What you should know about ENERGY If it wasn't for Thomas Edison, we'd all be watching TV by the light of a candle. MFMcGraw Ch07 - Energy - Revised: 2/21/10

This lecture will help you understand: Energy Work Power Mechanical Energy : Potential and Kinetic Conservation of Energy MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 What is energy? The ability to do work. Energy Mover of things Observed when it is being transferred or being transformed Not spooky life force stuff. There is no such thing as good or bad energy, (Sorry Cesar Millan). MFMcGraw Ch07 - Energy - Revised: 2/21/10

Circular definition of work/ energy Property of a system that enables it to do work. Work So, the word work, in science terms, means something transferred energy somehow. Units for work and energy in science terms are Joules, in human terms they are Calories or kWh MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Derivation of W into E MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Work involves force and distance (but really displacement). Work = Force  displacement W = F x d J = N x m (units) J = kg * m/s2 x m = kg m2/s2 whenever useful mechanical work is done: Something changes the energy associated with that thing MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 But there is a catch The force must be in the same direction as the motion Is this dude doing any work? What if he was dragging the bag? MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 But there is a catch You might be saying, “Grist you are nuts. If I carried a heavy bag a long way I would get tired. So, I did work!” Nope! You may have exerted effort, but you did not change the energy of the bag. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Can you do negative work? Yes, when F & d are in opposite directions. Ex: setting down a bag of groceries. W = F cos ϑ d = F cos (180) d = F (-1) d = -Fd = -W MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Work CHECK YOUR NEIGHBOR If you push against a stationary brick wall for several minutes, you might get tired, but did you do any work? A. yes. no. Both of the above. None of the above. A. on the wall. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 An Example of Work A weightlifter raising a barbell from the floor does work on the barbell. Increasing it’s height increases its potential energy. If he dropped it on your foot it would do work on your foot. Does he do work while holding it over his head? Does he do work while holding it up and walking around? Does he do work if it’s on the ground and he drags it around? MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Power Power: Measure of how fast work is done In equation form: MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Units of Power Unit of power WATT is the unit of power? Named after James Watt, developer of the steam engine Horsepower Named after a horse that probably carried heavy stuff or pulled things, also eater of grass and maker of manure 750 Watts = 1 horsepower MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Power Examples A worker uses more power running up the stairs than climbing the same stairs slowly. T or F If he were to run the 2 flights of stairs would he produce more power than running just one? Yes / No If you drove your car twice as far as usual, would your car’s engine become more powerful? Yes / No An engine with twice the power can do twice the work of one engine in the same amount of time. T / F MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Power CHECK YOUR NEIGHBOR A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of A. energy. momentum. power. impulse. C. power. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Mechanical Energy Mechanical energy is due to position or to motion, or both. There are two forms of mechanical energy: Potential energy – position Kinetic energy - motion MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Potential Energy Stored energy potential for doing work Example: A stretched bow has stored energy that can do work on an arrow. A stretched rubber band of a slingshot has stored energy and is capable of doing work. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Potential Energy—Gravitational Potential energy due to elevated position. Example: water in an elevated reservoir Cat at the top of a tall tree MFMcGraw Ch07 - Energy - Revised: 2/21/10

Potential Energy—Gravitational Equal to the work done in lifting it MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Kinetic Energy Energy of motion Depends on the mass of the object and speed squared KE = ½mv2 If object speed is doubled  kinetic energy is quadrupled. If you double the speed you drive you quadruple you stopping distance. (slow down!!) MFMcGraw Ch07 - Energy - Revised: 2/21/10

Law of 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. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Kinetic Energy and Momentum Compared Kinetic energy is not momentum!!!! Since we are not doing a unit on momentum, ask Grist or Dickson what the difference is if you are curious. MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Energy for Life Body is a machine, so it needs energy. Average people need 2000 Calories per day. This is equals 2.2 kWh about 25 cents worth of energy from LPEA MFMcGraw Ch07 - Energy - Revised: 2/21/10

Ch07 - Energy - Revised: 2/21/10 Cool energy fact Example: Photovoltaic cells on rooftops catch the solar energy and convert it to electricity. More energy from the Sun hits Earth in 1 hour than all of the energy consumed by humans in an entire year! MFMcGraw Ch07 - Energy - Revised: 2/21/10