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FORCES
AND THE LAWS
OF MOTION
What is a force?
• A force is a push or a pull and
can cause an object to move,
stop, or change direction.
• When 2 forces act together in
the same direction, they are
added. Opposite direction, you
subtract.
• Unbalanced force = movement
• Balanced force = no movement
Forces
What Opposes Motion?
• Friction – will cause a moving
object to slow down and/or
stop. Acts in opposite
direction.
• A. Sliding friction – solid slides
over solid
• B. Rolling friction – wheels or
ball bearings are added; lessens
friction.
• C. Fluid friction – where fluids
slow down falling objects.
More on Friction
Newton’s Laws
• 1665 – 1666 Isaac Newton
developed 3 laws of motion.
• Inertia – the tendency of
objects to remain in motion
or stay at rest. Related to
mass: bigger object =
harder to move or stop.
Law # 1
• 1st law – an object at rest
will remain at rest; an
object in motion will remain
in motion; unless acted
upon by a force.
• Ex: you’re in a car moving
45 mph and you have to
slam on the brakes; the car
stops but you keep moving
forward.
Law # 2
• 2nd law –
• Force (1 N) = Mass (kg) x
Acceleration
• Force is measured in newtons.
• Ex: How much force is needed to
accelerate a 1400 kg car
2m/sec/sec?
1400 x 2 = 2800 N
Newton’s
st
1
and
nd
2
laws…
Law #3
• 3rd law – For every
action, there is an equal
and opposite reaction.
• Ex: walking (you push
down and back but you
move up and forward); a
rocket.
Newton’s 3rd law…
Just cuz it’s funny…
Which law does this represent?
And…why?
Let’s take a poll?
(don’t write any of this)
• If you could drop an elephant
(assume he wouldn’t be hurt) and
a golf ball off the Eiffel Tower;
which would hit the ground first?
• Let’s test your theory… (with a
book and a pencil off the
desk…which hits first?)
Gravity
• Acceleration of a falling object is due to
the force of gravity is 9.8 m/sec/sec.
• 1 meter = 9.8 m/sec/sec
• 2 meters = 19.6 m/sec/sec
• 3 meters = 29.4 m/sec/sec
• Galileo proved this first using
cannonballs of different sizes.
Gravity
Then why doesn’t everything fall
at the same rate?
• Air resistance
• Did you know that terminal
velocity is where sky divers
“float” because the force of
air holding them up equals
the pull of gravity.
Elephant and a feather!
Skydiving…
What did Newton conclude…?
• Newton’s Law of
Universal Gravity –
ALL objects have the
force of gravity.
• Bigger it is…more
gravity it exerts.
Weight and Gravity
• Weight – the measure of the pull of
gravity on an object.
• You weigh less on a mountain top
verses sea level. Why?
• Weight = mass x 9.8m/sec/sec
• Ex: What would the weight be of an
object with a mass of 10kg?
• 10 x 9.8 = 98 N
Weight…
The Wright Brothers
Do NOT write!
• December 17, 1903 Wilbur
and Orville Wright stand
on a deserted beach near
Kitty Hawk, NC. Orville
climbs in their creation and
a 12 horsepower gas engine.
They are about to try
something that no one else
has ever succeeded at
before. They are going to
fly their machine.
HISTORY IS MADE…
Fluid Pressure
• Pressure –is a push/force
and is exerted equally in
all directions.
• Pressure = Force/Area
• Hydraulic Devices – used
to raise heavy objects.
Ex: barbers’ chairs,
brakes on cars, and
rescue ladders.
Differences in Pressure
• Fluids move from high to low
pressure.
• Ex: a vacuum cleaner works by
creating a low pressure zone
causing the high pressure area
outside to rush in carrying all
the particles with it.
How does this affect humans?
Pressure and Gravity
• Pressure and
Gravity –the
pressure of any
fluid varies with
depth. Higher
altitude = lower
pressure and
lower depths =
higher pressure
Shoulder Partner
• Why would this
happen?
• When divers go too deep
and then resurface too
quickly their body rids
of nitrogen bubbles too
quickly causing “the
bends”. This is quite
painful and dangerous.
Divers must surface
slowly to prevent this.
The Bends
Buoyancy
• Buoyant force- upward
force that acts against
the downward weight
of the object.
• Archimedes Principle the buoyant force on
an object is equal to
the weight of the fluid
displaced.
Salt Lake
Floating…
• An object floats when it
displaces a volume of
fluid whose weight is
greater than or equal to
its own weight.
Archimedes Principle
More on floating…
• An object will also float if the density of
that object is less than the density of the
fluid.
• Density of water is 1g/cc
• Wood is .8 g/cc
• Aluminum is 2.7 g/cc
• Which floats the wood or the aluminum?
So…what really controls
buoyancy?
Applying the Concepts…please
explain the following scenarios.
Why can this fly?
Why did one sink and the other
did not?
Flight
• Bernoulli’s principle – (in simple
terms) says that moving air creates
lower pressure than the area around it.
• Ex: smoke goes up a chimney because
wind blows by the top, you have to run
to fly a kite.
• An airplane wing is designed so that
air passing over the wing travels faster
than air passing beneath it. Causing
high pressure under the wings to hold
the plane up…
DEMO
More on Bernoulli’s
Hovercraft race…