…and with charm.
You’ve probably noticed that I like puns; when I spent a lovely Saturday afternoon participating in a charm bracelet walk with a couple of friends in nearby Snohomish, a certain quark pun of questionable propriety ran through my mind. However, what really got my science-gears turning was the sight of skydivers as they descended into the nearby fields.
Air resistance is a topic I’ve tackled on many different occasions, including a post on this very blog. (If you haven’t tried the activity described at the end of the post, go for it now!) Parachutes work because air, being a fluid, exerts a force on objects that move through it. As the surface area of an object increases, the air has more space to push on it, thus increasing the total force exerted on the object. A parachute has a large surface area and a relatively low mass, meaning that the large upwards force due to air resistance is only minimally canceled out by a smaller downward force due to gravity. After a quick review of freebody diagrams, we can see that the net force on the parachuted skydiver is actually upwards, yet he continues to move downwards.
This counterintuitive detail trips up many students. What we have to remember from Newton’s Second Law (Force=mass*acceleration) is that our net force shows the us the direction of our acceleration, not our motion. Let’s define the downward direction to be positive. An upward net force indicates a negative acceleration; since acceleration is a measure of how quickly an object’s velocity is changing, and our skydiver is not changing direction, a negative acceleration represents a reduction in speed. Our analysis of the forces shows us that the skydiver is slowing down, and that’s exactly what we see.
You can experience this phenomenon even if you have a fear of heights. Like air, water is a fluid and exerts a force analogous to air resistance. Tape rocks or small weights along one edge of a clean trash bag. Take it to a pool (preferably one owned by friends of yours) that is about five feet deep (four, if, like me, you are a shade under five feet tall). Start running through the pool, then spread your arms out and hold the trash bag behind you, weighted end down, like a cape. The water will exert a force on the trash bag and you’ll slow down.
(This should go without saying, but PLEASE don’t try this unless you have permission.)