Riddle #472

Question: You have two coconuts and you want to find out how high they can be dropped from a 100 story building before they break. But you only have $1.40 and the elevator costs a dime each time you ride it up (it's free for rides down).

How can you drop the coconuts to guarantee you will find the lowest floor they will break at, while starting and ending at floor 1?

Note: They break when dropped from the same height and they don't weaken from getting dropped.

Riddle Discussion

By: THWeaver on 22/1/16

Ignore the elevator and instead stand outside of the building. Take a rock and approximate its volume through a rough estimation of measurement, then find a small pool, semi-cylindrical pool of water; place the rock in and measure the displacement of the water. Using the values of density and volume you just found, calculate the mass of the rock. Next, calculate the force due to gravity upon the rock through F = mg. Place the rock upon one of the coconuts. Continue to do this until the coconut is near to breaking. Once you have completed it, sum the forces of the rocks. Make certain to approximate the mass of the coconut as you did the rock.\n\nNow, understanding that F = mg + (mgh / 2d), you can solve for the correct distance (height) by estimating the height of the building and dividing by the number of floors. Go floor by floor (or infer as you note the resulting force) until you get a force close to (preferably greater than) the summed forces of the rocks on the coconut.\n\nSpeaking of which, you do need to estimate the height of the building; I almost forgot that. Find a nearby lamppost and measure both its height and the length of its shadow; immediately measure the length of the building's shadow afterwards. Using the ratio of (where B is building and L is lamppost) B shadow / B height = L shadow / L height you can approximate the height of the building. As mentioned before, divide this by the number of floors in the building to get an interval of heights to test. Note: you do have to assume that each floor of the building is nearly equal in height for the last step to work properly.\n\nHuzzah for physics!

By: musab on 12/3/15

I think the answer is wrong because when you drop second coconut you have to ride down to get coconuts so it cost more than 1.40. I can get at most floor 47. I can go floor 22 then floor 32 with first two coconuts then floor 38 and 42 with second 2 coconuts. I cannot go beyond 47 because I already use 74 dime after first and second try and 66 dime left. After I guarantee floor 42 I ride down and get 2 coconuts and go floor 46 if first coconut does not break then I ride up to 47 and drop last one even it does not break since I need to ride down to get the coconut I cannot ride up again becaues just 19 dime left. If at floor 46 first coconut breaks then drop second one at floor 44 if it breaks than the answer is 43 otherwise 44.

By: tardis221b on 7/1/14

Go up the stairs

By: AtomicBond on 14/12/13

Take the stairs

Similar Riddles

The Tide (medium)

By Fred

Question: At low tide 14 rungs of a ship are above the water, the rungs are 15 centimeters apart. each 15 mins the water rises 15 centimeters. after 2 hours and 30 mins how many rungs are above the water?

Question: During what month do people sleep the least?

Question: When is 99 more than 100?

Riddle #3363 (medium)

Question: When you're given one, you'll have either two or none?

Question: A Japanese ship was sailing in the Pacific Ocean. The Japanese captain of the ship put his diamond chain and Rolex watch on a shelf, went to get a shower and returned ten minutes later. Now listen carefully, as I will only tell it once: When he returned, both the chain and the watch were missing!! He called the crew of his ship together. There were four of them. A British guy was the cook of the ship. The captain asked him: "Where were you the last ten minutes?" And the cook answered "I was in the cold storage room to select the meat for lunch". A Sri Lankan was the house keeping guy. The captain repeated his question to him, and learnt that the Sri Lankan was at the top of the ship correcting the flag which had been put upside down. An Indian guy was the engineer maintaining the ship. Same question, and the Indian told that the he was in the generator room checking the generator. A French guy also served on the house keeping crew. Same question, and the French told that he was sleeping after the night shift. Within ten seconds the smart captain caught the thief. Who was the thief? How did the captain find him? Source: Puzzlevilla