vce1994's Question Thread!

[vce1994]

hey guys

started doing practice exams, so got a few questions. Any help would be appreciated

First Question: If a boat is travelling at 5.0 m/s down a river with a current flowing at 4 m/s, what is the fastest possible time for the boat to travel from one side to the other? Distance between sides is 120m

Boat is travelling south and current is going east.

Answer says its just 120/5, but wouldnt the current have an effect and change the speed of the boat (using vector addition)?

[schnappy]

Is the boat going at 5 m/s downstream or upstream?

Edit: Sounds like down.

I would have thought that would mean it can move at 1 m/s indepedant of the current, so 120/1.

When it says make it across the river, it doesn't mean in a line perpendicular to the river bank. You don't need to do anything with vectors, don't assume you have to do work against the current, just go straight ahead - the fact that you'll go downstream a bit doesn't matter.

[luffy]

hey guys

started doing practice exams, so got a few questions. Any help would be appreciated

First Question: If a boat is travelling at 5.0 m/s down a river with a current flowing at 4 m/s, what is the fastest possible time for the boat to travel from one side to the other? Distance between sides is 120m

Boat is travelling south and current is going east.

Answer says its just 120/5, but wouldnt the current have an effect and change the speed of the boat (using vector addition)?

The current is in the EAST direction. Think of it logically - if you were travelling North and there was a current dragging you east. Would it have any impact on your ability to travel north? Of course not, it would just sway you east a bit, but you would travel the same amount of distance north as you would if there was no current.

- Its the same in your question, except the direction is south.

EDIT: You should probably note that in this question, the boat is not trying to get to a fixed point. If there was a "single" point that you needed to get to at 120m South, then yes, the current would have an impact as the boat would have to force itself west. However, in this question, its not a fixed point, but a whole heap of points which are all 120m South of the origin. Thus, it should be 120/5 and the current will have no impact.

[vce1994]

Downstream, ill upload the exam so its easier to see
its q1

Edit: Oh really? so for the vector triangle, the hypotenus would be 5?

[schnappy]

There is no triangle.

[vce1994]

wouldnt you need one for question 2?

[schnappy]

What's question 2? You didn't ask about it.

[vce1994]

sorry, its the next question which says that the boat goes directly from Point A to Point B Due South on the other bank.
It asks for the bearing needed in order to reach Point B

[Shark 774]

I don't think this is part of the new course. That's what my teacher told me anyway.