Energy

Energy Universe is made up of matter and energy. Energy is the mover of matter. Energy has several forms: –Kinetic –Potential –Electrical –Heat –etc.

Work Now instead of a force for how long in time we consider a force for how long in distance. Work = Force  Distance W = F d The unit for work is the Newton-meter which is also called a Joule.

How much work? A weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters A weight lifter pushes on a wall with a force of 1000 Newtons for 15 sec?

Power Power is equal to the amount of work done per unit time. The unit for power is the Joule/second which is also called a Watt.

Light Bulbs and Appliances How much energy does a 100 Watt light bulb use in one hour? How about a 40 Watt light bulb?

Mechanical Energy When work is done on an object, the object generally has acquired the ability to do work. This "ability to do work" is called energy and it has the same units as work….Joules. Two Types of Mechanical Energy –Potential Energy and Kinetic Energy

Potential Energy The energy that is stored is called potential energy. Examples: –Rubber bands –Springs –Bows –Batteries –Gravity

Gravitational Potential Energy PE = Weight  height PE = m g h Question: –How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

Kinetic Energy Kinetic Energy is the energy of motion. Kinetic Energy = ½ mass  speed 2 Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second?

Work/Energy Relationship If you want to move something, you have to do work. The work done is equal to the change in kinetic energy. Work =  KE

Example Question When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h? (a) 2 times (b) 3 times (c) 9 times (d) 27 times (e) half as far

Conservation of Energy Energy cannot be created or destroyed......it may be transformed from one form into another......but the total amount of energy never changes. Demos –Galileo's incline –Bowling ball pendulum

Pendulum Example Problem At what point is the kinetic energy the highest? –(A) At the bottom of its path –(B) At the ends of its path –(C) Somewhere in between

Pendulum Example Problem At what point is the potential energy the highest? –(A) At the bottom of its path –(B) At the ends of its path –(C) Somewhere in between

Example Problem A 100 kg mass is dropped from rest from a height of 1 meter. How much potential energy does it have when it is released? How much kinetic energy does it have just before it hits the ground? What is its speed just before impact? How much work could it do if it were to strike a nail before hitting the ground?

100 kg 1 meter nail

Machines - An Application of Energy Conservation If there is no mechanical energy losses then for a simple machine... work input = work output ( F d )input = ( F d )output Examples - levers and tire jacks

Efficiency Useful energy becomes wasted energy with inefficiency. Heat is the graveyard of useful energy.

Comparison of Kinetic Energy and Momentum Kinetic energy is a scalar quantity. Momentum is a vector quantity.

Example Questions A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work? (a) 10 lb weight (b) 20 lb weight (c) same work for each lifting (d) not enough information is given to work the problem

An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest? (a) 180 Joules (b) 2700 Joules (c) 36 Joules (d) 5 Joules (e) 180 N

A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton. meters is (a) 20 (b) 24 (c) 16 (d) 80 (e) 5

An object of mass 2 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest? (a) 180 Joules (b) 2700 Joules (c) 36 Joules (d) 5 Joules (e) 900 Joules

Example Questions A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 5 ft. Which lifting required the most work? (a) 10 lb weight (b) 20 lb weight (c) same work for each lifting (d) not enough information is given to work the problem

A 1 kg mass is lifted 1 meter. The change in potential energy of the weight is (a) 1 N (b) 9.8 N (c) 1 J (d) 9.8 J (e) 9.8 m/s 2

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