Work, Power, & Machines What is work ?

Work, Power, & Machines What is work ?

Work, Power, & Machines What is work ? The product of the force applied to an object and the distance through which that force is applied. 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 YES YES

NO YES Calculating Work All or part of the force must act in the direction of the movement. 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. Labeling work: w = F x d Newton X meter (N m) Which also = (kg x m2) s2 The Joule

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

Input Force Output Force FO FI Force Force applied by applied to a machine 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.

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

Usually input machines distance/output distance 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)

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. First Class Levers

Fulcrum is located between the effort and resistance. Makes work easier by multiplying the effort force AND changing direction. Examples: Second Class Levers

Resistance is found between the fulcrum and effort force. Makes work easier by multiplying the effort force, but NOT changing direction. Examples: Third Class

Levers Effort force is located between the resistance force and the fulcrum. Does NOT multiply the effort force, only multiplies the distance. Examples:

Levers!!!!!!!!!!! Mechanical advantage of levers. Ideal = input arm length/output arm length input arm = distance from input force to the fulcrum output arm =

distance from output force to the fulcrum 2. The Wheel and Axle A lever that rotates in a circle. A combination of two wheels of different sizes. Smaller wheel is termed the axle.

IMA = radius of wheel/radius of axle. 3. The Inclined Plane A slanted surface used to raise an object. Examples: ramps, stairs, ladders IMA = length of ramp/height of ramp

Can never be less than one. 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

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 MA = Count # of ropes that apply an upward force (note the block and tackle!) Fe A combination of two or more simple

machines. Cannot get more work out of a compound machine than is put in.

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