PHYSICSACT and PSAE Test Prep

April 2011

Energy

Energy• Defined as the capacity of doing 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

Chemical energy

Nuclear energy

Mechanical energy

Gravitational energy

Thermal energy

Mechanical energy

Electrical energy

Electromagnetic energy

Kinetic Energy(energy in motion)

Two Types of Energy

Potential Energy (energy at rest)

In the form of In the form of

or

or

or

or

or

or

Formula KE =½ mv2

Formula PE = mgh

Potential EnergyPotential 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• Convection is 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)

Conduction, Convection and Radiation

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

CircuitsSeries Circuit

Parallel 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

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)

MotionOne 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.

MotionOne 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 MotionVertical 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 td = 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/s2 )

• 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 vertical motion.

Newton’s Laws of Motion• 1st 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 MotionExamples:

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 forceMass 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

• 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

Fluid Dynamics

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

Directly proportional Positive Slope

Exponential - Slope not constant

Up Graphs or

Directly proportional Negative Slope

Inversely Proportional Slope not constant

Down Graphs

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)