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I can't wrap my head around this basic problem

D

dxkj

Lifer
A boat is traveling upstream at 14km/h with respect to the water of the river.

The water is flowing at 9km/h with respect to the ground.

What is the velocity of the boat with respect to the ground.


Wouldn't the velocity of the boat be -5 with respect to the ground?


<----------14km/h
------>9km/h

5km/h upstream, and since downstream is positive, upstream would be negative?


Im just trying to make sure my logic is right here.
 
If upstream was negative, they would have said the boat was going -14 km/h. I dunno about you, but my speedometer doesn't go any lower than 0 km/h. 5km/h is the speed of the boat with respect to ehg round..
 
Upstream would be against the flow of water.

The magnitude of the answer is 5, I'll leave it to you determine whether it's positive or negative.

Viper GTS
 
i think the question was not worded very well, they should have just said speed, since velocity includes direction.
 
Wink:


If the river was moving at 7 instead of 9, then both the boat and the river would have 7km/h with respect to the ground, but be going in opposite directions... why wouldnt you slap negative on one?
 
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
 
Originally posted by: dxkj
Wink:


If the river was moving at 7 instead of 9, then both the boat and the river would have 7km/h with respect to the ground, but be going in opposite directions... why wouldnt you slap negative on one?
Click to expand...

DUDE, Unless the water is moving faster than the boat, you wont have a negative speed.
 
Originally posted by: dxkj
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
Click to expand...

Except that they specified that the boat was traveling UPSTREAM.

Viper GTS
 
Originally posted by: Viper GTS
Originally posted by: dxkj
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
Click to expand...

Except that they specified that the boat was traveling UPSTREAM.

Viper GTS
Click to expand...

Right... so if downstream is positive, upstream should be negative.


Sheesh, forget that I asked. You people are saying 14-9 = 5.... yes that makes sense, but you subtracted 9 when it was a positive value relative to the ground, so to fix that you have to change the sign on the result.


-14+9 = -5

 
Originally posted by: dxkj
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
Click to expand...

The question gave you speed. Speed does not include direction. 10kph wrt object X and 20kph wrt object X may or may not be in the same direction. Speed is distance over time. Distance and time can not be negative, hence speed is always positive.

If you want your answer to include direction and therefore fit the definition of velocity, the boat is moving "5kph in the upstream direction". There was never anything in the problem that suggested the upstream and downstream directions corresponded to positive and negative directions on any axis, so don't make up positive and negative numbers for your answer.
 
Originally posted by: notfred
Originally posted by: dxkj
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
Click to expand...

The question gave you speed. Speed does not include direction. 10kph wrt object X and 20kph wrt object X may or may not be in the same direction. Speed is distance over time. Distance and time can not be negative, hence speed is always positive.

If you want your answer to include direction and therefore fit the definition of velocity, the boat is moving "5kph in the upstream direction". There was never anything in the problem that suggested the upstream and downstream directions corresponded to positive and negative directions on any axis, so don't make up positive and negative numbers for your answer.
Click to expand...

edit:

its asking me for velocity though.... hrm, ok thanks
 
Well, the question used the word velocity. But it referred to both the flow of water and the movement of the boat as positive numbers. So technically you could call it positive or negative, but they didn't specify which direction should be considered positive. You assumed that downstream was positive.

Since the question is worded in terms of directions like upstream and downstream, you should answer in the same way:
The boat is moving 5km/h upstream in relation to the ground.
 
Originally posted by: dxkj
Originally posted by: notfred
Originally posted by: dxkj
Originally posted by: notfred
14 - 9 = 5

Maybe you're just not cut out for middle school math.
Click to expand...

being able to add two numbers together, and understanding what they meant by the question are two different things.


I understand that the number is 5... but they started out by saying the water was moving at 9 in respect to the ground, if the boat is moving at 5 in respect to the ground that would make it sound like they were going in the same direction, in respect to the ground...
Click to expand...

The question gave you speed. Speed does not include direction. 10kph wrt object X and 20kph wrt object X may or may not be in the same direction. Speed is distance over time. Distance and time can not be negative, hence speed is always positive.

If you want your answer to include direction and therefore fit the definition of velocity, the boat is moving "5kph in the upstream direction". There was never anything in the problem that suggested the upstream and downstream directions corresponded to positive and negative directions on any axis, so don't make up positive and negative numbers for your answer.
Click to expand...

edit:

its asking me for velocity though.... hrm, ok thanks
Click to expand...

Velocity simply means speed and direction. You don't need to say +/- 5, you can simply say 5 km/h upstream.

That is a velocity.

Viper GTS
 
I can't believe the full comprehension of this problem took 20 posts. 😛

I suggest dxkl take a break from his homework. 🙂
 
Originally posted by: dxkj
Wink:


If the river was moving at 7 instead of 9, then both the boat and the river would have 7km/h with respect to the ground, but be going in opposite directions... why wouldnt you slap negative on one?
Click to expand...

Because it tells you one is upstream, one is downstream.
 
Originally posted by: Eli
I can't believe the full comprehension of this problem took 20 posts. 😛

I suggest dxkl take a break from his homework. 🙂
Click to expand...

Right, I just needed to add on upstream or downstream


Sorry, been doing homework for about 4hrs 🙁
 
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