Conservation of Energy Lec 07: Energy Energy, Work, Power Mechanical Energy Conservation of Energy Types of Energy

KE = (1/2) (mass) (velocity)2 Energy Things moving must have energy Kinetic Energy (Joules) KE = (1/2) (mass) (velocity)2 Example: A 20,000-kg train is moving at 22 m/s (about 50 mph). How much is its kinetic energy?

Work = (Force ) (Distance ) Amount of Energy moved by a force (Watt) Work = (Force ) (Distance ) W = F d

Work Example: You are lifting 10 books from the floor to the bookshelf, 1 m above. Each book weighs 10.0 N. How much work do you do in lifting them?

Energy and Work Work = - (mg) x (15 m) Example: If you carry your 5-kg backpack from the parking lot to the fourth floor of the building (15 m above), how much is the work done by the gravitational Force? Work = - (mg) x (15 m) Example: How much is the work when you carry your 5-kg backpack back to the parking lot? Work = (mg) x (15 m) Question: How much is the total work?

Power How fast the Work is being done Power Power = (Work) / (Time) Example: You leave a 25-W light bulb on for 10 minutes. How much energy in Joules has it consumed?

PE = (mass) (g) (change in elevation) Potential Energy PE = (mass) (g) (change in elevation) PE = mgh Example: How much is the PE of the ball?

Puzzles P1. You do work when pushing a cart with a constant force. If you push the cart twice as far, how much is the work? P2. How much is the Kinetic Energy of a 2-kg object moving at 3.0 m/s? P3. You run a 100-W light bulb on for 1 hour. How much energy have you consumed? P4. What costs more to run: a 100-W light bulb on for 1 day or a 1,000-W hair-dryer run for 10 minutes? P5. A typical grade school pitcher can throw a baseball at 80 km/h, but only a few professional athletes have the extraordinary strength needed to throw a baseball at twice that speed because they need to impart the ball much more energy. By what factor?

Conservation of Energy Energy moves and is transformed from one form to another but never appears or disappears “unaccountably” Example: You drop a 100-g rock from height of 2 m. How much is its potential energy at the top? How much is its potential energy at the bottom? How much is its kinetic energy at the bottom in the absence of friction?

Conservation of Energy Energy moves and is transformed from one form to another but never appears or disappears “unaccountably” Example: You drop a 100-g rock from height of 2 m. By the time it reached ground, the force of air resistance removed 1.0 J. How much is the kinetic energy just before the ground? How much is the speed of the rock?

Ordered vs. Disordered Energy Question: Which of the following is an example of ordered energy? The energy of a hurricane? Energy of steam coming of hot pot Energy delivered by the electrical company?

(FORCE) x (distance) = (force) x (DISTANCE) Simple machines: RAMP (FORCE) x (distance) = (force) x (DISTANCE)

Simple machines: PULLEY The arrangement allows the load to be lifted by half as much force!

Simple machines: Efficiency (useful energy output) Efficiency = (total energy input) Example: A certain machine is 30% efficient. If the amount of energy you got out of it was 30 Joules, you paid for 30 J (b) 70 J (c) 100 J (d) Don’t know

Puzzles P1. You throw a 10-g pebble upward with velocity of 6 m/s. How high will it reach in the absence of air resistance? P2. You throw a 10-g pebble upward with velocity of 6 m/s. If the air-resistance takes about 0.05 J of its energy, how high will it go? P3. You are lifting a 5-kg object using a pulley. With how much force do you need to pull?