You may have heard that if you drop a penny from the top of Empire State Building, it could kill somebody. At first thought, this seems logical enough. The Empire State Building is, after all, a mighty 381 metres tall. Anything dropped from that height would be dangerous, right? Wrong. Even though the observation deck of the Empire State Building is 381 metres high, a penny dropped from there would only achieve a terminal velocity of around 19 kilometres per hour, hardly enough to kill somebody.
As you probably know, all freely-falling objects experience an acceleration due to the force of gravity towards the surface of the earth. In a vacuum, this acceleration would be 9.8 metres per second per second. But since the earth has an atmosphere, many different factors affect the acceleration of falling objects. Wind speed, air pressure, temperature and humidity all affect the acceleration of a falling body. Another crucial factor in calculating the acceleration is the air resistance acting on the body. Air resistance is the upward resistive force offered by molecules of the atmosphere to counteract the force of gravity. This resistance largely depends on the shape and configuration of the object. For example, a large sheet of paper would experience a much greater upward force because of its great surface area and light weight. However a small, dense metal sphere of iron would experience much less upward force.
Since a penny weighs only 2.5 g but has a diameter of 19 mm, it is, like a sheet of paper, affected greatly by air resistance. When falling, the penny continuously flips, and this further reduces its acceleration towards the earth. When the penny finally does reach the earth, it will be travelling at only around 19 kilometres per hour. That may cause a sharp sting to the head, but it is certainly not enough to kill.
Even though pennies are not capable of killing when dropped from a height, other objects are. Nuts and bolts, used commonly in construction, are heavier and more compact compared to pennies. This means that if they are dropped from a height, they would achieve a greater terminal velocity (and would be more dangerous) than a penny. That is why construction workers always wear hard hats at work sites. A stray nut or bolt could kill seriously injure them.