Transcript PHYSICS ACT and PSAE Test Prep

PHYSICS
ACT and PSAE Test Prep
April 2011
Energy
• Defined as the capacity of doing
Energy
work
• When work is done HEAT is
released
• First Law of Thermodynamics or
The Law of Conservation of Energy
• Energy cannot be created nor
destroyed but may change form
Two Types of Energy
Potential Energy
(energy at rest)
In the form of
Chemical energy
or
Formula
PE = mgh
Kinetic Energy Formula
(energy in motion) KE =½ mv2
In the form of
Electromagnetic energy
or
Nuclear energy
Electrical energy
or
or
Gravitational energy
Mechanical energy
or
Mechanical energy
or
Thermal energy
Potential Energy
•
Potential
Energy
Chemical – Gain or loss of energy due
to breaking of chemical bonds
• Nuclear – Release of energy due to
division (fission) or combination
(fusion) of the subatomic particles in an
atom’s nucleus
• Gravitational – Stored energy by virtue
of it’s location relative to the center of
the Earth
• Mechanical – Energy an object has
because of its motion and or position
Kinetic Energy
• Electromagnetic – Light
energy associated with
photons comprising
wavelengths along an
electromagnetic
spectrum
• Electrical – Energy
associated with the
movement of electrons
through a wire or circuit
• Thermal – Energy
created by the
movement or vibrations
of an object’s atoms
Energy Transfer
• Heat is the transfer of energy between two
objects
• Second Law of Thermodynamics states
that heat will always transfer in one
direction: from a higher temperature to a
lower temperature
• Heat gets transfers by one of three
methods:
– Conduction
– Convection
– Radiation
Conduction, Convection and Radiation
Conduction, Convection and
• Convection isRadiation
the movement of heat
through fluids (remember gas laws)
(ex:Boiling water)
• Conduction is the transfer of heat
through a substance dependent upon
thermal conductivity and difference in
temperature (ex:metal spoon in a pot of
boiling water)
• Radiation is the transfer of heat
through wave energy (ex: warmth on
your face by the sun)
Electrical Conductivity
• Current – the rate of
flow of electrons
• Dependent upon
materials’ desire to
hold onto
(insulators) or get
rid of (conductors)
electrons
Good
Conductors
Good
Insulators
Diamond
Vacuum
Silver
Air
Copper
Wood
Aluminum
Rubber
Iron
Porcelain
Ceramics
Lead
Glass
Water
Carbon
Decreasing
Conductivity
Decreasing
Resistance
Circuits
Series Circuit
In a series circuit
current flows
through each
component one
after the other…if
one part of the
circuit breaks the
whole thing shuts
down
Parallel Circuit
In a parallel circuit
there are branches
off the main
circuit…if one
component breaks
the rest will continue
to work
Light
• Light is radiant energy in the form of waves
of differing wavelengths ()
• What we term “light” is the portion of the
electromagnetic spectrum that the human
eye can detect
Characteristics of Light
• Electromagnetic waves: theory that light is
considered to be a wave composed of
oscillating electric and magnetic fields
• Fields are perpendicular to the direction the
wave moves, thus the waves are transverse
waves
• Frequency and wavelengths account for
differences in colors and distinguishes visible
light from invisible radiation (ex. X-rays)
Reflection and Refraction
• Reflection is the change in
the direction of light (the
angle of incidence is
always equal to the angle of
reflection)
• Refraction is the bending of
light due to change in the
speed of light through
different mediums
Mirrors
In a convex mirror
(mirror that curves
outward) the image is
smaller and closer to the
mirror then the object is
In a concave mirror
(mirror that curves
inward like a “cave”)
the virtual image is
larger and farther away
than the object is
Waves
• Light and sound are produced by energy
which is traveling in the form of a wave
• Waves are the transfer of energy from a
vibrating source to a receiver (no matter is
exchanged)
• The speed of a wave is dependent upon the
medium in which it is flowing
• Formula: v = f
Two Types of Waves
• Transverse: Motion of
the wave is at right
angles to the direction
in which the wave is
moving (shaking a
slinky up and down)
• Longitudinal: Particles
move along the same
path that the wave is
moving ( push a slinky
in and out)
Motion
One Dimensional
• Moving horizontally without acceleration.
•
Examples: Puck sliding on ice, bus moving
at constant speed.
• Equation: velocity = distance divided by time
v = d/t
• Terms:
•
Speed = how fast an object travels
•
Velocity = speed with a direction.
• Misconception:
•
Speed and velocity are the same
• Truth:
•
Velocity is speed combined with direction.
Motion
One Dimensional
• Acceleration = the rate at which velocity
changes.
•
3 ways to accelerate a car
•
Gas Pedal – positive acceleration
•
Brake Pedal – negative acceleration
•
Steering Wheel – change of direction
• Misconception:
•
Acceleration is simply an increase in
velocity.
• Truth:
•
Acceleration occurs whenever velocity
(speed and direction) changes.
Vertical
Motion
Vertical Motion
• What happens when we fall?
•
Falling means accelerating
downward.
•
Acceleration due to gravity is 9.8
m/s2 or 32 ft/s2
• This means that the velocity of a
falling object increases by 9.8 m/s
every second.
• To make life easy round off this
number to 10 m/s2
Vertical Motion
Equations for falling (assuming a dropped object)
v = 9.8 t which is almost 10 t
d = ½ 9.8 t2 which is almost 5 t2
For example:
After one second v = 10 m/s and d = 5 m (16 ft)
After two seconds v = 20 m/s and d = 20 m (64 ft)
After three seconds v = 30 m/s and d = 45 m (144
ft)
Horizontal and Vertical
Motion
(2 Dimensional Motion)
• Horizontal motion + vertical motion =
projectile motion
• What happens horizontally stays horizontal.
What happens vertically stays vertical.
• Horizontal = no acceleration (constant
velocity)
• Vertical = acceleration downward (9.8 m/s 2 )
• Misconception:
The curved path of a projectile is very
complicated.
• Truth:
The path of a projectile is just the combination
of constant horizontal motion and changing
•
•
•
•
•
•
•
Newton’s Laws of
Motion
st
1 Law – Law of Inertia: An object in motion will
stay in motion and an object at rest will stay at
rest unless acted upon by an outside force.
What this means: No force is needed for motion.
Friction is a force that stops motion. Balanced
forces means zero acceleration.
2nd Law: An unbalanced force produces
acceleration in the direction of the force.
What this means: Force creates acceleration. F =
ma.
For a constant force larger masses accelerate at a
slower rate.
3rd Law: For every action there is an equal an
opposite reaction.
What this means: You cannot touch without
being touched.
Newton’s Laws of
Motion
Examples:
1st Law : People not wearing seatbelts fly through
front window after collision with another car
2nd Law : More force is needed to accelerate a
semi-truck then a bicycle
3rd Law : Rocket being launched into space
Misconceptions:
Constant motion requires force.
Even if no force acts on a moving object, it will
eventually stop.
Inertia is a force
Mass is not the same thing as weight (weight is
dependent upon gravitational pull)
Simple Machines
• Machines are tools used to make work
easier
• There are 6 simple machines which
may be combined in a variety of ways
in order to make more complex
machines
• The six simple machines are: Lever,
Inclined Plane, Wheel and Axle, Screw,
Wedge, and Pulley
Lever
• Lever: A bar which is free to move
about a fixed point called a fulcrum
• Ex. See-saw, Crowbar, Claw end of a
hammer
First, Second and Third Class
Levers
• First class: Fulcrum is between Effort and
Resistance (ex. See-saw)
• Second Class Lever: Resistance is between
Fulcrum and Effort (Paper cutter)
• Third Class Lever: Effort is between Fulcrum
and Resistance (Hockey Stick)
Inclined Plane
• Inclined Plane: A plane surface set at
an angle (not 90°) against a horizontal
surface
• Allows for a smaller force over a longer
distance. This helps to overcome
resistance
• Ex: Ramp, Chisel, Snow plow
Wheel and Axle
• Wheel and Axle: A lever
that rotates in a circle
around a center point (or
fulcrum)
• The larger wheel (outside)
rotates around the smaller
wheel (axle)
• Ex: Cars, Bicycles, Ferris
wheels, Skateboards, Gears
Screw
• Screw: Inclined plane
wrapped around a
cylinder
• Modified wedge to yield
large mechanical
advantage (ability to
make work easier) in
minimum space
• Ex: Screws
The closer the
threads the
greater the
M.A.
Wedge
• Wedge: A moving inclined
plane
• The sharper the wedge the
greater the M.A.
• Ex: Axe, knife
Pulley
• Pulley: A rope, belt or chain
wrapped around a grooved wheel.
• Used to change the direction or
amount of a force
• Ex: Flagpole, blinds
Fluid Dynamics
• Fluids:
• Misconception: Fluids are liquids
• Fact: Fluids are atoms or molecules
which are free to flow past one
another in a substance
• A fluid may be a liquid or a gas
• There are four main properties of
Fluids
– Density
– Pressure
– Bouyancy
– Viscosity
Density
• Density is defined
as mass per unit
volume
• Formula: d = m/v
• If the density of
an object exceeds
the density of the
liquid they are
placed in the
object will sink
Pressure
• Pressure is defined as Force per unit of area
• Formula P = F/a
• Air pressure is the force exerted upon an object
by air molecules.
• Air pressure may be measured with a device
called a Barometer
• Air pressure decreases as altitude increases,
whereas pressure in a liquid increases as depth
increases
• Pascal’s Principle: Pressure applied to a fluid in
a closed container is transmitted equally to every
point of the fluid and to the walls of the container
Buoyancy
• Buoyancy: A force that acts upward on an
object submerged in a liquid or floating on
the liquid’s surface
• Archimedes’ Principle: Any body partially or
completely submerged in a fluid is buoyed
up by a force equal to the weight of the fluid
displaced by the object
Viscosity
• Viscosity: The
measure of a
material’s
resistance to flow
• Bernoulli’s
Principle: As the
speed of a moving
fluid increases, the
pressure within the
fluid decreases
Graphs
Up Graphs
Directly proportional
Positive Slope
Exponential Slope not constant
Down Graphs
Directly proportional
Negative Slope
Inversely Proportional
Slope not constant
or
Graphs
• Up graphs = Both variables are increasing
or decreasing
• Down graphs = One variable increasing
while the other is decreasing
• Straight Line = Slope is constant (x to y
ratio should be the same for all
comparative data)
• Curve = Slope is not constant (x to y ratio
changes)