im really stuck on this Q..please help!

Hi

Im doing a Physics unit and I have never studied it before! :s-smilie:

We have to use the Understanding Physics book by Jim Breithaupt and im stuck on this question:

Q1.9
A boat has a speed of 1.5 m s-1 in still water. It is used to cross a river 500m wide along which there is a strong current speed of 0.9 m s-1. The boat is directed towards the opposite bank but the water current carries it downstream.
Calculate:

a) the boat's speed relative to the bank
b) the distance the boat is carried downstream from its point of departure to its point of arrival at the opposite bank.
c) the time taken to cross

help would be very much appreciated. every time I look at the question i feel dizzy :s-smilie:

..pleaaase enlighten me.

many thanks :smile:

Reply 1

Glutamic Acid

17

I'm not sure about a (I think it may just be finding the resultant vector), but I'll try b and c.

I think it's best to work out c) first. Time = Distance / Speed (you're only interested in the 1.5ms^-1 bit because the vertical component doesn't affect it). So it's 500 / 1.5

Now do b). You want the distance downstream, which is given by speed x time. Which you both have, using the speed x 0.9 ms^-1 as you're interested in this component.

Reply 2

EdwardCurrent

10

hijabi

Hi

Im doing a Physics unit and I have never studied it before! :s-smilie:

We have to use the Understanding Physics book by Jim Breithaupt and im stuck on this question:

Q1.9
A boat has a speed of 1.5 m s-1 in still water. It is used to cross a river 500m wide along which there is a strong current speed of 0.9 m s-1. The boat is directed towards the opposite bank but the water current carries it downstream.
Calculate:

a) the boat's speed relative to the bank
b) the distance the boat is carried downstream from its point of departure to its point of arrival at the opposite bank.
c) the time taken to cross

help would be very much appreciated. every time I look at the question i feel dizzy :s-smilie:

..pleaaase enlighten me.

many thanks :smile:

Consider that the force the water moving downstream applies to the boat acts perpendicuarly to the boat's motion across the river. So, it does not affect the velocity of the boat as it moves across the river (ie. its still water speed). Was this what was throwing you?

a) is just looking for the magnitude of the resultant velocity vector as Glutamic Acid said. Use Pythagoras.

For b) you could also just set up a pair of simultaneous equations, if you didnt want to do c) first.

Reply 3

Drummy

10

a) its the vector sum - just use Pythag

for b) and c) Glutamic is correct

im really stuck on this Q..please help!

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