Phy 101: Fundamentals of Physics I Chapter 7 Lecture Notes

James Prescott Joule ( ) Inventor & scientist Interested in efficiency of electric motors Described the heat dissipated across a resistor (now known as Joules’ Law) Showed that heat is produced by motion Credited with establishing the mechanical energy equivalent of heat Participated in establishing the “Law of Energy Conservation”

Work Effort times output To calculate work: Work = (Force Applied)*(Distance Traveled) or W = F. d Only force that is applied in the direction of distance traveled can do work Work is a scalar quantity Units (SI) are Joules (J)

Energy The capacity of something to do work Types of energy: –Mechanical –Heat –Electrical –Chemical –Nuclear Energy cannot be created nor destroyed Work can be re-described as a measure of energy transfer (i.e. chemical to mechanical energy)

Power Rate at which work is performed Or Rate at which energy is consumed Units of power are units of energy per time –SI: Joules/sec (or Watts, W) –Other: horsepower (1hp = 750 W) To calculate power: Power = work/time or Power = energy/time Note: when an object has constant velocity Power = (force) x (velocity)

Kinetic Energy Energy due to an object’s motion (in order to have kinetic energy an object must have motion) Product of ½ times mass times velocity squared A scalar quantity To calculate kinetic energy (KE) KE = ½ x mass x velocity 2 or KE = ½ mv 2 Units (SI) are J (or kg. m 2 /s 2 )

Gravitational Potential Energy Energy due to an object’s position (when it is within a gravitational field) Represents the potential work gravity could perform if an object were let go A scalar quantity To calculate potential energy (PE) PE = mass*gravitational acceleration*elevation or PE = mgh Units (SI) are J (or kg. m 2 /s 2 )

Work-Energy Theorem The work performed on an object (the net work) is equal to its change in kinetic energy: W =  KE Individual forces can do work even though no net work is performed on an object Does this make sense in terms of Newton’s Laws?

Conservation of Mechanical Energy Mechanical energy in a system remains constant if there is no heat loss (due to friction) Energy may change form but the total amount of it stays the same, for example: –An object in free-fall gains as much KE as it loses PE during its descent

Machines Two fundamental types: –Convert one form of mechanical work into another (create “mechanical advantage”) –Transform energy into work Efficiency (%) is a measure of a machine’s performance: Efficiency = (work performed/energy input) * 100% No machine can be more than 100% efficient

Aristotle Revisited According to Aristotle’s theory, forces cause motion (violent motion) To Aristotle, an object’s speed was simply the ratio of the force exerted on the object divided by its resistance or Speed = “force”/“resistance” Aristotle was not too far off: –His “force” is really power –His “resistance” is really the applied force acting on the object (which must be equal to the resistance/frictional force when v is constant!!) Speed = power/force Of course, Aristotle did not consider accelerating objects!!