Transcript Chapter 5


Motion
◦ An objects change in position relative to a reference
point
 Reference Point: The object that appears to stay in
place

Speed
◦ The distance traveled divided by the time interval
during which the motion occurred.
 Average Speed = total distance/total time

Velocity
◦ Not to be confused with speed: it is the speed of an
object in a particular direction
 Example: a man is walking 1 m/s west.
◦ Resulting Velocity: when two or more objects are
combined.
 Example: A bus is traveling 15 m/s east. A man stands
up and walks forward at 1 m/s east. The Resulting
Velocity is 16 m/s east.
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Acceleration
◦ The rate at which velocity changes over time; an
object accelerates if its speed, direction, or both
change
 Positive Acceleration: an increase in velocity.
 Negative Acceleration (aka: deceleration): a decrease in
velocity
◦ Centripetal Acceleration: the acceleration that
occurs in a circular motion
 Example: Windmills that are a changing directions
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Force
◦ A push or a pull exerted on an object in order to change
the motion of the object; force has size and direction
◦ Newton: The SI Unit for force
◦ Net Force: the combination of all of the forces acting on
an object.
 Example: Bill is pushing a box with 25 N and Sally is pulling
the box with 30 N. Their Net Force is 55 N, in the direction
they are traveling.
◦ Balanced Force: When the forces of an object produce a
Net Force of 0 N.
 Unbalanced Force: when the Net Force does not equal zero.
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Friction
◦ A force that opposes motion between two surfaces that
are connect.
 Example: A book sliding across the table.
◦ Kinetic Friction: the friction between moving surfaces
 Kinetic mean moving: Like my watch…
◦ Static Friction: You observe static friction when you push
an object and it does not move (large furniture).
 Static means not moving: Static Electricity
◦ Lubricants: substances that are applied to surfaces to
reduce friction.
 Example: Oil for an Engine
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Gravity
◦ A force of attraction between objects that is due to
their masses
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The Law of Universal Gravitation
◦ Part 1: Gravitational force increases as mass
increase. Vice Versa
◦ Part 2: Gravitational forces decrease as distance
increases. Vice Versa

Weight
◦ A measure of the gravitational force exerted on an
object; its value can change with the location of the
object
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Mass
◦ A measure of the amount of matter in an object; its
value does not change.

Objects fall to the ground at the same rate
because the acceleration due to gravity is the
same for all objects.
◦ The rate at which objects accelerate to Earth is
9.8m/s/s
 So, for every second that an object falls, the objects
downward velocity increases by 9.8m/s.
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Calculating the change of velocity of a falling
object can be written like this:
◦

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v=gXt
means change
Terminal Velocity
◦ The constant velocity of a falling object when the
force of air resistance is equal in magnitude and
opposite in direction to the force of gravity.
 Air Resistance: is the force that opposes the motion of
objects through air
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Free Fall
◦ The motion of a body when only the force of gravity is
acting on the body
 Orbiting objects are in free fall
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Centripetal Force
◦ The unbalances force that causes objects to move in a
circular path
 Centripetal mean “toward the center”
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Projectile Motion
◦ The curved path that an object follows when thrown,
launched, or otherwise projected near the surface of
Earth
 Angry Birds
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Newton’s 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.
◦ What are some of the reasons we can see the
affects of the second part of Newton’s First Law
here on Earth?
 Examples…
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Inertia
◦ The tendency of an object to resist being moved or,
if the object is moving, to resist a change in speed
or direction until an outside force acts on the
object.
 Example: Because of inertia you slide toward the side
of a car when the driver turns a corner
 Draw Example
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The acceleration of an object depends on the
mass of the object and the amount of force
applied.
◦ Part 1: Acceleration depends on Mass
 The acceleration of an object decreases as its mass
increases and that its acceleration increases as its
mass decreases.
◦ Part 2: Acceleration depends on Force
 An object’s acceleration increases as the force on the
object increases, vice versa
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An apple with a mass of 0.102 kg with a
Force of 1N will fall at the same rate as a
watermelon with a mass of 1.02 kg with a
force of 10 N.
◦ A = f/m or F = mXa
◦
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Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first
◦ All forces act in pairs – action and reaction
 Example: shuttle blasting off, rabbit jumping, baseball
hitting a bat
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Momentum
◦ A quantity defined as the product of the mass and
velocity of an object.
◦ Example:
 Imagine a compact car and a large truck traveling with
the same velocity (speed + direction). The drivers of
both vehicles put on the brakes at the same time.
Which vehicle will stop first
 Answer:

Calculating Momentum
◦ Momentum is expressed as “p”
 P=mXv
◦ Example
 What is the momentum of an ostrich with a mass of
120 kg that runs with a velocity of 16 m/s north?
 Answer:
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Law of Conservation of Momentum
◦ The law states that any time objects collide, the
total amount of momentum stays the same. This is
true for any collision if no other forces act on the
colliding objects. This law applies whether the
objects stick together or bounce off each other.
 Example of “stick together” – football players tackling
each other
 Example of “bounce off each other” – bowling ball
hitting pins.
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Conservation of Momentum and Newton’s
Third Law
◦ Explain in your own words:
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Work: the transfer of energy to an object by
using a force that causes the object to move
in the direction of the force
◦ Transfer of energy – kinetic energy
 One object moves another object
◦ Difference between work and force
 With force, you can push hard on an object, but it
doesn’t move.
 An object must move for there to be work done.
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Work or No work
◦ Pushing a box in the direction you are walking
________________
◦ Carrying a backpack forward
________________
◦ Lifting groceries up
________________
◦ Carrying the groceries to the counter
________________
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Calculating Work
◦ W=F X d
 F: Force, measured in Newton’s (N)
 D: distance, measured in meters (m)
 W: work, measured in Joules (J)
◦ Example
 W=30N X 5m = 150J
 W=30N X 10m = 300J
 Notice that the further the distance the more work is done
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Power: the rate at which work is done or
energy is transformed
◦ Example: You can sand a piece of wood by hand, or
by using an electric sander. The electric sander is
faster, so it uses more power
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Calculating Power
◦ P=W/t
 P: power, measured using Watts
 W: Watts, Measured in J/s
 t: time, Measured in seconds
◦ Example:
◦ It takes you 10s to do 150J of work on a box to
move it up a ramp, what is your power output?
 Answer: ___________ W
 Notice the “s” in the Watts is cancelled out by the time, so
you do not include it in the answer
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Machine: a device that helps do work by
either overcoming a force or changing the
direction of the applied force.
Work Input: the work done on a machine; the
product of the input force and the distance
through which the force is exerted.
◦ How much force you put into it, in a direction
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Work Output: the work done by a machine;
the product of the output force and the
distance through which the force is exerted.
◦ The amount work done by the machine
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Force-Distance Trade-Off
◦ If you pick a box straight up, 1 meter with 450 N of
force, the outcome is 450 J. (450N X 1m)
◦ If you use a ramp that is 3 meters long, it only
takes you 150 N of force to move the box, the
outcome is still 450 J. (150N X 3m)
 Notice when you use a machine, a ramp, it uses less
force to move the box, but the same results occur.
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Mechanical Advantage: a number that tells
how many times a machine multiplies force.\
◦ Examples:
 Pulley helps lift objects up
 Levers help pry things apart
◦ Calculating Mechanical Advantage
 MA=output force/input force
 Example: MA=500N/50N = 10
 There is no unit. You said say the machine multiplies its
force by 10.
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Mechanical Efficiency: a quantity, usually
expressed as a percentage, that measures the
ratio of work output to work input
◦ Calculating Mechanical Efficiency
 ME = work output/work input X 100
 The unit is percent, %
◦ Machine can never be 100% efficient, because every
machine has some work input, which causes
friction, therefor using some energy.
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Lever: a simple machine that consists of a bar
that pivots at a fixed point called a fulcrum
◦ First Class Lever: The fulcrum is between the input
force and the load, always change the direction of
the input force. Ex: push down = load goes up
◦ Second Class Lever: The load is between the
fulcrum and the input force, the direction of the
input force and load are the same. Ex: push up =
goes up
◦ Third Class Lever: The input force is between the
fulcrum and the load, the diction of the load and
input are the same. Ex: a hammer
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Pulley: a simple machine that consists of a
wheel over which a rope passes over
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Wheel and Axel: a simple machine consisting
of two circular objects of different sizes; the
wheel is the larger of the two objects
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Inclined Planes: a simple machine that is a
straight, slanted surface, which facilitates the
raising of loads; a ramp.
Wedge: a simple machine that is made up of
two inclined planes and that moves; a knife
Screw: a simple machine that consists of an
inclined plane wrapped around a cylinder
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Compound Machine: a machine made of more
than one simple machine
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Law of Electric Charges: States that like
charges repel, or push away, and opposite
charges attract
Electric Force: the force of attraction or
repulsion on a charged particle that is due to
an electric field
Electric Field: the space around a charge
object in which another charged object
experiences an electric field

Ways to Charge an Object
◦ Friction: happens when electrons are “wiped” from
one object onto another
◦ Conduction: happens when electrons move from
one object to another by direct contact
◦ Induction: happens when charges in an uncharged
metal
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Electrical Conductors: is a material in which
charges can move freely
◦ Examples:

Electrical Insulator: a material in which
charges cannot move freely
◦ Examples:

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Static electricity: electric charge at rest;
generally produced by friction or induction
Electric Discharge: the release of electricity
stored in a source - lightning
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Electric Current: is the rate at which charges
pass a given point
◦ A/C: continually shift from flowing in one direction
to flowing in the reverse direction
 Example: Electricity from the outlet in a house
◦ D/C: the charges always flow in the same direction
 Example: electricity from a battery
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Voltage: the potential difference between two
points; measured in volts
◦ How much work is needed to move a charge
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Resistance: in physical science, the
opposition presented to the current by
material or device
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Resistance, Thickness, and Length
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Cells: in electricity, a device that produces an
electric current by converting chemical or
radiant energy into electrical energy