Relative Velocity

Y_Shun SG

a) 25s; 50m
b) 53.1degree, 31.3s

How to do b), especially the time taken part.

^tamago^ SG

atan(BC/AB) = atan(40m/30m) = 53.1º

eagle SG

from there,

upwards speed = 1.2 sin 53.1 degrees

time taken = 30 / ( 1.2 sin 53.1 ) = 31.3 s

mrcoconut SG

how to get part a?
Laughing

sorry monday exam le, i still so blur

Y_Shun SG

don't get this part lehs... Neutral Neutral Confused

eagle SG

it is resolving 1.2m/s into vertical and horizontal component.

The horizontal component no need to bother because rather troublesome to calculate. And if we purely take the vertical distance, we only need to consider the vertical component.

This is the beauty of vectors:
vertical component of velocity only travels in the vertical direction
likewise for horizontal

eagle SG

I lazy to do

Y_Shun, you post your answer for him?

^tamago^ SG

actually ah, why sin ah? Laughing

i noe it leads to the answer but why v-component is defined by sin not cos when v-component is on the adjacent side?

mrcoconut SG

actually i know how to do le!

just worked it out thanks!

but this is not O level standard right?
like quite easy leh

eagle SG

i assume the 53.1 degrees is with respect to the horizontal line, and clockwise

didn't really bother to check what you do... lazy Smile

lpx88 SG

w8.....i abit blur....

Edit: nvm i noe which angle liao..

Y_Shun SG

omgs i dun understand leh.

mrcoconut:

^tamago^ SG

yeah same here, but v-component is adjacent to this 53.1º angle. Neutral

can't be answer key wrong? Confused

Y_Shun SG

BAC = 53.1?

lpx88 SG

That's wad i got too...........

mrcoconut SG

this is O level standard?

why so easy ah?

the diagram is already given or we must draw ourselves?

lpx88 SG

how did u get 31.3 secs?

mrcoconut SG

i did everything. but i never find the last time nia.

but big improvement leh! Very Happy

Y_Shun SG

-_-

its the last part tht is hard lols.

lpx88 SG

The speed of 2.33m/s..shoudn it be the same for both guys?

Assuming speed of river is constant

And they have the same speed in still water?