Forces and Motion

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Energy
Energy is the ability/potential to do work (move
matter).
When matter interacts energy is transferred back
and forth.
Something with a lot of energy could do a lot of
work Or To do a lot of work (moving something
massive), it requires a lot of energy
2
Energy associated with the motion and position of
objects is called mechanical energy. It is split
into two types, kinetic energy and potential
energy.
Kinetic energy is the energy of motion,
describing how an object moves. Potential energy
is the energy of position, describing the
potential an object has to move as a result of
its position in the universe.
Mechanical energy Kinetic energy Potential
Energy
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Kinetic energy depends on the mass of an object
and how fast it is moving (its velocity).
Potential Energy comes in two forms, elastic
potential and gravitational potential.
Elastic potential energy is an objects potential
to spring back after being stressed
somehow. e.g- stretching rubber band, drawing
back a bow, compressing a spring
Gravitational potential energy is an objects
potential to move as a result of gravity (to
fall). It depends on the height of an object and
its weight (mass and force of gravity)
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When an object falls or an elastic is released,
its potential energy is converted into kinetic
energy, but the total amount of energy stays the
same.
When the ball hits the ground and stops moving,
its mechanical energy is gone. Where did it go?
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Energy that appears to be lost is really just
transferred into a new form. There are seven
forms of energy (e.g. thermal, sound,
radiant/light)
Energy is converted from one form to another
constantly and is involved in nearly every event
or occurrence in the universe
This year well be focusing on motion.
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Forces and Motion
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How can we describe motion?
How can we measure it?
We can measure how fast an object is moving, how
much its speed is changing, or how hard it will
be to stop.
How fast something goes is actually how far it
goes in a period of time. This is speed.
Speed distance / time
The base unit used for speed is the unit for
distance (meters) and time (seconds) together, so
meters/second or m/s. -prefixes can be added to
the meters (e.g. kilo-, hecto, centi.
milli) -and seconds can be converted to minutes
or hours
Since few things move at a constant speed,
calculations are done of an objects average speed
not its instantaneous speed.
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Speed is often shown graphically on distance vs.
time graphs.
These are done on a traditional coordinate plane,
but you usually only need to use Quadrant I.
Time is always the x-axis Distance is always the
y-axis
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Velocity is more useful than speed, it always has
the same numerical value, but it also tells you
in which direction the object is moving.
Example Speed 2 m/s Velocity 2 m/s south
Directions are given using the same directions as
appear on a compass.
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What is acceleration?
Acceleration describes how an objects velocity
is changing (speeding up, slowing down, or
changing direction).
If acceleration is greater than 0, then speed is
increasing. If acceleration is 0, then speed is
staying the same. If acceleration is less than 0,
then speed is decreasing.
A car in cruise control can be going 100km/h, but
it will have an acceleration of 0, since its
speed isnt changing.
Since it describes how much m/s has changed in a
certain amount of time (s), the unit is m/s².
The formula is average acceleration Final
velocity initial velocity
Time or a ?v / t
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Since acceleration can also indicate a change in
direction, an object going in a circle is
constantly accelerating even if its speed doesnt
change.
A graph of acceleration would be similar to one
for velocity, but instead of distance vs. time it
would be velocity vs. time.
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Try a few calculations
A car pulls out of the driveway, accelerating
from parked to 12 m/s in 4 seconds. A person
after a sprint slows down from 8 m/s to 0 m/s in
4 seconds. A person running uphill slows down
from 8 m/s before the hill to 4 m/s on the hill
in 1 second. A baseball thrown at 15 m/s slows
down to 12 meters per second in 2 seconds. A
football punted straight up slows down from 19.6
m/s to 0 m/s as it climbs in 2 seconds. The
same football then falls, speeding up from 0 m/s
to 19.6 m/s before it hits the ground in 2
seconds.
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Challenges
A car drives 10 meters in 2 seconds. 5 seconds
later it is going 20 m/s. What was its
acceleration? If a car accelerated at 10 m/s²,
starting at 15 m/s, how long would it take to get
to 65 m/s? If the breaks on a car allow it to
accelerate at -15m/s². How long would it take it
to come to a complete stop if going 60 m/s?
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How hard an object will be to stop, how much
power it has as a result of its motion, is
related to its amount of kinetic energy.
Kinetic energy ½ mv²
Momentum (m)(v)
Just like energy is conserved, momentum is too.
The law of conservation of momentum states that
when objects collide the total amount of momentum
remains the same. -the momentum of each object
can change, but the sum will remain the same.
What happens when two objects of equal mass and
velocity collide? What happens when a large
object and small object at equal velocity
collide? What happens when a moving object
collides with a stationary object? What changes
when objects collide, mass or velocity?
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What is a force?
A force is a push or pull.
A force tries to move an object in a particular
direction, so they not only have a size
(magnitude or strength) but a direction.
The unit for force is the Newton (N).
Any change in the state of motion of an object is
the result of the sum of all the forces acting on
it.
The result, the combination of all these forces
is called the net force.
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Forces that act in the same direction add
together. Forces that act in opposite directions
take away from each other. Forces acting at
angles can partially add or subtract from each
other, depending on the angle.
If the net force works in one direction more than
others the object will move. If they add up to
zero, then it will not.
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Two of the most common and important forces are
friction and gravity. -gravity works whenever
two objects with mass are near each
other. -friction works whenever two objects
touch each other.
If two objects are touching and a force is
applied, friction works in the opposite direction
to resist movement. -if friction is strong
enough to keep the object from moving it is
called static friction. -After the object starts
moving it is called kinetic friction.
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The strength of friction depends on how much the
objects are being pressed together (usually
weight pressing it to the ground) and the
structure of the surfaces in contact.
Rough surfaces create more friction than smooth
surfaces.
Energy used fighting friction is wasted energy,
so most machines are designed to minimize
friction (car oil).
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Gravity is the attractive force that works
between bodies of mass.
The law of universal gravitation states that
gravitational force increases as mass increases
and the distance between objects decreases.
Force of gravity (Gravitational constant)
__(Mass1)(Mass2)__ (Distance between
objects)²
Your weight is the force of gravity between you
and the earth.
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How fast an object falls is a result of the force
of gravity.
As an object falls it gets closer to earth, so
the force of gravity increases. Increased force
means that the object will accelerate.
Free falling objects all accelerate at a constant
rate (ignoring wind resistance). -9.8 m/s² g
The velocity of a free falling object is this
value multiplied by time. ?v (g)(t)
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Orbital motion is the net of gravity and the
forward motion an object already has (its
inertia).
Projectile motion- How far a ball thrown
forwards goes before hitting the ground is
decided by the net forces of gravity and
horizontal velocity (how hard you threw it).
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Newtons First Law of Motion
An object at rest remains at rest, and an object
in motion remains in motion at constant speed and
in a straight line unless acted on by an
unbalanced force.
Basically, an object wont accelerate in anyway
unless a force makes it.
This is sometimes called the Law of Inertia,
because it describes an objects resistance to a
change in its state of motion.
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Newtons Second Law of Motion
The acceleration of an object depends on the mass
of the object and the amount of force applied.
The law is summarized by the following equation.
F ma
So force equals mass times acceleration
What happens to force if mass increases?
Decreases? What happens to force if acceleration
increases? Decreases? What happens to
acceleration if force increases?
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Newtons Third Law of Motion
Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first.
If you jump off of something high, it hurts
because the ground hits you as hard as you hit it.
A rocket works because as it pushes the gases
downwards, they push it upwards.
This is why hitting things hurts
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Fluid substances also exert force on any object
in them. -This is why some things float while
others sink and why temperature changes with
elevation.
What forces are acting on an object floating in
water?
The two main ones are gravity and buoyant force.
Gravity, or the objects weight, pushes it
down. Buoyant force pushes it up.
Weight depends on the mass of the object. Buoyant
force depends on its volume. So, whether an
object floats or not depends on its density.
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Archimedes Principle- The buoyant force acting on
an object is equal to the weight of the fluid
displaced by the object but acts in an upward
direction.
If an object is less dense than water, displaced
water will weigh more, so buoyant force will be
stronger than gravity and the object will float.
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Since fluids are composed of active molecules, at
least compared to solids, they are constantly
moving, pushing outwards, and applying force to
molecules (objects) around them.
The amount of force per unit area is called
pressure.
Since fluid molecules have a lot of energy, the
smaller a space you squeeze them into, the more
pressure they will exert. -e.g. putting too
much air into a balloon or tire.
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Our atmosphere is a giant mixture of gases (a
fluid), so it exerts pressure on everything in
and around it.
An air molecule at a low elevation has a lot of
air molecules above it, and their weight is
pressing down on it. This means that it is being
squeezed into a smaller space and pushed closer
to the molecules around it, which means that it
will exert more pressure on the objects around
it.
As you increase in elevation, this atmospheric
pressure decreases. As atmospheric pressure
decreases, temperature also decreases, and volume
increases.
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Water (a fluid) also exerts pressure on any
objects in it. Just like air, the deeper you go
the more pressure you feel.
Our bodies are adapted to the amount of pressure
present at sea level. If we go up into the air or
below water our bodies have to acclimatize.
However, the range that our bodies can
acclimatize to is pretty small.
Our bodies are filled with fluids, especially the
air in our lungs, so when we change elevations
the pressure in those parts of the body also
change, and they do not work properly.
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We can change the amount of pressure a fluid
exerts by changing its speed.
Bernoullis Principle- the pressure in a fluid
decreases as the fluids velocity increases.
This allows a plane to fly, if the air moving
over the wing is going faster than the air going
below it the pressure below will push the plane
up. -This upward force is called lift.
Bernoullis principle also explains why a curve
ball curves or a two seam fastball tails.
Independent research- Another principle of fluid
dynamics is Pascals Principle-. Hydraulics work
because of the properties it describes.