Originally Posted by

**Harry1W** The question reads:

"A glider is moving with a velocity $\displaystyle v = (40, 30, 10) $ relative to the air and is blown by the wind which has velocity relative to the earth of $\displaystyle w = (5, -10, 0) $. Find the velocity of the glider relative to the earth."

My argument goes that as the velocity of the wind relative to the earth increases, and the velocity of the glider relative to the air, increase, so does the velocity of the glider relative to earth. So if we let $\displaystyle v_E $ represent the velocity of the glider relative to the earth,

$\displaystyle v_E = v + w $.

Therefore, in this case the velocity of the glider relative to the earth,

$\displaystyle v_E = (40, 30, 10) + (5, -10, 0) = (45, 20, 10) $

However, the answer booklet has the expression for $\displaystyle v_E $ as follows:

$\displaystyle v_E = v - w $

giving $\displaystyle v_E = (35, 40, 10) $ which I suppose must be the right answer. Could somebody explain this result to me? Thank you.