Relative Velocity (Old Syllabus)

• Snoopyies

  17 Oct 08, 21:23

  I am re-take "O"level old syllabus add maths this year but relative velocity so difficult.

  I can't figure out how to draw the diagram for relative velocity. It is damn tough.

  Don't know how to draw and solve. Help please.

  1   To a cyclist travelling due south on a horizontal road at 7 m/s, the wind appears to be blowing from north-east. Given that the wind has a constant speed of 12m/s, find the direction from which the wind is blowing. The answer is 0.20.6 degrees.

  2.  An aeroplane can fly at 300 km/h in still air. If a steady wind 60km/h is blowing from the direction 030 degrees, determine

  a. the direction in which the aeroplane must head in order that it actually flies due north. The answer is 0005.7 degrees.

  b. the aeroplane's actual speed. The answer is 247 km/h

  3. A boattravels due north at a speed of 20 km/h. To the man in the boat, the wind appears to blow at 10km/h from the bearing of 030 degrees. Find the actual velocity of the wing. The answer is 12.4 km/h at 156.2 degrees.

  Thanks a lot. XD.

   

• Snoopyies

  18 Oct 08, 12:52

  Help please. Moderators ? Anyone ? Please help.

• tinuviel07

  18 Oct 08, 13:15

  i would have helped but i don't wanna upload.. where's all the people

• Mikethm

  18 Oct 08, 15:06

  Here is the solution for the 1st one

  

• Mikethm

  18 Oct 08, 15:12

  But solutions are not important to you at this point in time. Blind sequence is what is important to you.  And since it is out of syllabus, I will share it here with you

  1. Draw the velocities information down INDIVIDUALLY.

  2. Play with the formula V(a-b) = Va -Vb.

  Here you have to make the unknown the subject. In the prior example, you were given V(a-b) and Vb. Thus you make Va = V(a-b) + Vb.

  Put your pen to paper. Draw in the direction of V(a-b) and THEN in the direction of Vb.
  NOW connect the start and end of what you have drawn and THAT is Va.

  3. Use E Maths to solve whatever you want.

   

  Step 1 is not necessary for question given in column vector form. Drawing is unnecessary as well.

• Snoopyies

  18 Oct 08, 16:03

  Jalat, not in the old syllabus also, cannot be lah. I'm a private candiate, last batch to take the old syllabus this year.

  Don't understand leh, how to tell wat's V(a-b), Va, Vb in the question. How to know where to draw the arrows. Die already. Sure fail. Never mind. also not fail before. XD.

   

• Mikethm

  18 Oct 08, 16:45

  Ya it is in your syllabus. And ya if you ain't done with the other topics... this topic should just give up.

  As for V(a-b), Va and Vb... you don't have ShingLee TYS meh?

  V(a-b) is the velocity of "a" as it APPEARS to "b". Thus "a" refer to the wind and "b" refer to the cyclist. And the question say "45 from the north east".

  Step 1 (understanding the information)
  So you draw an arrow STANDALONE pointing toward SouthWest AS the wind COME from the NorthEast.

  And the cyclist (b) is moving south... SO YOU DRAW an arrow ALONE pointing SOUTH.

  Now you know the "arrows" for V(a-b) and Vb

  Step 2 (Construction of diagram)

  AND YOU NEED TO FIND Va.

  Using the formula Va-b = Va  -Vb <----- LOOK AT THE SHINGLEE TYS!!!

  Since you NEED to find Va. MAKE IT THE SUBJECT IN THE EQUATION.

  Va = Va-b + Vb

  Now JUST PUT THE TIP OF YOUR PEN TO PAPER. DRAW IN THE DIRECTION OF Vb, STOP, DON'T LIFT YOUR PEN. CONTINUE NOW IN THE DIRECTION OF Va-b.

  LIFT YOUR PEN UP AND LOOK!!! YOU NOW HAVE 2 SIDES OF A TRIANGLE. THE 3rd SIDE WILL BE your Va.

  In my diagram. I represented the apparent velocity (Va-b) AS Va. The velocity of the cyclist as Vc ( Vb in the formula) and THE velocity of the wind as Vw( Va in the formula).

  FROM HERE ON, IT IS ELEMENTARY MATHEMATICS.