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Physics Primer

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.

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Physics Primer

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  1. Physics Primer

  2. Definitions Energy - the ability to do work Work - the transfer of energy by applying a force through a distance But what is a “force”?

  3. Position Position - orientation and distance an objectis 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

  4. Velocity Defn. - time rate of change ofdisplacement; 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 secondsto go the 400 meters down the dragstrip?

  5. Speed Some books say that velocity is speed + direction. WRONG! Distance traveled Average speed = Elapsed time Displacement = Distance traveled Displacement on racetrack is 0, while distance travelled is not

  6. Acceleration Defn. - time rate of change of velocity;is a vector quantity; SI unit ism/s2 Dv Average acceleration = Dt Accelerations can occur without changing the magnitude of velocity; Ex. Object going in circle at constantrate

  7. 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?

  8. Newton’s Second Law F = ma Relates kinematic variables to dynamic ones Can measure accelerations  calculate forces Note: SI unit is newtons, English is poundsIncorrect 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/s2?

  9. Newton’s Third Law “For every force, there is an equal and opposite reaction force.” Often misunderstood; actually means that one object actingon a second object will have the second object act on it Mule pulls on cart. Cart pulls back onmule with equal and opposite force.“Why pull?”, says mule, if force willbe negated.

  10. Get Back To Work Work - the transfer of energy by applying a force through a distance W = F x d if F is constantDW = Fn x Dd if F varies Lifting box: F = mgDistance lifted = h W = mg x h = mgh

  11. Simple Machines Allow for the same amount of workto be done, but with smaller forces Trade-off of using a smaller force isthat 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/LDistance pushed up plane = L

  12. Power DE Power = = rate of energy usage Dt Can deliver the same amount of energy to a system using lesspower, but it takes a longer amount of time Our Western mindset usually screams for more powerEx. 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?

  13. TYPES OF POTENTIAL ENERGY: Gravitational Chemical Nuclear 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

  14. EXAMPLES: Water behind a dam A rock at the top of a steep hill Example: Gravitational potential energy Potential energy due to gravity If the water or rock drops, gravity operates over a distance, thereby doing work. This work converts the potential energy to kinetic energy.

  15. 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 Kinetic energy 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

  16. Fig. 5: Diagram of falling box Example • 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 mv2

  17. 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 subatomicparticles that contain charge; equal amounts of + and - results in neutrality Designation of positive and negative iscompletely arbitrary If like charges repel and unlike attract, what holds protons andneutrons together?

  18. Currents - Moving Charge Oersted (1820) discovered that a currentcreates a magnetic fieldThis means a current has a force on a magnet Newton’s Third Law means that amagnet has a force on a moving chargeDirection of force is perpendicular tovelocity and magnetic field

  19. Generator Electric generator: rotate coils of wire perpendicularto magnetic field Magnet Layers of wire coils

  20. 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  P = (Dq V )/ DtBut I = Dq/Dt  P = I V

  21. 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 per person has stayed about the same over the last 30 years Source: Dept. of Energy, http://eia.doe.gov/

  22. One Case: Crude Oil We get energy from many different sources. One of the moreimportant ones we will discuss is crude oil. What are the implications of this graph? Whathistorical eventsoccurred during thistime that relate tocrude oil? Source: Dept. of Energy, http://eia.doe.gov/

  23. 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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