Insane physics calculations figured out by reddit IAMA

Cass Anderson

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Today over on Reddit, readers were treated to some top notch mathematical wizardry. Physicist/author Aaron Santos did an IAMA where users were able to submit asinine questions of which Santos would do the mathematical calculations for. Examples include ‘gallons of liquor consumed in a lifetime’ (4.4 m3)and ‘number of biblical sins committed in a lifetime,’ (60,000 sins). You can read the entire IAMA here, but we’ve compiled 9 of the best and most outrageous calculations for you here.

How fast do you have to throw a burrito so it catches on fire?

Answer: I’m gonna guess a burrito has a similar flashpoint (i.e. the temperature at which it ignites) to wood, which would put it around 300 degrees Celsius (~570 Kelvin). There’s a lot of water in food, so I’ll assume they have similar heat capacities (~4 J/g K). As such, a 0.5 kg burrito would need to gain 500 kJ of heat energy to ignite. The energy lost due to friction for a burrito will be about the same magnitude as that for a baseball. I’m assuming all the energy lost to friction goes into heating the burrito. (Numerical Assumptions: Drag coefficient ~ 0.3, Area ~ 9 square inches, air density 1.2 kg/m2, burrito catchs on fire in 1 second.) This will be about (0.0003 kg s/m) x (velocity)3. This gives about 1000 m/s.

What is the kinetic energy of a growing finger nail?

Answer: I’ll assume the fingernail has a thickness of 0.2 mm and an area of 1 cm2. If it’s about as dense as water, then this would make its mass 20 mg. My nails grow about 2 mm per week. Using these, you can estimate a kinetic energy of 10-22 Joules.

If I gain a kilo of weight here on earth, how much more will my wife be attracted to me?

Answer: Assuming you’re 80 kg and she’s 60 kg and you’re standing 1 m apart, she’ll be attracted to you with an extra 4×10-9 Newtons of force. That makes you about 1.2% more attractive to her.

Is it more dangerous to own and regularly ride a motorcycle, or regularly use cocaine?

Answer: A quick web search shows that in the U.S., about 4000 cocaine-related deaths occur each year. There is a similar number of motorcycle related deaths. The fraction of cocaine users over the age of 12 is about 0.7 percent. There are about 6 million motorcycle riders in a U.S. population of 300 million, meaning that roughly 2 percent of Americans are motorcycle riders. Given a similar numer of deaths, but about 3 times as many riders as cocaine users, it’s likely that cocaine use is more dangerous.

How many Hot Pockets would you need to burn to release the amount of energy released by a standard US issue hand grenade?

Answer: I’m going to assume the source below is correct. (Not necessarily a safe assumption.) It lists about 800 kJ of energy for the grenade. A hot pocket has about 300 Calories. A Calorie is just another unit of energy. 300 Calories equal to about 1kJ of energy. (Food calories are 1000 times bigger than the physics calorie.) You’d need about 800 hotpockets to equal 1 hand grenade. If this number is surprisingly low, remember you have to consider rate. All the energy of a grenade is released instantly, were as it takes a while to burn all the hot pockets.

How many semesters would it take to gather enough pubes from a dormitory floor to make a size large sweater?

Answer: OK…now this is my kind of question. I’ll assume 1 cm long pubes spaced 2 mm apart covering a total area of 20 square inches. Laid end-to-end, that gives a total pube length of about 60 m for each person. At about about 10 microns thick with a density of 1 g/cm3, you’d have a total mass of about 64 mg of hair. The mass of a sweater might be 0.3 kg. From this, you can see that you’d need about 5000 people or roughly 100 dorm floors assuming 50 people per floor.

How large would the wings of a pegasus have to be to allow a horse to actually fly?

Answer: We need to consider two things here: wing area and wing flapping rate. I did a similar problem for Mothra’s wingspan. Horses weigh about 500 kg, which gives a downward gravitational force of about 5000 N. If you assume her wings flap 2 meters down and do so once every second, she’d need winds that were about 1000 m2 in area. A 2 meter wide wing would need to be about 5 football fields long.

How many house cats would it take to crush an average man?

Answer: Let’s assume 2000 kg (roughly the mass of a car) can crush a man. If your cat weighs 5 kg, it would take 400 cats to crush a man.

If I am a poor college student trying to donate blood/plasma/sperm for food money, how much should I make per year if I attend the maximum times?

Answer: You can make up to $50 per for blood/plasma donations. Let’s assume you donate once per month. I’ve also seen sperm donation banks that pay up to $1000 per month (assuming you’ve got that kind of genes they’re looking for.) The catch is you can’t masturbate of have any other type of sex. If you meet all the criteria, you’ve got yourself about $17,000 per year. Most of that comes from the sperm.