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Bunuel
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Bunuel
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MegB07
Hi why re we adding the speeds when going in the same direction?
Because when the boat goes from A to B, the current is pushing it forward in the same direction.

  • Boat’s own still-water speed = x mph
  • Current’s speed = 3 mph
  • Effective speed with the current = x + 3 mph

That’s why we add the speeds.

On the way back, the current resists the motion, so the effective speed is x - 3 mph.
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Hi MegB07


If the relative speed concept is used, then you are right, it should be subtracted. And thus Ship speed will be 33 miles/Hour. But here in that case the return journey will take less time than the initial journey, which is against what the question stem says.

This question is an application of speed, distance than relative velocity.


MegB07
Hi why re we adding the speeds when going in the same direction?
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Hi Bunuel
Could you please explain why using the formula Sr=(2*S1*S2)/(S1+S2) doesn't work for the round trip's average ? I am trying to use this formula as the distance is the same for both runs but can't get the correct answer.
Thank you
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Omnoui
Hi Bunuel
Could you please explain why using the formula Sr=(2*S1*S2)/(S1+S2) doesn't work for the round trip's average ? I am trying to use this formula as the distance is the same for both runs but can't get the correct answer.
Thank you

The formula actually does work here.

For the round trip, the two actual speeds are

  • with current: 30 mph
  • against current: 24 mph

So the average speed is

(2 * S1 * S2)/(S1 + S2) =

= (2 * 30 * 24)/(30 + 24) =

= 1440/54 =

= 26 2/3 mph.
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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