Archive for October, 2009

Wakes and relativity

Thursday, October 22nd, 2009

It is a bit of a stretch to draw an analogy between a sailboat’s wake and special relativity. A picture of a wake and the space-time diagram of relatively can be drawn to appear similar. This is an amusing coincidence, not a deep insight.

CH#8Fig8-8

The boat on the left produces a wake with a fixed angle. This angle does not depend on the speed of the sailboat. Fast or slow, the wake angle is the same. Outside this angle, there are no wake waves. If one only watched the water, the boat would not be noticed outside the wake.

The picture on the right shows that an event that happens at the point marked with the star cannot be observed until light from the event has time to get to an observer. Outside this “light cone” the event cannot be noticed. Just as the wake angle does not depend on the boat speed, the shape of the relativity diagram depends only on the speed of light. It does not depend on the speed of the object that produced the event, or the observer who detects it.

Of course there are differences. Light travels a lot faster than a wake.

Capsize made easy

Wednesday, October 21st, 2009

In the final analysis, a capsize is a result of unbalanced torques. For small sailboats, the torque provided by the sailor’s position at the edge of the boat can provide the torque needed for stability. When a sailboat tips too far, the sailor’s torque is diminished as he is brought closer to the center of the boat. It takes a lot a practice to capsize as elegantly as this MC sailor.

flatcrop

When the wind is astern, the required restoring torque is small. The sailor can rest in comfort.

Almostcrop

At this point, excessive heel means the sailor is contributing essentially no restoring torque.

overcrop

This sailor’s final position clinging to the bottom might help a little. But it is too late.