Energy

1. Energy Physics

2. Energy • Energy comes to us from the sun. • Persons, places, and things have energy, but we only observe the effects of energy when something is happening. • Energy can be transferred from one form to another and from one place to another.

3. Work • The concept of force x distance is work. • When work is done two things always occur: • The application of a force • The movement of something by that force • Twice the load equals twice the work because it requires twice the force. • Twice the distance also equals twice the work.

4. Types of Work • Work falls into two general categories. • The first category is work done against a force. • You do work on something when you force it to move against the indulgence of an opposing force. • Ex. Pulling a bowstring • The second is work done to change the speed of an object. • Ex. Speeding a car up.

5. Units of Work • The unit of measurement for work combines a unit of force, N, with a unit of distance, m. • The resulting unit of work is the newton-meter, or joule, named after James Joule. • One joule of work is done when a force of 1 N is exerted over a distance of 1 m. • There are also kilojoules, kJ (1000 J) and megajoules, mJ (1,000,000 J).

6. Power • The definition of work does not include time. • Power is the rate at which work is done. • Power = Work Done/Time Interval • Twice the power means twice the work can be done in the same amount of time. • The unit of power is called the watt, after James Watt. • One watt of power is expended when one joule of work is done in one second. • One horsepower is equal to 0.75 kW.

7. Mechanical Energy • Something that enables an object to do work is energy. • Energy is measured in joules. • Energy occurs in many forms • Two of the most common forms of mechanical energy are: • Potential energy: energy do to position • Kinetic energy: movement of something

8. Potential Energy • An object may store energy by virtue of its position. • The energy that is stored and held in readiness is called potential energy (PE) because in the stored state it has the potential for doing work. • Ex. Spring or Rubberband • Chemical energy is also potential energy. • Any substance that can do work through chemical action possesses potential energy.

9. Work and Potential Energy • Work is required to elevate objects against the earth’s gravity. • The potential energy due to elevated position is called gravitational potential energy, GPE. • The amount of GPE an object possess is equal to the work done against gravity in lifting it or, • GPE = weight x height = mgh • Height is the distance above some chosen reference level such as the ground.

10. Kinetic Energy • If an object is moving then it is capable of doing work. • It has energy of motion, or kinetic energy, KE. • The kinetic energy of an object depends on the mass of the object as well as its speed. • It is equal to half the mass multiplied by the square of the speed or, • Kinetic Energy = ½ mass x speed2 • KE = ½ mv2

11. Kinetic Energy and Work • The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest, or the work the object can do while begin brought to rest. • The equation is net force x distance = KE, • Or Fd = ½ mv2 • Therefore, because speed is squared the kinetic energy is quadrupled.

12. Conservation of Energy • Energy changes from one form to another. • It transforms without net loss or gain. • The study of various forms of energy transformations led to the law of conservation of energy. • The law states, “energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy never changes.” • In any system, one quantity doesn’t change: energy.

13. Machines • A machine is a device used to multiply forces or simply to change the direction of forces. • The concept that underlies every machine is the conservation of energy. • If friction and heat is neglected than, work input should equal work output, or • (Force x distance)input=(Force x distance)output

14. Simple Machines • There are six types of simple machines: • The inclined plane • The wedge • The screw • The lever • The pulley • The wheel and axle

15. Wedge • The wedge is a form of the inclined plane. • A wedge is an inclined plane that moves to raise an object. • A wedge is usually a piece of wood or metal that is thinner at one end. • The longer and thinner the wedge, the more force it exerts. • Ex. Knife or Zipper

16. The Screw • The screw is an inclined plane wrapped around a central bar or cylinder to form a spiral. • A screw multiplies the force by acting through a long distance. • The mechanical advantage of the screw increases as the threads are closer together. • Ex. Bolt and nut, and jar lids

17. The Inclined Plane • The inclined plane is a flat, slanted surface. • It is a simple machine with no moving parts that works by increasing the distance over which the force is exerted and thus multiplying the force. • The most common use is probably as a ramp.

18. The Lever • A lever is a rigid bar that is free to pivot, or move about, a fixed point. • The fixed point is called the fulcrum. • When a force is applied to part of the bar by pushing or pulling, the lever swings on the fulcrum and overcomes the resistance force.

19. Types of Levers • There are three types of levers: • First class lever: Resistance force and effort force are on opposite sides of the fulcrum • Second class lever: the resistance force and effort force are on the same side with the effort force further away from the fulcrum. • Third class lever: the resistance force and effort force are on the same side with the resistance force further away from the fulcrum

20. Pulley • A pulley is a rope, belt or chain wrapped around a grooved wheel. • A pulley can function two ways: • Change direction of the force • Change amount of the force • A fixed pulley (attached to a stationary object) changes the direction of the effort force. • A movable pulley is attached to the object you are trying to move and thus the effort force is multiplied. • Fixed and movable pulleys can be combined to accomplish even more work.

21. Wheel and Axle • A wheel and axle is a simple machine made up of two circular objects of different sizes. • The wheel is the larger object and it turns about the smaller object called the axle. • Because the wheel is larger, force is multiplied when it is applied to the axle. • The mechanical advantage depends on the radius of the wheel and axle systems. • Ex. Screwdriver, Ferris Wheel

22. Mechanical Advantage • The ratio of output force to input force for a machine is called the mechanical advantage. • Neglecting friction, the mechanical advantage can also be determined by the ratio of input distance to output distance.

23. Efficiency • An ideal machine would be 100% efficient. • However, in any machine, some energy is transformed into atomic or molecular kinetic energy aka heat. • Efficiency will always be less than one. • Efficiency is expressed as, • Useful work output/total work output

24. Rube Goldberg • Rube Goldberg was a Jewish American cartoonist who became famous for his Rube Goldberg machines. • He drew cartoons for the many newspapers including the New York Evening Journal and won the Pulitzer Prize in 1948. • Rube Goldberg machines were developed to poke fun at the way useful devices are supposed to make life easier.

25. Rube Goldberg Machines • A Rube Goldberg machine is an exceedingly complex machine that performs a simple task in a convoluted way. • In 1987, Purdue University began the National Rube Goldberg machine contest in which college teams build machines around a simple theme.

26. An Example

27. The Task • Your task is to build a Rube Goldberg machine to complete the simple task of making a cup of coffee. • Your machine must be small enough to fit through the classroom door and complete the task in less than nine minutes. • You must use at least 15 steps to complete the task and use each of the six simple machines. • You will also have to submit a drawing and written description of your machine outlining each energy transfer.