Energy Work and Power Energy and Work n

Energy, Work and Power

Energy and Work n Energy is the ability to do work ¨ Ex. Kinetic, Thermal, Gravitational, Elastic, Chemical, Electric, Magnetic, Radiant… n Work is the transfer of energy

James Prescott Joule (1818 – 1889): - English physicist - Discovered relationship btw. heat & mechanical work (energy) - Conservation of Energy Theorem

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Power n Power is the rate at which work is done [N. m] , [J] Joule [W] watt [s] second James Watt (1736 – 1819): - Scottish inventor/mechanical engineer. - Improved the efficiency of steam engines. - Introduced horse power (750 watts).

n If a hair dryer does 3000 J of work to heat the air every two seconds, what is its power?

n A 613. 0 kg mass is placed on a forklift that can generate 950 W of power. What is the constant speed of the forklift while lifting this load?

n A HD TV consumes 5. 0 k. W∙h of energy in 30 minutes. ¨ What is its power? ¨ How much does it cost to watch TV for 2 hours if the cost of electricity during peak time is $0. 12/ k. W∙h

Mechanical Energy

Mechanical Energy is a combination of two fundamental types of energy: n n

Mechanical Energy is a combination of two fundamental types of energy: n Kinetic energy (the energy of motion) n Potential energy (energy that is stored) - Gravitational Potential energy

Kinetic Energy n n The kinetic energy of an object of mass m, in kg, and speed v, in m/s:

Kinetic Energy The work done by the net force causes a change in speed n The kinetic energy of an object of mass m, in kg, and speed v, in m/s: n

Gravitational Potential Energy n Ex: A mass m is lifted from a height h 1 to a height h 2 at a constant speed.

Gravitational Potential Energy n “stored energy” in an object at a particular height w. r. t. a reference point. Ex: A mass m is lifted from a height h 1 to a height h 2 at a constant speed.

Gravitational Potential Energy n The gravitational potential energy Eg of an object of mass m, in kg, that is at height h, in m, above the reference point is: n g = 9. 8 m/s 2

Gravitational Potential Energy n The gravitational potential energy Eg of an object of mass m, in kg, that is at height h, in m, above the reference point is: n g = 9. 8 m/s 2

W. E. T. (Work-Energy Theorem) n The total work done on an object equals the change in the object’s kinetic energy OR gravitational potential energy, but NOT BOTH.

Ex 1: What happens to the kinetic energy of an object when work is done on it?

Ex 2: Calculate the work done to speed up a 1500 kg Honda Civic: a) From rest to 20 km/h b) From 80 km/h to 100 km/h

Ex 3: What role does gravitational potential energy play in the production of electricity by Ontario Power Generation?

Ex 4: Ontario Power Generation at Niagara Falls operates under a normal head of about 55 m (height from which water falls). If about 4. 54 x 108 kg of water falls every minute, how many mega-joules of energy are created by the falling water in a hour?