Thursday, October 6, 2011

The Physics of Waterskiing

Ever wonder about the physics of water skiing?

Well, when you are water skiing physics comes into play quite a bit.  When you try to get up on a ski, it’s important to keep your ski at a fairly precise angle to the water so that as you’re pulled forward the water hitting your ski creates a downward force, enabling you to stand up.  When the upward force of the water on your ski is equal to the downward force of gravity, you can effectively stay afloat.

When you are being pulled behind the boat, the force of tension in the rope is also acting on you.  When there is constant tension in the rope, you will travel at the same speed as the boat that is towing you.  However, if you’ve ever been water skiing you know that the skier is often traveling at a speed faster than the boat.  How is this possible? Well, this is where centripetal forces come into play. The rope keeps the skier in a circular path around the boat.  Because of this circular motion, the skier experiences acceleration toward the center of the circular path, just as we saw in lab with the swinging mass.  This centripetal acceleration means that the skier can be skiing at speeds quite a bit faster than the boat is traveling-- making wipe outs that much more painful!

Finally, Bernoulli's principle factors in as well.  As the velocity of fluid flow increases, the pressure decreases.  Therefore, the speed of the boat must increase when the skis have less surface area (so, if you're doubling skiing, the boat can pull you significantly slower than if you are single skiing).  If you've ever tried to barefoot ski, this makes complete sense and explains why the boat has to pull you a such a fast speed in order to keep you going-- because after all, your feet are much, much smaller in surface area than a ski.  And this is why when you fall while barefoot skiing, it can be pretty painful!