Energy Physics

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.

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.

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.

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).

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.

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

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.

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.

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 speed 2 –KE = ½ mv 2

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 mv 2 Therefore, because speed is squared the kinetic energy is quadrupled.

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.

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

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

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

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

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.

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.

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

Pulley A pulley is a rope, belt or chain wrapped around a grooved wheel. A pulley can function two ways: 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.

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

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.

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

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 Rube Goldberg machines were developed to poke fun at the way useful devices are supposed to make life easier.

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.

An Example

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.