Mechanics 1 (vectors)

Hi people! I'm struggling on one of the questions my teacher has given so and was wondering if anyone could help. The question is A man is able to paddle a canoe at the speed of 3ms-1 in still water. He wants to cross a river which is 20m apart. The flow of the water is 1.5ms-1 which is parallel to the river.
It tell me to say at what angle to the river should he paddle to get straight across
I have no idea how to do this

Reply 1

Daniel Freedman

7

In order to stay straight, he must paddle with a velocity of 1.5m/s in the opposite direction to that which the river is flowing. You know his speed is 3m/s. You can therefore draw a right-angled triangle, with hypotenuse 3. The other two sides are the two perpendicular components of his velocity (i.e 1.5 and 3/2 root3 [irrelevant]). Now use some trig to find the angle.

Reply 2

Chaoslord

16

Daniel Freedman

In order to stay straight, he must paddle with a velocity of 1.5m/s in the opposite direction to that which the river is flowing. You know his speed is 3m/s. You can therefore draw a right-angled triangle, with hypotenuse 3. The other two sides are the two perpendicular components of his velocity (i.e 1.5 and 3/2 root3 [irrelevant]). Now use some trig to find the angle.

what? xD

hyp = 3 (speed of canoe)

op = 1.5 (it is better to understand this as the speed of the water rather then the perpendicular components of velocity)

Spoiler

edit: i see what your saying, it was a bit confusing sorry xD

Reply 3

JBKProductions

OP

7

Daniel Freedman

In order to stay straight, he must paddle with a velocity of 1.5m/s in the opposite direction to that which the river is flowing. You know his speed is 3m/s. You can therefore draw a right-angled triangle, with hypotenuse 3. The other two sides are the two perpendicular components of his velocity (i.e 1.5 and 3/2 root3 [irrelevant]). Now use some trig to find the angle.

Theres something i dont understand about this question do they mean the agle on the opposite end where he is going to or at the start? i dont know which end of the river to meausre the angle
and i dont understand why you have put 3 as the hyp if you dont mind explaining. I normally would put 3 as the adjacent to work out the hyp to find out its resultant velocity. Sorry for all these questions, i hope you wont mind explaining them

Reply 4

Daniel Freedman

7

It doesn't really matter what angle you give, as long as you tell them where you're measuring from.

Well the question states that the man is able to paddle the canoe with a speed of 3m/s. The biggest the hypotenuse can be is therefore 3 - if the adjacent was 3, the hyoptenuse would be larger, but the man isn't able to paddle that quickly.

Reply 5

JBKProductions

OP

7

Daniel Freedman

It doesn't really matter what angle you give, as long as you tell them where you're measuring from.

Well the question states that the man is able to paddle the canoe with a speed of 3m/s. The biggest the hypotenuse can be is therefore 3 - if the adjacent was 3, the hyoptenuse would be larger, but the man isn't able to paddle that quickly.

ok thanks

Mechanics 1 (vectors)

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