AP Physics Question

Test Preparation AP Test Preparation
1

<p>Alright, i have a pHysics test tommorwo, and i was going over some problems, and i saw this ..wHich seems kinda hard</p>

<p>A boat capable of making 9.0 Km/h in still water is used to cross a river flowing at a speed of 4.0 Km/h
(a) at what angle must the boat be directed so that its motion will be staright across the river?</p>

<p>Alright i really dont get this i mean am i supposed to find the resultant of adding The 9 and 4 Vectors and then finding their angle or what?</p>

<p>Help needed...
(haha the test is tommorow...)</p>

2

<p>let's say you plot it in the cartesian plane. the 4.0 km/hr vector is facing 270 degrees. You have to find where 9.0 km/hr vector has to be so that the resultant vector's direction is 0 degrees. Then, find the angle between the two vectors</p>

3

<p>Thanks Man...But just TO make sure though, cant i just assume that the 9 and the 4 form a right angle triangle (So that The Apparent Speed could be staright) Where 9 is the Hypotenuse and 4 is the other side, And then find the angle by using Like Inverse Trig ( Cos^-1 or Sin^-1 Depending on how the triangle is drawn) ?</p>

4

<p>assume vertical is down/up the river, horizontal is across</p>

<p>the 4 km/h current must be balanced by a 4 km/h vertical velocity in the opposite direction. </p>

<p>draw it out:</p>

<p>9 * cos(x) = 4, cos x = 4/9, cos^-1 (4/9) = x</p>

5

<p>boridi is correct in his method</p>

6

<p>uhh i havent taken calc or physics yet so this is obscenely confusing to me. i'm in awe.</p>

7

<p>Thanks alot guys......</p>

<p>O and greendayfan, you dont NEED calculus for this..i never took it either..lol</p>

8

<p>that problem is a basic trigonometry question, but u need to understand how to cancel forces, which is taught near the beginning of physics.</p>

9

<p>xylem..yeah i havent taken trig or precalc either...lol.</p>