7 Energy

Work W = Fd (F parallel to d) W = 0 (F perpendicular to d) Work is change in energy Energy is the capacity to do work Energy can be positional or motional

Energy Positional Energy is called Potential Energy, e.g. stretched spring Motional Energy is called Kinetic Energy SI Units for Work and Energy: J = Nm = kgm2/s2

Work-Energy Theorem Work = D(KE) Examples: Positional-Motional Energy Transformations: Bow and Arrow Mass on Spring Inclined Plane

Conservation of Energy Energy cannot be created or destroyed; it may be transformed (by work) from one form into another, but the total amount of energy never changes. Energy Forms include: thermal, elastic, chemical, electromagnetic, nuclear, kinetic.

Machine A device that changes an applied force by increasing it, decreasing it, or changes its direction. Examples of Machines: pulley, lever, jack, inclined plane.

Efficiency The percent of the work put into a machine that is converted into useful work output. efficiency = (work output)/(energy input)

Review Questions

1. Front car 2. Middle car 3. Rear car 4. Other In which car will you be moving the fastest at the very bottom of the incline? This question can be restated: Where is the train when it has its maximum speed? Which car will be at the bottom at this instant? 1. Front car 2. Middle car 3. Rear car 4. Other Ch 7-3 Thanks to Joe Delaney.

1. Front car 2. Middle car 3. Rear car 4. Other The middle car is at the bottom when the train is moving the fastest. 1. Front car 2. Middle car 3. Rear car 4. Other Ch 7-3 Thanks to Joe Delaney. Answer: 2 The middle car. Of course, at any moment all cars have the same speed, but the gravitational potential energy of the system of cars is least when the middle car is at the bottom—when the center of mass of the system is lowest. Lowest potential energy means highest kinetic energy. So sit in the middle car for the fastest ride at the very bottom!