Chapter 4.1- Work & Power

What is Work?  For work to occur two things must happen:  Work is done when an object moves in the same direction in which the force is exerted. 1) The object has to move some distance as the result of your force. 2) The object has to move in the same direction as the applied force – only the force acting IN THE SAME DIRECTION as the object’s motion is performing work.

- What Is Work? The Meaning of Work  Work is done on an object when the object moves in the same direction in which the force is exerted. QUESTION: IF YOU PULL AN OBJECT HORIZONTALLY, WHAT PART OF YOUR FORCE DOES WORK? __All of your forces contributes to work

Calculating Work  The amount of work done depends on BOTH the AMOUNT OF FORCE exerted and the DISTANCE THE OBJECT MOVES.  The amount of work can be determined by multiplying FORCE times DISTANCE: Formula: W = (F)(d) W = work F = Force d = Distance

Calculating Work  SI Unit of Work =  When force = NEWTONS (N)  When distance = METERS  Joule (N*m)  Named after James Prescott Joule a physicist from the mid 1800’s who studied work  1 Joule =1 Newton of force to move an object 1 meter  Joule is abbreviated with a J

Practice: 1.How much work is performed if you move a 50 N object.5m? 2. What if the object was 100 N?

Power  Work is UNAFFECTED BY TIME  Power is the rate (time) at which work is done.  how fast or slow work is done  Power equals the amount of work done on an object in a unit of time.  With more power you can do the same amount of work in less time.

Calculating Power  Formula to calculate power: Power = Work Time OR Power = force x distance Time Practice!!!!!!!!!

When work is measured in joules, and time in seconds: SI Units for power: Joule/second (J/sec) 1 Watt = Joule/Sec. (J/s) James Watt & His Steam Engine

Units of Power  One watt is a small amount of power  Power is more often expressed in KILOWATT  Kilowatt = 1000 watts  Horsepower: English Unit (not an SI unit)  1 HP = 746 Watts Corvette

QuestionAnswer What is work? Work is done when an object moves in the same direction in which the force is exerted. How can you calculate work? What is power? Work = Force X Distance Power is the rate at which work is done. P = work / time Quick Summary:

Chapter 4.2 How Machines Do Work?

What Is a Machine? i. A device that allows you to DO WORK IN A WAY THAT IS EASIER.

Input and Output Work  The amount of input work done by the gardener equals the amount of output work done by the shovel.

Input & Output work  Input force = the FORCE you exert on the machine.  Output force = the FORCE the machine exerts ON THE OBJECT

Calculating Input & Output work  Input work  Input force x Input distance  Output work  Output force x Output distance  When you use a machine, the amount of output work can never be greater than the amount of input work.

What Is a Machine?  A machine makes work easier by changing at least one of three factors.  A machine may change the amount of force you exert  the distance over which you exert your force  or the direction in which you exert your force.

QUESTION: How does the cable system on a weight machine make raising the weights easier? The cable system enables you to raise the weights more conveniently by changing the direction to make work easier.

Mechanical Advantage  If you compare the input force to the output force, you can find the advantage of using a machine.  A machine’s mechanical advantage is the number of times a machine increases a force exerted on it.

Calculating Mechanical advantage  Mechanical advantage = Output force input force Examples: A machine that increases force: output force is greater than input force. ex. a can opener

 For a machine that increases distance the output force is less than the input force. ¨ Ex. A wooden spoon.  For a machine that changes direction the input force will equal the output force ¨ Ex. A weight pulley machine ¨ The mechanical advantage of these types of machines will always be 1.

Mechanical Advantage ¨ 400 N Interpreting Data: ¨ If an 80-N input force is exerted on Ramp 2, what is the output force?

Efficiency of Machines  So far you learned that input work is equal to output work. This does not happen is real life.  WHY??? FRICTION In every machine, some work is wasted overcoming friction. Force must be applied to overcome the friction of the machine.

Efficiency of Machines Input Work is ALWAYS GREATER than Output Work 1. Force must be applied to OVERCOME THE FRICTION of the machine itself Expressed as a percentage 1. To calculate the efficiency of a machine, DIVIDE the OUTPUT WORK by the INPUT WORK and multiply the result by 100 percent. a.Efficiency = Output Work / Input Work x 100

Calculating Efficiency  You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower?  Read and Understand  What information have you been given? Input Work (W input ) = 250,000 J Output Work (W output ) = 200,000 J

Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = _?_ What formula contains the given quantities and the unknown quantity? Efficiency = Output work/Input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent.

Real & Ideal Machines 1.Real machines  Efficiency is always less than 100% 2.Ideal machines  Efficiency is always 100% ( these machines do not exist)

QUESTION: Why is output work always less than input work in real situations? Because friction exists in every machine and reduces the machines efficiency.

wedge Input force Output force