Physics Primer Definitions Energy the ability to do
Physics Primer
Definitions Energy - the ability to do work Work - the transfer of energy by applying a force through a distance But what is a “force”?
Position - orientation and distance an object is from some origin; measurement of position requires a coordinate system If the position does not change, the object is easily found Displacement - change in position; if position is designated with the vector r, then displacement is Dr
Velocity Defn. - time rate of change of displacement; is a vector quantity; SI unit = m/s Displacement Dr Average velocity = = Elapsed time Dt Instantaneous velocity = limit (average velocity) Dt 0 What is the average velocity of a dragster that takes 5. 5 seconds to go the 400 meters down the dragstrip?
Speed Some books say that velocity is speed + direction. WRONG! Average speed = Distance traveled Elapsed time Displacement = Distance traveled Displacement on racetrack is 0, while distance travelled is not
Acceleration Defn. - time rate of change of velocity; is a vector quantity; SI unit is m/s 2 Dv Average acceleration = Dt Accelerations can occur without changing the magnitude of velocity; Ex. Object going in circle at constant rate
Newton’s First Law Really, Galileo’s “An object at rest, or in a state of constant motion, will continue in that state unless acted upon by an unbalanced force. ” Inverse of statement is very important: if an object is acceleration, then a net force is operating on it, even if you cannot see the reason for the force. Is there a force operating in this picture, and if so, from what direction?
Newton’s Second Law F = ma Relates kinematic variables to dynamic ones Can measure accelerations calculate forces Note: SI unit is newtons, English is pounds Incorrect to say that X pounds = Y kilograms Not all forces are constant What force is needed to accelerate a 1000 kg car to 5 m/s 2?
Newton’s Third Law “For every force, there is an equal and opposite reaction force. ” Often misunderstood; actually means that one object acting on a second object will have the second object act on it Mule pulls on cart. Cart pulls back on mule with equal and opposite force. “Why pull? ”, says mule, if force will be negated.
Get Back To Work - the transfer of energy by applying a force through a distance W=Fxd DW = Fn x Dd if F is constant if F varies Lifting box: F = mg Distance lifted = h W = mg x h = mgh
Simple Machines Allow for the same amount of work to be done, but with smaller forces Trade-off of using a smaller force is that the force is applied through a longer distance Box lifted straight up a height h, force supplied is F = mg Force of gravity down inclined plane is F = mg sinq = mgh/L Distance pushed up plane = L
Power = DE = rate of energy usage Dt Can deliver the same amount of energy to a system using less power, but it takes a longer amount of time Our Western mindset usually screams for more power Ex. SUV’s require more powerful engines; larger homes require more powerful a. c. How much power do you expend by climbing 3 flights of stairs (10 m) in 10 seconds?
Potential energy Energy stored within the force between two objects separated by a distance; if objects are allowed to move, force is applied through distance = work done TYPES OF POTENTIAL ENERGY: Gravitational Chemical Nuclear
Example: Gravitational potential energy Potential energy due to gravity EXAMPLES: Water behind a dam A rock at the top of a steep hill If the water or rock drops, gravity operates over a distance, thereby doing work. This work converts the potential energy to kinetic energy.
Kinetic energy ENERGY OF MOTION A moving object has momentum. If it hits another object, it will transfer energy to it by applying a force through a distance, i. e. work Some of the bullet’s kinetic energy is transferred to the apple during the collision Kinetic energy of falling water is converted to motion of turbines when water falls on them
Example Fig. 5: Diagram of falling box • Potential energy at highest point is given by mgh • As it falls, all of this potential energy is converted to kinetic • Right before it hits the ground, all of that is left is kinetic energy • Thus, mgh = ½ mv 2
Charge Defn: Property of matter that determines how it will interact with other matter via the electrostatic force; opposite charges attract, like repel All atoms are composed of subatomic particles that contain charge; equal amounts of + and - results in neutrality Designation of positive and negative is completely arbitrary If like charges repel and unlike attract, what holds protons and neutrons together?
Currents - Moving Charge Oersted (1820) discovered that a current creates a magnetic field This means a current has a force on a magnet Newton’s Third Law means that a magnet has a force on a moving charge Direction of force is perpendicular to velocity and magnetic field
Generator Electric generator: rotate coils of wire perpendicular to magnetic field Magnet Layers of wire coils
Electrical Power We know that P = DE/Dt from the definition of power The amount of energy delivered by a current is equal to the amount of charge delivered times the voltage, DE = Dq V But I = Dq/Dt P=IV P = (Dq V )/ Dt
U. S. Energy Consumption Over the last 50 years, our consumption of energy has increased (except for after energy crises) Because of more efficient devices, our consumption person has stayed about the same over the last 30 years Source: Dept. of Energy, http: //eia. doe. gov/
One Case: Crude Oil We get energy from many different sources. One of the more important ones we will discuss is crude oil. What are the implications of this graph? What historical events occurred during this time that relate to crude oil? Source: Dept. of Energy, http: //eia. doe. gov/
Import Countries Since the mid-1970’s, we have increased our dependence of oil imports on non-OPEC countries Why? We have increased our reliance on oil from Central and South America, as well as Canada and Africa Why?
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