estimation! a skill every engineer should cultivate
TRANSCRIPT
ESTIMATION!
A skill every engineer should cultivate
Time to solve problem
Acc
urac
y of
ans
wer
Detailed analysisEstimation
Vol = d3 = 1 Vol = (4/3d/2)3 = .52
Estimation is…
Quick models
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Quick measurements
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Quick calculations
Estimation
Keep things simple…very simple
An estimation challenge
What does a 5 gallon pail of liquid weigh?
Density of water: 1.0Kg/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)
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1. Observation
• Notice the chairs (size/material/strength)• Notice a pop can (material/thickness/how-
formed)• Notice a staple (how it changes shape/how
it grips the paper)
2. Model the physics…simply
• F = mA• Everything is a cantilevered beam• Coffee cup is 1-D heat transfer• Vibration is 2nd 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?
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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)
LENGTH0.5 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)
3E8 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?
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3. Calculations
What a good estimator thinks of calculators….
Round off
• 8 = 10
• 23 = 20
• 32.2 fps2 = 30
• pi = 3
Conversions25 Microns = .001 in.1 mph = 1.47 fps1 atm = 14.7 psi = 760 mmHg1 g = 32.2 fss1 BTU = 778 ft-lbs1 hp = 550 ft-lbs/s = 746 watts1 lb = 4.4 Newtons
Or, remember equivalent points on scale:
32 deg F = 0 deg C (-40 = -40)15 mph = 22 fts = 264 ips60 mph = 88 fps1/2 psi = 1 ft of water
If you don’t know the units….
EXAMPLE:
Someone says, “The pressure needed to promote skin breakdown is 100 mmHg”
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 9v battery?
(I weigh about 165 lbs)
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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?
POWER1 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 9v alkaline battery)