ESTIMATION!
A skill every engineer
should cultivate
How many ping-pong balls
can fit into this room?
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Accuracy of answer
Estimation
Time to solve problem
Detailed analysis
Vol =
d3 =
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: 1.0Kg/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/howformed)
• Notice a staple (how it changes shape/how
it grips the paper)
2. Model the physics…simply
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•
•
•
•
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)
1s
("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)
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?
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•
•
•
•
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
Conversions
25 Microns = .001 in.
1 mph = 1.47 fps
1 atm = 14.7 psi = 760 mmHg
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 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?
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 9v alkaline battery)