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Unit 2b    

Unit 2b

Physical Science Chapter 15

Work, Power, and Simple Machines

15.1  Work:   A force acting through a distance.

        Work = Force x Distance = Fd (Newton-meters (N-m) or Joules (J) )

Work is measured in newton-meters or a unit called a joule   (J).

         1 newton-meter = 1 joule

15-2   Power:  the rate at which work is done.

         Power is work per unit time.

Power =   Work      or       Force x Distance =     F*d
                Time                          Time         t

The unit of power is the watt (W)             1 W = 1J/sec = 1 newton-meter/sec

Machines:  Any instrument that makes work easier is called a "machine."
Machines make work easier by:

changing the size or direction of the force put into a machine.

       f small using machine* Dlarge using machine > F large force output by machine *  Dsmall distance moved

Efficiency:    A measure of how much of the work put into a machine is changed to useful work put out by the machine.  Efficiency is a comparison of work output to work input.  (Always less than 100% !!) "Work out" can never  be greater than "Work in"!!
Due to energy converted to friction and loss of efficiency.

   Win   =   Work Input:   Work done on the machine.

   Wout  =   Work Output:   Work done by the machine.

      Efficiency = work output   x   100% =     Wout  x 100 %  
                         work input                Win


Chapter 16:  Energy, Forms and Changes

16-1    Nature of Energy
Energy is the ability to do work.  Energy is needed to exert a force over a distance in order to move an object.

                            Energy is measured in "Joules" (J).    

You need energy to do work, however objects can gain energy because work is being done on them.   

There are five (5) main forms of energy:
•   Mechanical Energy - energy associated with motion.  Example:  

•   Heat energy - energy involving atoms and molecules.  Example:

•   Chemical Energy - energy in the bonds that hold molecules together. Example:

•   Electromagnetic Energy - movement of electricity and light, ray, and wave energy.

•   Nuclear Energy - dealing with strong force and binding energy.  Usually released in the form of light and heat.  Example:

16-2   Kinetic and Potential Energy
Energy is the ability to do work.      

There are two (2) states of energy:
  Kinetic energy  - "Energy of Motion"   Example:  

                       K.E. = ½ m v2 = ½ * mass * velocity squared    (Joules or J

  Potential energy - "Energy of Position"  Example:

                             P.E. = work needed to get object in it's current state (Joules or J)

Energy dependent on height is called gravitational potential energy.  (G.P.E.)

       G.P.E. = Weight x Height = mgh = mass (kg)  * gravityacceleration (m/s/s)* height (m)

16-3    Energy   Conversions

Energy can be transferred from one object to another and energy can be changed from one form to another.
The most common form of energy conversion is the changing of energy from kinetic energy to potential energy and from potential energy to kinetic energy.

All forms of energy can be converted to other forms!

16-4  Conservation  of  Energy:   ENERGY IS NEVER  LOST (never)!!

Law of conservation of energy:
"Energy can be neither created nor destroyed by ordinary means."

Einstein showed us that if matter is destroyed, energy is created, but the total amount of mass and energy is conserved.

16-5          Physics and Energy

A moving object has momentum (mass x velocity).  Mass and velocity are used to measure "kinetic energy."   Momentum is a form of kinetic energy and must be conserved because energy is conserved.


A force is required to change the motion of an object. A force does work on an object and therefore it changes the energy of the object.

Power is the rate at which work is done.  So power must be the rate at which energy is consumed.

Thus far no one has discovered a way to violate the law of conservation of energy.