# Lethal pennies and other misconceptions

4/13/2016

Here is a list of some common misconceptions in STEM subjects and their explanations.

Physics:
A penny dropped from the top of the Empire State Building could gain enough momentum to kill a person standing on the sidewalk below. First of all, this especially doesn’t work because the Empire State Building tapers from top to bottom, so the penny would just hit the side of the building soon after being dropped. But let’s say it falls from a building of equivalent height and does make it to the ground. The penny still won’t have enough momentum to kill someone. The terminal velocity of a penny (aka maximum speed the penny can possibly achieve in free fall) is between 30 and 50 miles per hour, depending on the conditions. Using the max 50 mph = 22.35 m/s and the mass of a penny (2.5 g = .0025 kg), this gives a momentum of .0025*22.35 = .0558 kgm/s (using the momentum equation p = mv). If you wanted to find the force the penny has, you would need to divide this number by the number of seconds it takes the penny to reach the ground, thus giving kgm/s^2, or Newtons, of force. As you can see, this force is miniscule and not deadly.

Lightning never strikes the same place twice. There is nothing that prevents lightning from striking in the same place more than once. On the contrary, places that are conducive for lightning strikes will generally attract them repeatedly. Some structures have lightning rods for this purpose, so that in the event of being struck the electricity will go to the rod and be directed to the ground instead of doing damage to the structure. In his column “Ask Tom Why,” WGN-TV chief meteorologist Tom Skilling estimated that the Sears Tower gets struck by lightning about 50 times per year.

Math:
If the same equally probable outcome has occurred several times, a different outcome is sure to come next. Examples of this would be rolling dice or flipping a coin.  If you’ve ever had a thought along the lines of, “It’s been heads five times so it should be tails next,” you’ve fallen for this misconception. Sometimes that’s exactly what happens, and it seems like it makes sense, but each outcome is in no way related to the previous or next outcome. Whatever comes next is completely chance.  If you’ve taken a probability class, you may have learned how to calculate the odds of each possible outcome or sequence of outcomes. But again, odds do not dictate what will actually occur. They only serve to inform your prediction.

This diagram explains Newton's third law applied to an airplane wing.

Fluid dynamics:
Air travels over both sides of an airplane wing in the same amount of time. It is true that air travels over differently shaped sides of the wing at different speeds, but not because it has to take the same amount of time. For the wing to create lift there needs to be a pressure difference, with the net pressure pushing up on the wing (i.e. higher pressure below the wing and lower pressure above). Bernoulli’s principle says that this is accompanied by a difference in speed. This is very similar to pipe flow: the faster water flows through a pipe, the lower the pressure in the pipe. Alternatively, Newton’s third law of action-reaction says that since the wing’s design deflects the airflow downward, the wing must be deflected upward. Neither of these have to do with the time the air takes to pass over the wing. There are many debates over which theory is the better one, but that’s another story.

This common theory is an incorrect explanation of lift.
Thermodynamics:
Temporarily turning down the heat in a building when it’s cold outside wastes more energy than keeping it at the same temperature. It is commonly believed that lowering the temperature at night and turning it back up during the day wastes more energy because the furnace has to work harder than normal every time it needs to heat the building back up. However, the heat lost by a warm building in the cold is proportional to the temperature difference between the inside and outside of the building. Less of a difference over the course of several hours adds to a significant energy savings, making it more energy efficient to turn down the heat at night than to leave it at the same temperature.