Work, Power, & Machines

What is work ? The product of the force applied to an object and the distance through which that force is applied. Work = Force (N) x Distance (m) 1 N*m = Joule (J)

Is work being done or not? Mowing the lawn Weight-lifting Moving furniture up a flight of stairs Pushing against a locked door Swinging a golf club YES NO

Do you do more work when you finish a job quickly? Work does NOT involve time, only force and distance. No work is done when you stand in place holding an object. Would you do more work if you ran up the stairs and skipped 2 at a time?

The Joule 1 newton-meter is a quantity known as a joule (J). Named after British physicist James Prescott Joule.

Power How quickly work is done. Amount of work done per unit time. If two people mow two lawns of equal size and one does the job in half the time, who did more work? Same work. Different power exerted. POWER = WORK / TIME

The watt A unit named after Scottish inventor James Watt. Invented the steam engine. P = W/t Joules/second 1 watt = 1 J/s

watts Used to measure power of light bulbs and small appliances An electric bill is measured in kW/hrs. 1 kilowatt = 1000 W

Horsepower (hp) = 745.5 watts Traditionally associated with engines. (car,motorcycle,lawn-mower) The term horsepower was developed to quantify power. A strong horse could move a 750 N object one meter in one second. 750 N

Machines A device that makes work easier. A machine can change the size, the direction, or the distance over which a force acts.

Force applied to a machine Forces involved: Input Force FI Force applied to a machine Output Force FO Force applied by a machine

Two forces, thus two types of work Work Input work done on a machine =Input force x the distance through which that force acts (input distance) Work Output Work done by a machine =Output force x the distance through which the resistance moves (output distance)

Can you get more work out than you put in? Work output can never be greater than work input.

Mechanical Advantage (MA) – expressed in a ratio WITH NO UNITS!! The number of times a machine multiplies the input force. MA = output force/input force

2 types of mechanical advantage IDEAL Involves no friction. Is calculated differently for different machines Usually input distance/output distance ACTUAL Involves friction. Calculated the same for all machines

Different mechanical advantages: MA equal to one. (output force = input force) Change the direction of the applied force only. Mechanical advantage less than one An increase in the distance an object is moved (do)

Efficiency Efficiency can never be greater than 100 %. Why? Some work is always needed to overcome friction. A percentage comparison of work output to work input. work output (WO) / work input (WI)

Simple Machines 6 different types of simple machines: 1.) the lever 2.) the wheel and axle 3.) the inclined plane 4.) the wedge 5.) the screw 6.) the pulley

1. The Lever A bar that is free to pivot, or move about a fixed point when an input force is applied. Fulcrum = the pivot point of a lever. There are three classes of levers based on the positioning of the effort force, resistance force, and fulcrum.

    INPUT FORCE (EFFORT FORCE) OUTPUT FORCE (Resistance Force) INPUT ARM (EFFORT DISTANCE) OUTPUT ARM (RESISTANCE DISTANCE) FULCRUM

First-Class Lever The position of the fulcrum identifies a first-class lever. The fulcrum of a first-class lever is ALWAYS located between the input force (effort) and the output force (resistance) Depending on position of fulcrum, MA of first-class lever can be greater than 1, equal to 1, or less than 1

Which ones are first-class levers?

Second-Class Levers Output force is located BETWEEN the input force and the fulcrum

Example of second-class lever When you lift handles of wheelbarrow, it rotates around its fulcrum Input distance greater than output distance Increased input distance means it takes LESS force from you to lift the load MA of a second-class lever is always greater than 1

Third-Class Levers Input force is located between the fulcrum and the output force The output distance over which the lever exerts its force is always larger than the input distance you move the lever through. Therefore MA for third-class levers is always less than 1

Wheel & Axle Consists of 2 discs or cylinders, each one with a different radius. wheel   axle Can have MA greater than or less than 1 Would the MA for the steering wheel be greater than or less than 1? Greater than 1: input distance larger than output distance

Inclined Planes Imagine how hard it would be to walk up the side of a steep hill. It would be MUCH easier to follow a gentle slope of a winding trail….why is this? What is happening to input distance if you decide to take the curvy trail instead of going straight up the hill? Because input distance is greater than output distance, in this case, the input force is decreased…..so it’s easier for you!

Inclined Planes Inclined plane: a slanted surface along which a force moves an object to a different elevation Input distance Output distance  

Inclined Planes What is the MA of the following inclined plane? 6m 1 m Mechanical Advantage = 6 MA for inclined plane will NEVER be less than 1

Wedges & Screws Similar to inclined planes BUT sloping surfaces can move. Wedge: a V-shaped object whose sides are two inclined planes sloped toward each other. A thin wedge of a given length has a greater MA than thick wedge of the same length. Why is this so? Length of wedge is divided by width of the opening. Smaller the opening (thickness), the larger your MA will be Examples: knife, axe, razor blade

Wedge Why did Samurai’s feel it was so important to keep their blade sharp? They wanted to have the highest MA possible! Sharpening the sword increases MA (just like having a thinner wedge in the wood worked better than a thicker one)

Screws Screw: an inclined plane wrapped around a cylinder Which screw would be easier to put into a piece of wood? A screw with the threads very close together or a screw with the threads somewhat far apart The screw with threads CLOSE together! Though it would take you less turns for the screw with fewer threads, it would require you to exert a greater force Screws with threads that are closer together have a greater MA threads

4. The Wedge An inclined plane that moves. Examples: knife, axe, razor blade Mechanical advantage is increased by sharpening it.

5. The Screw An inclined plane wrapped around a cylinder. The closer the threads, the greater the mechanical advantage Examples: bolts, augers, drill bits

6. The Pulley A chain, belt , or rope wrapped around a wheel. Can either change the direction or the amount of effort force Ex. Flag pole, blinds, stage curtain

IMA = number of supporting ropes Pulley types FIXED Can only change the direction of a force. MA = 1 MOVABLE Can multiply an effort force, but cannot change direction. MA > 1 IMA = number of supporting ropes

MA = Count # of ropes that apply an upward force (note the block and tackle!) Fe

Compound Machines A combination of two or more simple machines. Cannot get more work out of a compound machine than is put in.