# Relative Velocity

#### sb

A small air plane is travelling at a velocity relative to the air of 100km/h [E]. A wind from the north is blowing at 20km/h.
(a) Determine the position of the airplane after 2.5 h of this motion.
(b) Find the average velocity of the airplave during the 2.5 h interval.

Thanks a Lot...

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#### HallsofIvy

Homework Helper
Draw a picture: Draw a vertical line (from north to south) of length 20 (whatever units you want) representing the velocity vector of the air relative to the ground. Draw a horizontal line to the right (east) from the tip of the previous line of length 100 representing the velocity vector of the airplane relative to the air.

The "velocity vector" of the airplane relative to the ground is the hypotenuse of the right triangle you have just drawn. You can use the Pythagorean theorem to find the length of that: the speed of the airplane relative to the ground. You can use tan(theta)= 20/100 to find theta, the angle the airplane flies relative to north.

Multiply by 2.5 to find the position after 2 and 1/2 hours.

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