Presentation on theme: "ENERGY What is Energy? Ability to do WORK Measured in Joules (calories or kilocalories)"— Presentation transcript:
What is Energy? Ability to do WORK Measured in Joules (calories or kilocalories)
What is Work? 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.
Work: Work is done on an object whenever a force is exerted on an object through some distance. W = 0, if F = 0 W = F d If the force is constant and parallel to the direction of movement then: W = 0, if d = 0 No work done if object doesnt move. No work done, whenever there is no force.
If the force on the object is in the direction the object moves, the work done is: W = Fx F x
If the direction of the force is opposite the direction the object moves, work is: W = -Fx F x
Force is NOT Work If the force is perpendicular to the direction the object moves, the work done is 0. If the object doesnt move, the work done is 0. F x W = 0
How is all energy divided? Potential Energy Kinetic Energy All Energy Gravitation Potential Energy Elastic Potential Energy Chemical Potential Energy
What is Potential Energy? o Energy that is stored and waiting to be used later
What is Gravitational Potential Energy? o Potential energy due to an objects position o P.E. (gravitational) = mass x height x gravity Dont look down, Rover! Good boy!
What is Elastic Potential Energy? Potential o Potential energy due compression or expansion of an elastic object. Notice the ball compressing and expanding
Work done by a spring = ½ Kx 2
What is Chemical Potential Energy? o Potential energy stored within the chemical bonds of an object
What is Kinetic Energy? An objects kinetic energy depends on: the objects mass. Kinetic energy is directly proportional to mass. the objects speed. o Kinetic energy is directly proportional to the square of the objects speed. o Energy an object has due to its motion K.E. = ½ mv 2
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 The work done on an object by the net force equals the objects change in kinetic energy. W net = KE
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?
Work done by Friction 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 JoulesFx
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-Fx 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!
Power is the rate work is done. Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt Power is a scalar quantity. Power = Work time W P t
What are (Simple) Machines? A machine is a mechanical device used to do work. Examples of simple machines: Inclined plane Lever pulley
(Simple) Machines A machine can never output more work (energy) than is put into it. At best, Work out = Work in Machine Work in Work out
What is Mechanical Advantage? Machines cant multiply work or energy, but they can multiply force. Mechanical advantage measures how much a machine multiplies force. MA = Force machine exerts Force you exert
How do we Measure Efficiency? 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. Efficiency = Useful work done Energy input x 100%