Most of us have memories of spending hours every day playing on the neighborhood swingset growing up — or if we were lucky enough, we had one in our own backyards to enjoy. They came in all shapes and sizes. Some had swings, slides, seesaws or monkey bars — various combinations of things to climb on or over or beneath. These served as the setting for great battles, epic stories and anything our imaginations could come up with.
We could spend all day playing on a swingset and never consider the science that makes it work.
Fun With Friction
Picture your favorite slide growing up — the one you knew forward and backward. You knew precisely how to hold your body to miss any bumps and gain the most speed on the way down. How hard did you hit the ground at the bottom? Now picture that slide again, only with something coating it that would reduce the friction between you and the metal or plastic surface. How far do you think you would fly before you hit the ground? Friction is what makes slides fun and safe.
Friction isn’t considered a fundamental force like gravity or electromagnetism, but it still occurs whenever two surfaces come in contact with each other. Scientists believe it’s due to the electromagnetic attraction between charged particles on opposing surfaces. When you’re building a slide, you need to choose materials that offer enough friction to keep you from flying off the end. They shouldn’t cause you to grind to a halt because the charged electromagnetic particles in your shorts interact too much with those on the slide.
Potential to Kinetic and Back Again
Most of us know how to swing. You kick off the ground and then move your legs in a rhythm to push yourself to new heights. Get the pattern right, and it wouldn’t be long before you were shaking the entire swingset and threatening to flip right over that top bar — though if you were anything like us, you definitely chickened out before you started inverting. Thankfully, those large commercial swingsets were usually well anchored in the concrete below the playground, so we didn’t have to worry about pulling it loose from the ground. That didn’t stop us from trying, though.
When you kick off on a swing without moving your legs, you’ve essentially turned yourself into a pendulum. A pendulum relies on gravity, inertia and centripetal force to maintain its movement. Unless it is acted upon by an outside force, it will never move further than the point where it was released. That’s Newton’s first law of motion in action. Once it reaches its peak, gravity will pull it back toward the ground, continuing the cycle until the gathered energy is expended. When you start kicking your legs, you become that outside force, switching back and forth between potential and kinetic energy. The higher you push your swing, the more potential energy you’ve created.
Understanding Air Resistance
Gravity isn’t the only force you have to contend with when you’re trying to reach the stars on a swingset. The very air you breathe is also working to slow you down and keep you from moving any faster than you already do. Air resistance is defined as “the forces that are in opposition to the relative motion of an object as it passes through the air.” Essentially, the faster you move, the harder it becomes to move through the air.
You can mimic this sensation by sticking your hand out a car window when it’s in motion. If you hold your hand flat, perpendicular to the ground, you’ll feel strong air resistance. If you put your hand parallel to the road, the resistance is still there, but it doesn’t have as great of an impact because you’ve reduced the surface area.
Air resistance and gravity make it harder to get the kind of speed most people want on swings and slides. You can counteract this a little by squeezing into as small a space as possible. The less surface area you have, the less wind resistance you’ll generate. It is also why bikers press themselves to the bars to pick up speed when they’re moving downhill.
Science Shouldn’t Detract From Fun
We probably spent years as kids playing on these swingsets without ever giving a passing thought to the science that created them or the physics that make them function. That isn’t a bad thing. Knowing how things work is great, but understanding the science shouldn’t detract from the fun.
It does make it easier to teach basic physics to a young crowd, though. If you ever find yourself having to explain the concepts of potential and kinetic energy, momentum or Newton’s first law of motion, just head out to the nearest swingset for the perfect real-life example.