the universe
are attracted to each
other through gravitational
force
– The magnitude of the
force depends on the
masses of the objects and
the distance between
them
Gravity depends on distance and mass
• Gravity increases as
mass increases
Large objects
have lots of
gravity
Small objects have
little gravity
• Gravity increases as
distance decreases
Close objects have
lots of gravity
Far objects have little
gravity
Mass vs Weight
• Weight is a measure of
gravitational force
– Weight changes as gravity
changes
• More gravity means more
weight
On the
Earth
• Less gravity means less
weight
On the
Moon
• Differences
– Weight is measured in
newtons (N), mass is in
grams (g)
– Weight changes when
gravity is different, mass
remains constant
– Weight is measured on a
scale, mass is measure
using a triple-beam
balance
13.2 Gravity and Motion
Gravity- A force of
attraction between
all objects due to
their masses.
Motion- An objects
change in position
over time when
compared to a
reference point.
• All objects drop at the
same rate due to
gravity.
– Doesn’t matter if
objects are large or
small
– Proven by Galileo
more than 400 years
ago
Air resistance
Acceleration due to gravity
Air resistance
• Acceleration due to
gravity can be
affected by a fluid
friction called air
resistance. The
amount of air
resistance depends
on the size and shape
of the object.
Acceleration due to gravity
Air Resistance
Speed due to Gravity
• All objects accelerate
toward the Earth at a
constant rate.
– 9.8 m/s2
• Remember, we are
ignoring air friction and
the size of the object
doesn’t matter
– Only time is a factor
(how long is the object
falling)
• Velocity of a falling
object
Time
(seconds)
Vg = g x t
Accel. due to gravity
(9.8 m/s2)
• Example #1
– A box falls from an airplane. The plane is
traveling 35,000 feet up at 540 miles per hour.
The box takes one minute to hit the ground.
What is its speed when it hits the ground?
• What do you need?
Vg = g x t
Vg = 9.8 m/s/s x 60 s
Vg = 588 m/s
• What is a penny’s
velocity if it falls for 2
seconds?
• What is a penny’s
velocity if it falls for 10
seconds?
Vg = g x t
Vg = 9.8 m/s2 x 2 s
Vg = 19.6 m/s
Vg = g x t
Vg = 9.8 m/s2 x 10 s
Vg = 98 m/s
• An object falls…
…for 1 second (9.8 m/s)
…for 2 seconds (19.6 m/s)
…for 3 seconds (29.4 m/s)
… for 4 seconds (39.2 m/s)
• Air resistance (air friction)
slows down acceleration
– As objects fall faster
air resistance
increases.
• Without air resistance
objects get faster and
faster due to gravity
– Free fall is when there
is no air resistance.
Terminal Velocity
• The point at which the
downward pull of gravity
and the upward push of
air resistance become
equal.
• The object no longer
accelerates, but falls at a
constant rate the rest of
the way down..
Free Fall
• Free fall occurs when • Objects in orbit are in
gravity is the only force
free fall because
acting on the object. It
there is no air
can only occur in a
resistance.
vacuum or in space.
• Sky divers are not
actually in free fall
before they release
their parachute, they
are still being affected
by air resistance.
Projectile Motion
• The curved path an
object follows when
propelled near the
Earth’s surface
Forward motion
• the downward pull of
gravity
• Forward motion
Gravity
– Involves…
When gravity is the only force
acting on the shuttle…
When forward motion is the only
force acting on the shuttle…
When both gravity and forward
motion forces act on the shuttle…
Pull of Gravity
Forward motion
Section 3
Newton’s Laws
• Newton’s 1st Law
– An object at rest
remains at rest and an
object in motion
remains in motion at a
constant speed and in
a straight line until
acted on by an
unbalanced force.
• Objects at rest
– Objects at rest will not
move unless forced to
• Objects in motion
– Objects that are
moving will continue to
move unless forced to
turn or stop
Forces
Balanced forces
• Forces have both size
and direction.
create no motion
40 N
Unbalanced forces
create motion
50 N
No Motion
10 N
50 N
• Friction’s Effect on
Newton’s 1st Law
– Due to the
unavoidable friction
experienced on Earth
(air friction, friction
with other surfaces)
objects cannot be
seen continuing to
move.
• Examples
– A soccer ball rolling
across a field
– A skateboard (you
don’t have to push all
of the time)
– Key word
• Inertia- the tendency of
an object to resist any
change in motion.
• Newton’s 2nd Law
– The acceleration of an
object depends on the
mass of the object and
the amount of force
applied.
• Small objects need only
a small force to move,
Large objects need a
large force
mass
F=mxa
force
acceleration
F
m
a
F
m
F
a
m
a
• What force is necessary • What is the
to accelerate a 1,250 kg
acceleration of a 7 kg
car at a rate of 40 m/s2?
mass if a force 56 N
is used to move it?
F=mxa
F = 1,250 x 40
F = 50,000 N
F
a
m
56N
 a  8 m /s /s
7kg
F
m
a
• What is the mass of an object if a force of
34N produces an acceleration of 4 m/s/s?
F
m
a
34 N
 m  8.5 kg
4 m/s/s
• Newton’s 3rd Law
– Whenever one object
exerts a force on a second
object , the second object
exerts an equal and
opposite force on the first.
• Action/reaction force pairs
can result in motion or no
motion
– The forces do not act on
the same object.
Otherwise, they would
always cancel out.
• Examples
– Swimming
• Action- pushing on the
water
• Reaction- water pushing
on the hands/feet move
you forward
– Kicking a ball
• Action- foot striking a ball
pushing the ball forward
• Reaction- ball striking the
foot trying to push the foot
back
Since the downward pull of gravity is constant on all objects (regardless
of size or forward velocity) even objects moving forward will fall at the
same rate as objects with no forward velocity.
• Key word
– Momentum- the property of a moving object
that depends on mass and velocity.
• The more momentum an object has the harder it is
to stop.
– Examples
» A full grocery cart verses and empty one.
» A loaded train verses a single car.
» An 8th grader verses a 6th grader.
• Newton’s First Law of Motion
• Newton’s Second Law of Motion
• Newton’s Third Law of Motion
Fluid and Pressure
• Pressure is the amount of
Force exerted on a given Area.
UNITS
Force = Newtons (N)
Area = square meters (m2)
Pressure = Pascals (Pa)
1 Pa = 1N/m2
F
A
P
F
P
A
Mt. Everest= 30,000 Pa
• Atmospheric pressure
- The weight of the Earth’s atmosphere exerts 101,300
N on each square meter (or 101,300 Pa) at sea level.
This is equal to 10 N per sq centimeter. We aren’t
crushed because our body fluids push out with an
equal amount of force.
Deepest ocean depth =
110,000,000 Pa
Deepest ocean depth = 110,000,000 Pa
Sea level= 101,300 Pa
Principles of Flight
Lift
Thrust
Flight requires
some fluid friction
to hold the plane
up
Drag
(air friction)
Gravity (weight)
• A wing (for a bird or a plane) creates LIFT because the
air pressure surrounding it is not the same on each side.
Air over the wing moves faster creating
less pressure on top of the wing.
If the difference between the pressures
above and below the wing (lift)
becomes greater than the weight of the
plane, it will lift off the ground.
A helicopter creates lift by
moving its wings around in a
circle very fast.
A huge cargo plane moves
slower and weighs a lot using
giant wings to create lift.
A fast plane creates lift even
with tiny wings (very little drag)
by going extremely fast.
Chapter 14, section 2
• Buoyancy is the
upward force a fluid
exerts on all matter
– If the upward force of
buoyancy is greater
than the downward
force of gravity
(weight) the object will
float
Determining Buoyant Force
• Archimedes
Principle: “the
buoyant force on an
object in a fluid is an
upward force equal to
the weight of the
Weight of
Displaced
volume of fluid that
Water= 3.0N
object displaces”
• Weight of an object
has nothing to do with Buoyant
the buoyant force.
Force=
3.0N
• The weight of the
displaced water that
matters.
This cruise ship is made of steel and weighs 100,000 tons (200 million
pounds). It floats because the bottom of the ship pushes 100,000 tons
(200 million pounds) of water out of the way.
A hot-air balloon must
displace a large
amount of heavier
cool air to float.
1000 pound
1000 pounds
balloon
air displaced
Amount
of air
displaced
A boat will float if it
displaces the proper
amount of water.
Negative Buoyancy
Weight of
displaced water
= 90 N
Object weight
100 N
Object will sink.
What happens if you change
the volume (increase)?
Neutral Buoyancy
Weight of
displaced
water = 100 N
Object weight
100 N
Object will not move up or down
(Neutral buoyancy).
What happens if you change
the volume (decrease)?
Positive Buoyancy
Weight of
displaced water
= 200 N
Object weight
100 N
Object will float up.
•
To begin preparing for the Chapter 12,
13 & 14 Test…
– Create a page that has all of the necessary
formula that we need to do the following
calculations
1. Speed
2. Acceleration
3. Speed due to gravity 4. Newton’s 2nd Law
5. Buoyancy
- Include examples, all needed units, etc