Ice Sailing

QUESTION:

An ice sailcraft is stalled on a frozen lake on a windless day. A large fan blows air into the sail. If all the wind produced by the fan strikes the sail and then bounces backward, the craft will move

backward (to the left).
forward (to the right).
not at all.

drawing of an ice sailcraft

ANSWER: B

You might think the craft wouldn't move—that the force of the wind on the sail would be balanced by the reaction force on the fan. That would be true IF the wind came to an abrupt halt upon striking the sail. But it doesn't. Instead, the wind bounces from the sail, producing a greater force on the sail than if it merely stopped. (Like any collision, more force is required to reverse direction than to start or stop.) The result is that there is a net force on the craft and a forward acceleration.

Why not simply turn the fan around and omit the sail?

Back to question