Interview Question

Software Engineer Interview(Student Candidate) Bangalore (India)

Puzzle: How do you weight an elephant without using a weigh


Interview Answer

27 Answers


Use a pool -> calculate the volume of water of the pool and the water level, put the elephant in the pool, take note of the level after the elephant after it is in the pool. By Archimedes principle, the volume of water dislocated is the same as the weight. So the weight of the elephant is the same as the difference in water volume.

Fernando on 2010-12-31

Not quite. The volume of the water is not the same as the weight of the elephant. You'd have to estimate the density of an elephant and multiply that by the volume of the water to get the mass, then multiply that by the acceleration due to gravity in water system (SI, English Customary, etc.) you're using. Luckily, mammals are mostly water (humans are around 70% water on average), so about 2/3 of the weight of the elephant would be equivalent to the weight of the water displaced. So you would have to estimate how dense the rest of the elephant is (since it'd be minerals and such, I'd say it's more dense than water) and follow the steps described above.

Kyle on 2011-01-02

Apply a known force to the elephant and measure the acceleration. Use physics to deduce the mass.

Stuart on 2011-01-04

As it is not specified that the International System of Units must be used, define the Elephant unit (E) as the weight of your elephant. Your elephant then weights exactly 1E.

Wouzz on 2011-01-05

Kyle I have to disagree with you. According to Buoyancy principle: Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. So if you calculate the displaced fluid weight, which is quite easy in water as it is very close or equal to 1kg/L, you can get very close to the real weight of the elephant.

Fernando on 2011-01-06

Hmm, I think Kyle is right... and so are you when you say "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object". IMHO elephants don't float...

Wouzz on 2011-01-07

Actually Kyle is right. If we put something in the water, such as an sealed box with a ten pound weight inside (does not float), it will displace a proportionate amount of water. However, if we fill it up with a 50 pound weight, the same amount of water will be displaced, however.
Fernando, you are using the Buoyancy principle. The Buoyancy principle is meant for objects that float (hence the word buoyancy-it means pressure to keep afloat). As Wouzz said, elephants don't float.

The Nav on 2011-01-07

You could just put a boat in water, measure displacement. Then put the elephant in the boat and take the difference. The displacement is going to equal mass as long as it's floating so you're good in a boat.

Kyle was right that if you just threw an elephant in a pool, you would have you figure out density and all that.

Matt on 2011-01-11

I would say bring a female elephant and let them enjoy life..

chodobhagat on 2011-01-11

Isn't that just an improvised "weighing machine"?

Ben on 2011-01-11

it sounds like you all are trying to determine whether an elephant is a witch (Monty Python reference).

Instead of dealing with the whole mess of a wet elephant, why not use other pneumatic tools like an inflatable platform, an air pump, and a pressure sensor. See how much air pressure is required to lift the elephant.

pt on 2011-01-11

My experience is this, you don't have to have a perfectly correct answer. The goal of the question is to see if you can think around a problem. Besides elephants, I have heard 747's and aircraft carriers used as the object. A good answer shows that you thought of an alternate way of measurement, so the water displacement and pressure sensor ideas work. PS, if they ask you to weigh a 747, I would answer "Land it on an aircraft carrier and measure the additional water displacement!"

EP on 2011-01-11

I would use a see saw to weight my elephant. Using my weight (212 lbs in work wear), move the elephant until we balance, and then compare my distance to the pivot point to Clancy's distance to the pivot. (I named my elephant Clancy).

The hardest part... finding a see saw strong enough to hold Clancy.

AZ on 2011-01-11

Archimedes Law of the Lever states: Magnitudes are in equilibrium at distances reciprocally proportional to their weights.

"Give me a place to stand on, and I will move the Earth."

ERIC on 2011-01-11

Elephants DO float, even in fresh water. They're blubbery.

Sami on 2011-01-11

Simple answer : Use a beam balance . Put elephant on one side and start throwing weights on the other side . When the beam is balanced you got the weight of the elephant equal to the sum of weights on the other !

MG on 2011-01-11

Just ask him...

Overthinker on 2011-01-11

Just find a person having a bad day and weigh him/her, help that person feel better, then weigh him/her again. The difference is the weight of the elephant you removed off his/her back. Works for monkeys too.

DaveH on 2011-01-11

one "weight"(s) by putting a weight on it

massive weight on my mind on 2011-01-11

note that the answer above is referenced to the question on THIS page which has been transcribed wrongly from the "home page" question (the "t" has been added)

massive weight on my mind on 2011-01-11

I would ask if it was an Indian elephant or an African elephant.

Monty on 2011-01-12

I would solicit bids and then sub-contract the task to a reputable, cost-effective elephant weight service vendor, and let THEM decide the best approach.They can use a weigh machine or use any other method that does not require ME to use a weigh machine. The question (and answer) are no different than any other variation of, "How do you accomplish a task when you don't have the needed resources?"

Sandy on 2011-01-24

by using first clas lever. keep an elephant at known distance from the fulcrum; make the effort distance long enough that your weight balances the weight of an elephant;
by using the formula
load*load distance = effor*effortdistance( principle of lever)
you already know load distance, effort distance and effort now you can calculate load, which is nothing but elephan'ts weight

Naresh on 2011-12-30

By using the momentum conservation law,
First I find a heavy enough object mith exact mass of m1 and then throw it with speed V1 to the elephant which is standing on a friction-less surface like ice or on plate with wheels under it to reduce the friction.
after they meet, the elephant will move. and the object will change its direction and speed.
Then I calculate the speed and direction of elephant and reflecting object, and finally I solve the vector equation below:
v1=vector of initial speed of object
v2=vector of final speed of object
M=elephant mass
V=vector of elephant speed

Pooya on 2012-06-18

elevate the elephant to height "h". it will have potential energy of "U=mgh".
hurl the elephant into a well which is vacumed well to avoid friction.
at the end of the well, there is a pool of water with exact mass of " m' " and temprature of T1. after elephant reaches the pool he will give all of his energy to water causing water to become warmer Q=U. so mgh=m'c(T2-T1)
with knowing g,h,m',c,T1 and T2 m can be calculated easily.

Pooya on 2012-06-18

place elephant on an iron plate and move it horizontally with steady speed v. and pass them through a constant magnetic field of B.
change B untill the elephant start to move upward.
read B and V.
F=V x B is the force of magnetic field ( x denotes vector multiplication) which is now equal to weght force W=mg. knowing g the gravity constant, solve m.

Pooya on 2012-06-18

use SEE SAW put elephant on one side and on other side some men when it get balanced weighs the men

asad on 2013-03-21

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