Chapter 14 Work, Power, & Machines

Sec Work & Power

What is work? Work is done when a force acts on an object in the direction the object moves Work (J)= Force (N) x distance (m) W = F x d SI unit: Joule  Joule = Nm

Work requires Motion If there is no motion…there is no work Some motion must be in the direction of the force Since there is no motion, there is no work

What is Power? Power is the rate at which work is done.  Doing work faster requires more power Power (w) = Work (J) / Time (s) P = W / t SI unit: watt  Watt = J/s

Horsepower 1 horsepower = 746 watts Based on the power output of a very strong horse

Sec Work & Machines

What is a Machine? A machine makes doing work easier. How?  Change the size of the force  Change distance over which the force acts  Change the direction

Increasing Force & Distance Work = Force x Distance Increasing the Force causes a decrease in distance Increasing Distance causes a decrease in force

Work Input & Work Output Work Input = Input Force x Input distance W in = F in x d in Work Output = Output Force x Output distance W out = F out x d out Due to friction: W in > W out

Ideal Machine Contains No Friction!!! W in = W out

Sec Mechanical Advantage & Efficiency

Mechanical Advantage The number of times the machine increases an input force 2 types of Mechanical Advantage  AMA (Actual Mechanical Advantage) Measure actual forces in a machine  IMA (Ideal Mechanical Advantage) Measure with out friction (in an ideal machine)

Calculating AMA Actual Mechanical Advantage = output force input force AMA = F out / F in

Calculating IMA Ideal Mechanical Advantage = Input distance Output distance IMA = d in / d out

AMA versus IMA The AMA is always greater than the IMA because friction is present. Engineers try to design machines with low- friction materials & lubricants.

Efficiency The percent of work input that becomes work output Efficiency = (W out / W in ) x 100% Due to friction: Efficiency is always less than 100%

Sec Simple Machines

6 Types of Simple Machines 1. Lever 2. Wheel & Axle 3. Inclined Plane 4. Wedge 5. Screw 6. Pulley

Lever A rigid bar free to move around a fixed point (fulcrum) IMA = Input arm / output arm  Input arm: distance between F in and fulcrum  Output arm: distance between F out and fulcrum

First Class Lever Fulcrum is located between the input force & output force Ex. Seesaw, scissors, tongs MA: can be: >1, <1 or =1

Second Class Lever Output force is between the input force & fulcrum Ex. Wheelbarrow MA is >1

Third Class Lever The input force is located between the fulcrum and output force Ex. Baseball bat, hockey stick, golf club MA is <1

Wheel & Axle Consists of two disks or cylinders  Each has a different radius  Wheel: outer disk  Axle: inner disk IMA = r wheel / r axle Gears are a modified wheel & axle

Inclined Plane A slanted surface along which a force moves an object to a different height IMA = distance / change in height

Wedge A v-shaped object whose sides are 2 inclined planes Thin wedge of a given length has a greater IMA then a thick wedge of the same length Examples: zipper, knife blades, door stop

Screw An inclined plane wrapped around a cylinder The closer the threads, the greater the IMA

Pulley A rope that fits into the groove in a wheel IMA = # of ropes supporting the load  Pull down - don’t count  Pull up - count Changes the direction of the force

3 Types of Pulleys Fixed: wheel attached to a fixed location Movable: wheel attached to object Pulley System: combination of fixed & movable pulleys Fixed pulley Movable pulley

Block & Tackle Pulley system containing both fixed and movable pulleys

Compound Machines A combination of 2 or more simple machines Example: Honda Ad