W5.3 Sailing With Vectors

Graphic - Photo for Sailing PIRA

An air track glider, seen in the photograph above, is equipped with a sail as seen in the photograph below. Air from the air gun shown in the photograph is directed onto the left side of the sail, as indicated in the drawing below.

Graphic - Sail diagram 1

When the air gun is turned on, what will the glider do?

  1. move to the right.
  2. move to the left.
  3. remain where it is.
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Hint

The answer might seem quite obvious ... and it is.

What is less obvious is to explain why in terms of the force acting on the sail. You should be able to use your knowledge of forces to explain this. A good starting point is to the direction of the vector that represents the change of velocity of the ‘wind’ from the hairdryer.

 

Solution

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Solution

The answer is (1): the air track glider will move to the right, as seen in this MPEG Video. Note that after the video begins you hear start of the air track blower then the start of the air gun blowing the "wind" into the sail.

When the air stream deflects off the sail, the direction of the change in the velocity of the air stream is (approximately) SW. This is also the direction of the change in acceleration of the air and (by Newton’s second law) the direction of the vector that represents the force that the sail exerts on the air, $\vec{F}_{AS}$. The force that the air exerts on the sail, $\vec{F}_{SA}$ is in the opposite direction, approximately NE, laballed as F on the righthand diagram.

Because the glider can only move along the air track, the component to the right in the diagram causes the glider to move to the right.

Graphic - Sail Diagram 2

This is the situation that allows a sailor to "tack", or sail with a component of motion against the wind.