Ch 8 Energy Notes ENERGY

What is Energy? Ability to do WORK Ch 8 Energy Notes Ability to do WORK Measured in Joules (calories or kilocalories) What is Energy?

What is Work? Work depends on: Ch 8 Energy Notes Work depends on: The amount of force applied to the object. The distance that the object moves while the force is applied. The direction of the force with respect to the direction the object moves. What is Work?

Work: Work is done on an object whenever a force is exerted on an object through some distance. If the force is constant and parallel to the direction of movement then: W = F d W = 0, if d = 0 No work done if object doesn’t move. W = 0, if F = 0 No work done, whenever there is no force.

Ch 8 Energy Notes If the force on the object is in the direction the object moves, the work done is: W = Fx F x

Ch 8 Energy Notes If the direction of the force is opposite the direction the object moves, work is: W = -Fx F x

Ch 8 Energy Notes If the force is perpendicular to the direction the object moves, the work done is 0. If the object doesn’t move, the work done is 0. F W = 0 x Force is NOT Work

How is all energy divided? Potential Energy Kinetic Energy Gravitation Potential Energy Elastic Chemical

What is Potential Energy? Energy that is stored and waiting to be used later

What is Gravitational Potential Energy? Potential energy due to an object’s position P.E. (gravitational) = mass x height x gravity Don’t look down, Rover! Good boy!

What is Elastic Potential Energy? Potential energy due compression or expansion of an elastic object. Notice the ball compressing and expanding

Work done by a spring = ½ Kx2

What is Chemical Potential Energy? Potential energy stored within the chemical bonds of an object

K.E. = ½ mv2 What is Kinetic Energy? Ch 8 Energy Notes An object’s kinetic energy depends on: the object’s mass. Kinetic energy is directly proportional to mass. the object’s speed. Kinetic energy is directly proportional to the square of the object’s speed. Energy an object has due to its motion K.E. = ½ mv2 What is Kinetic Energy?

Kinetic energy is a scalar quantity. Ch 8 Energy Notes Kinetic energy is a scalar quantity. When the kinetic energy of an object changes, work has been done on the object. Work is a scalar quantity. What is the kinetic energy of an object with mass of 1 kg, moving at 1 m/s? 0.5 Joule.

Work and Kinetic Energy Ch 8 Energy Notes The work done on an object by the net force equals the object’s change in kinetic energy. Wnet = DKE Work and Kinetic Energy

Law of Conservation of Energy If no external forces act on a system, the total energy of the system will remain constant. Mechanical Energy = PE + KE

What are different Forms of Energy/Energy Transformations?

Your work = Fx = (10 N)(1 m) = 10 Joules Suppose you have to exert a force of 10 Newtons to push a book 1 meter across a horizontal table at constant velocity (as shown above). Since your force is in the direction the book moves, the work you do on the book is given by: Your work = Fx = (10 N)(1 m) = 10 Joules Work done by Friction

Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules The work-energy equation would lead you to think that doing 10 Joules of work on the book should increase the kinetic energy of the book by 10 Joules - but that doesn't happen (the speed of the book is constant). What's going on? A friction force opposes the motion of the book. This force must also be 10 Newtons (Since the book moves at constant velocity, the net force on it must be zero.). The friction force pulls in the opposite direction from the direction the book moves, the work done by friction is given by: Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules This means that the friction force removes 10 Joules of energy from the book. So, while you were adding 10 Joules of energy to the book, friction was busy taking the 10 Joules of energy away from the book. This is why the kinetic energy of the book does not change!

DWork Power = time W P t Power is the rate work is done. Ch 8 Energy Notes Power is the rate work is done. Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt Power is a scalar quantity. DWork Power = W time P t

What are (Simple) Machines? Ch 8 Energy Notes A machine is a mechanical device used to do work. Examples of simple machines: Inclined plane Lever pulley What are (Simple) Machines?

Workout = Workin (Simple) Machines Ch 8 Energy Notes A machine can never output more work (energy) than is put into it. At best, Workout = Workin Machine Workin Workout (Simple) Machines

What is Mechanical Advantage? Ch 8 Energy Notes Machines can’t multiply work or energy, but they can multiply force. Mechanical advantage measures how much a machine multiplies force. Force machine exerts MA = Force you exert What is Mechanical Advantage?

How do we Measure Efficiency? Ch 8 Energy Notes The efficiency of a machine tells how much of the energy (work) that goes into the machine actually does useful work. It is usually expressed as a percent. Useful work done Efficiency = x 100% Energy input How do we Measure Efficiency?