I. Energy and Work (p.124-131) Energy Work Conservation of Energy Ch. 4 - Energy I. Energy and Work (p.124-131) Energy Work Conservation of Energy

W = Fd A. Work Work transfer of energy through motion force exerted through a distance W = Fd W: work (J) F: force (N) d: distance (m) 1 J = 1 N·m Distance must be in direction of force!

A. Work Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ? WORK: W = F·d W = (30 N)(1.5 m) W = 45 J

A. Work GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ? A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ? WORK: W = F·d F = m·a F =(40kg)(9.8m/s2)=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. “d” is not in the direction of the force.

Example problems You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do? How much work do you do when you lift a 100 N child 0.5 m? The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 40 m while the brakes are being applied, how large is the force that the brakes exert on the car? How much work is done in lifting an object that has a mass of 5 kg a vertical distance of 2 m?

Machines A device that changes the force or increases the motion from work. 2 Types of machines: Simple machine Compound machine

Simple Machine A machine that does work with only one movement of the machine 6 types: lever, pulley, wheel and axle, include plane, screw, wedge

Compound Machine A machine that is a combination of two or more simple machines Ex: scissors – 2 wedges and 2 levers bicycle

Efficiency Machines can increase force OR increase speed (no machine can increase both at the same time) You always put more work into a machine than you get out of that machine

Efficiency Problems You do 20 J of work in pushing a crate up a ramp. If the output work from the inclined plane is 11 J, then what is the efficiency of the inclined plane? The input work on a pulley system is 75 J. If the pulley system is 84% efficient, then what is the output work from the pulley system? Workers do 8000 J of work on a 2,000N crate to push it up a ramp. If the ramp is 2 m high, then what is the efficiency of the ramp?

B. Energy ENERGY Without energy nothing would ever change. The ability to cause change. THERMAL internal motion of particles ENERGY MECHANICAL NUCLEAR motion of objects changes in the nucleus ELECTRICAL joules (J) CHEMICAL motion of electric charges bonding of atoms

B. Energy Kinetic Energy (KE) energy due to motion depends on mass and velocity 80 km/h 50 km/h Which has the most KE? Which has the least KE? 80 km/h truck 50 km/h motorcycle

Kinetic Energy

Kinetic Energy Examples A jogger with a mass of 60 kg is moving forward at a speed of 3 m/s. What is the jogger’s kinetic energy? A baseball will a mass of 0.15 kg is moving at a speed of 40 m/s. What is the baseball’s kinetic energy? A 1500 kg car doubles its speed from 50 km/h to 100 km/h. By how many times does the kinetic energy from the car’s forward motion increase?

B. Energy Potential Energy (PE) stored energy depends on position or configuration of an object Which boulder has greater gravitational PE? What other ways can an object store energy?

Potential Energy Elastic potential energy – energy that is stored by compressing or stretching an object (spring, rubber band, etc) Chemical potential energy – energy that is due to chemical bonds Gravitational potential energy – energy that is due to the gravitational forces between objects

Potential Energy Examples A 4 kg ceiling fan is placed 2.5 m above the floor. What is the potential energy of the ceiling fan? An 8 kg history textbook is placed on a 1.25 m high desk. What is the potential energy of the textbook relative to the floor?

C. Conservation of Energy Law of Conservation of Energy Energy may change forms, but it cannot be created or destroyed under ordinary conditions. EX: PE  KE (apple in a tree) mechanical  thermal (friction, wheels of car) chemical  thermal (reaction that creates heat)

C. Conservation of Energy PE  KE https://www.youtube.com/watch?v=IqV5L66EP2E View pendulum animation. View roller coaster animation.

C. Conservation of Energy Mechanical  Thermal View rolling ball animations. View skier animation.

Power Rate at which energy is converted Power = Watts (W) = J/s

Power Examples You transform 950 J of chemical energy into mechanical energy to push a sofa. It took you 5 s to move the sofa, what was your power? If a runner’s power is 400 W as she runs, how much chemical energy does she convert into other forms in 10 minutes? One horsepower is a unit of power equal to 746 W. How much energy can a 150 hp engine transform in 10 s?