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Energy, Work, Simple Machines, and Power

Energy - The "something" that enables work to be done.

Kinetic energy - energy of motion.

Potential energy - stored energy or energy of position. A common form of potential energy we deal with is gravitational potential energy (PE). PE is the potential energy of an object due to itís position relative to another object, the earth is often "the other" object. As a result the usual equation in physics for PE is
Conservation of energy - energy cannot be created nor destroyed; it can only change form.

We usually define mechanical energy as the sum of all the kinetic and potential energy. Since the energy of an object must be conserved, the kinetic energy used to place a book on a shelf is stored in the book as potential energy, all the while the mechanical energy staying constant.

Work is the amount of energy required to move an object, or released by the motion of an object, a certain distance; motion must be in the same direction of the applied force. If work is done on an object by its surroundings, the work is positive. When work is done by an object on its surroundings, the work is negative. The sign of the work indicates the direction of motion of energy.

The Work-energy theorem states that work equals the change in kinetic energy

Work is the applied force multiplied by the distance traveled, both in the same direction.

UNITS of energy and work
USCS -->; calorie/Calorie
SI --> N.m or Joule (J)
Any device that makes work easier to perform is known as a machine. A simple machine uses only one movement. A machine makes work easier by altering the force (magnitude or/and direction) that must be applied to the situation in order to get the same amount of work out of the machine as you would without the machine.

The force applied to the machine is called the effort force. The force applied by the machine is called the resistance force.

For example to raise a 100 N box 1 m, 100 J of work must be done. If you were to lift the box straight up, you would have to exert 100 N of effort force to move the 100 N resistance force of the boxís weight. If you used a machine such as a ramp, you may be able to reduce your effort force to 50 N as you push the box up the ramp, but you would have to push the box 2 m. This is because 100 J of work is still required to get the job done. The machine just makes it easier (in terms of force you must exert.)

Work input is the amount of work (energy) that goes into the machine. This work comes from the force that is applied to the machine, effort force.
Work output is the amount of work (energy) that comes out of the machine. This work is used to overcome the resistance force.

Machines cannot increase the amount of work that is put into them. This would violate the law of conservation of energy. The work input must always be larger than or equal to the work output in order for the machine to work properly. If the work input equals the work output the machine is said to be ideal, or 100% efficient.

The efficiency of a machine is defined as the ratio of work output to work input.

The closer the work out is to the work in, the more efficient the machine is.

Another important measure of a machine is the number of times it changes the effort force. This is called the mechanical advantage of the machine.

Simple Machines

There are two main categories of simple machines: the inclined plane and the lever. All simple machines fall into one of the two categories. With each type of machine is an equation for its ideal mechanical advantage (IMA), a 100% efficient machine. An ideal machine assumes the absence or neglecting of friction and other extraneous forces. The basic equation for IMA is

Where de is the effort distance and dr is the resistance distance.

Look at each of the IMA equations below and identify how each part corresponds with the basic equation.

Inclined planes

The ramp: an incline used to assist lifting

The wedge: Double incline that may move

The screw:  incline (threads) wrapped around a post


The lever: 3 classes of levers
1) First class - Fulcrum is between effort and resistance forces: shovel.
2) Second class - Fulcrum at one end, effort force is at other end, resistancein middle: Wheelbarrow.
3) Third class - Fulcrum at one end, resistance force at other end, effort inthe middle: Rake, broom.
The wheel and axle; gears: rotational lever

The pulley: the pulley is the fulcrum, the strands are the effort and resistance arms.

IMA = number of supporting strands POWER - The rate at which work is done.
The SI unit of power is the watt (W) which is simply a Joule/second