ESTIMATION A skill every engineer should cultivate

ESTIMATION! A skill every engineer should cultivate

How many ping-pong balls can fit into this room? Timer

Accuracy of answer Estimation Time to solve problem Detailed analysis

Vol = d 3 = 1 Vol = (4/3)p(d/2)3 =. 52

Estimation is… Quick models + Quick measurements + Quick calculations

Estimation Keep things simple…very simple

An estimation challenge What does a 5 gallon pail of liquid weigh? Density of water: 1. 0 Kg/L or 1 pound/pint (“a pint’s a pound”) 1 gallon = 8 pints = 8 lbs 5 gallon pail = 5 x 8 = 40 lbs (4% error, exact = 41. 74 lbs) Or… 1 qt ~ 1 liter 4 qts/gal 5 gal = 20 qts ~ 20 liters 20 Kg (1 Kg/L) 20 Kg x 2. 2 lb/Kg = 44 lbs (5% error) Timer

1. Observation • Notice the chairs (size/material/strength) • Notice a pop can (material/thickness/howformed) • Notice a staple (how it changes shape/how it grips the paper)

2. Model the physics…simply • • • F = m. A Everything is a cantilevered beam Coffee cup is 1 -D heat transfer Vibration is 2 nd order DEQ Pendulum is small angle

An estimation challenge Each time an automobile tire turns, the tire material flexes and extends in a stress cycle as it takes the weight of the car. How many stress cycles does an average automobile tire go through each year? Timer

3. Measurements • Develop a set of self-calibrations over wide range • Time, length, weight, velocity, … • Low extreme, middle (human experience), high extreme • For middle range, used body-centered measuring tools • No need to run to rulers or scales

TIME 1 ns (time it takes light to travel a foot) 100 ms (human reaction time) 0. 5 s (4 ft drop of an object) 1 s ("a-thousand-one-a-thousand-two. . . ") minutes, hours, days, years (you know these)

FREQUENCY 4 Hz (beat with your hands) 440 Hz (concert A on the piano) 1 KHz (high for a mechanical resonance 92. 5 MHz (your favorite FM radio station) 500 MHz (digital IC switching frequencies)

LENGTH 0. 5 micron (modern IC feature) 70 micron (human hair) (. 003") . 004 in (paper thickness) 1 in (quarter) (actual = a little over 15/16") 6 in (dollar) (actual = 6 3/32") 9 in (my finger span) 1 ft (standard floor tile) 6 ft (my outstretched arms) 100 m (football field) 1 mi (4 times around the track, 1 min in a car) 240, 000 mi (distance to moon) 92 million mi (distance to sun)

VELOCITY 1 fps (follow your finger) 60 mph (car) 200 mph (airplane take off speed) 3 E 8 m/s (speed of light)

ACCELERATION 0. 3 g (car 0 -60 in 10 sec) 0. 6 g (airplane 0 -200 in 15 sec) 1. 0 g (earth) 6. 0 g (30 -0 mph in ¼ sec) 200 g (4 ft drop test, 0. 25 inch crush zone) ? ? ? (fast elevator)

FORCE/WEIGHT 1 oz (4 quarters? Measure it (weigh a roll)) 1 lb (box of nails, tub of coleslaw, pint of water) 10 lbs (big bag of flour) 25 lbs (bag of dog food) xxx lbs (body weight) 1 ton (a car) ? ? (big trucks) ? ? (airplane, ships)

TORQUE 1 ft-lb (12” bar with tub of cole slaw hanging off the end 200 ft-lb (me hanging off a 15” bar) PRESSURE 4. 3 psi (10 ft column of water, water is ½ psi per foot) 9 psi (playground ball) 14. 7 psi (ground level air pressure) 100 psi (pneumatic automation, shop air) 5000 psi (high pressure hydraulics)

Pressure at your outdoor tap? • • • Hose shoots about 30 ft up Water is ½ psi per foot water is 15 psi at nozzle Assume shooting process 50% efficient water is about 30 psi inside hose

An estimation challenge How many gallons of water flow under the Washington Avenue bridge each hour? Timer

3. Calculations What a good estimator thinks of calculators….

Round off • • 8 = 10 23 = 20 32. 2 fps 2 = 30 pi = 3

Conversions 25 Microns =. 001 in. 1 mph = 1. 47 fps 1 atm = 14. 7 psi = 760 mm. Hg 1 g = 32. 2 fss 1 BTU = 778 ft-lbs 1 hp = 550 ft-lbs/s = 746 watts 1 lb = 4. 4 Newtons Or, remember equivalent points on scale: 32 deg F = 0 deg C (-40 = -40) 15 mph = 22 fts = 264 ips 60 mph = 88 fps 1/2 psi = 1 ft of water

If you don’t know the units…. EXAMPLE: Someone says, “The pressure needed to promote skin breakdown is 100 mm. Hg” try www. megaconverter. com

How to become better • Give yourself estimation problems all the time • Start memorizing conversions • Build a personal library of measurements • Be familiar with simple physics models • And… THROW AWAY YOUR CALCULATOR!

An estimation challenge How high can I be lifted by the energy in a 9 v battery? (I weigh about 165 lbs) Timer

Some to take home • What’s the weight of the stainless steel skin on the Weisman Museum? • What’s the pressure under each leg of your desk chair? • How many times is the ‘e’ key pressed in the 3 year lifetime of a computer? • How long does it take to make an ice cube? • What’s the bite pressure on your teeth when eating an apple? • What’s the volume of a ton of bricks?

POWER 1 watt (flashlight bulb) 100 watt (bright house bulb) 10, 000 watt (movie lamp) 200 hp (big car engine ENERGY 80 ft-lbs (PE of you standing) 5 watt-hrs (fresh 9 v alkaline battery)