The Motor Boat

Discussion in 'Physics & Math' started by Motor Daddy, May 31, 2013.

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  1. Pete It's not rocket surgery Registered Senior Member

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    Sure, let's make it full throttle. The speedometer reads 8km/hr at max power.
     
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  3. Motor Daddy Valued Senior Member

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    Max power? What's that? The 8km/hr is a maximum relative speed of the boat to the water (as measured by the boat's speedometer) at WOT!
     
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  5. Pete It's not rocket surgery Registered Senior Member

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  7. Motor Daddy Valued Senior Member

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    So it's settled, the max speed is 8km/hr, not 8+3.27.
     
  8. Pete It's not rocket surgery Registered Senior Member

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    The max speed relative to the water is 8 km/hr.

    Now add (or subtract for going upstream) the speed of the current to get the speed relative to the embankment.
     
  9. Motor Daddy Valued Senior Member

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    The speedometer only measures the boat's speed relative to the WATER. What does the embankment have to do with it? Who cares what the embankment thinks, it's the relative speed of the boat to the water that counts, and we already measured that at WOT to be 8km/hr. If an embankment happens to be in motion at the same time then so be it, but that has nothing to do with the relative speed of the boat to the water.
     
  10. Pete It's not rocket surgery Registered Senior Member

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    How long does it take the boat to go 5km up the river?
     
  11. Motor Daddy Valued Senior Member

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    5km up river meaning how much time does it take for the boat to traverse 5km of water?
     
  12. Pete It's not rocket surgery Registered Senior Member

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    No, meaning how long does it take to pass 5km of embankment. How long does it take to get from the boat ramp to your favourite fishing spot under the bridge 5km away. How long does it take to finish a 5km race.
     
  13. Motor Daddy Valued Senior Member

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    What does that have to do with the 8km/hr relative speed of the boat to the water that we already measured?

    Edit: Do you recommend that we do all calculations according to the embankment so there are no misconceptions and everything "adds up" right?
     
  14. Pete It's not rocket surgery Registered Senior Member

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    You add (or subtract) the speed of the current to the speed over water (the speedometer reading) to get the speed relative to the embankment.

    The speed of the current is 3.27km/hr.
    The reading on the speedometer is 8km/hr.
    How long does it take the boat to go 5km up the river?

    The original problem tell us that the speed relative to the water is 8km/hr, so we have to use that.
     
  15. Motor Daddy Valued Senior Member

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    Are you saying the boat's speed relative to the water is 8km/hr in each direction of travel in current water?
     
  16. Pete It's not rocket surgery Registered Senior Member

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    Yes.

    Please answer this question:

    The speed of the current is 3.27km/hr.
    The reading on the speedometer is 8km/hr.
    How long does it take the boat to go 5km up the river?
     
  17. IncogNegro Banned Banned

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    Hey thanks! Too bad Pete deleted the first funny one. Now this thread is boring because MD owns it and can play all day with the moderators minds...
     
  18. Motor Daddy Valued Senior Member

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    The current may be 3.27km/hr according the the embankment, but that is a relative speed like the 8km/hr of the boat relative to the water is. According to the boat and water there is no current, only relative speed between the water and the boat, and that relative speed at WOT is 8km/hr. There is no 3.27 in the relative speed of the boat to the water. The 3.27km/hr comes from the fact that the embankment has a relative motion to the water. That is, unless of course we measure with light, and then it's all crystal clear! After all, the definition of the meter is the length of the path that light travels in vacuum in 1/299792458 of a second. How would you be able to measure a meter unless you used light to measure with? How would you measure time without a clock? Speedometers measure relative speed. They are not clocks and they are not rulers.
     
  19. Pete It's not rocket surgery Registered Senior Member

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    Correct.
    Correct again.

    So, what is the speed of the boat relative to the embankment? That's the speed you need in order to calculate how long it will take to get to your destination 5km away.

    No more diverting, MD. Answer this simple question:

    The speed of the current is 3.27km/hr.
    The reading on the speedometer is 8km/hr.
    How long does it take the boat to go 5km up the river?
     
  20. Motor Daddy Valued Senior Member

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    What current? The only 3.27km/hr is that of a relative speed between the embankment and the water. How do you get a current from a relative speed?

    Now a different relative speed between the boat and the water. Where is the current?

    Which way is "up the river?" Each direction of travel has a relative speed. Each direction the embankment travels along the water has a relative speed, so how do you define "up the river?" You seem to be "up the river" (without a paddle.)

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  21. AlexG Like nailing Jello to a tree Valued Senior Member

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    That's what the current is. How fast the water flows, relative to the stationary embankment.

    Up river is, by definition, the direction opposite of the flow of the water.
     
  22. dumbest man on earth Real Eyes Realize Real Lies Valued Senior Member

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    What if there was a really long moving sidewalk/treadmill and you take a ride on it with an electric bicycle. The bikes speed is "fixed" at 8 mph.

    You travel to a point 5 miles away - turn the bike around and return to your starting point. The round trip takes 90 minutes. What speed is the sidewalk/treadmill moving ?

    Okay now - you take your choice of whether you want to make the first leg of said 5 mile journey traveling with or against the direction of said sidewalk/treadmill, it doesn't matter.
    Honestly, Motor Daddy, it's not like someone is asking you to figure out the square root of all the right angle corners in a spherical room using a slide rule that has been super glued shut !!
    P.S. remember I am the dumbest man on earth.
    P.P.S. Since you all speak of relativity - can darkness move faster than the speed of light ???
     
    Last edited: Jun 11, 2013
  23. Pete It's not rocket surgery Registered Senior Member

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    Motor Daddy, do you really want to say that you don't know what a current is, and that you can't tell which way is upstream and which way is downstream?

    If so, then I don't believe you. Your argument earlier in the thread indicates that you are perfectly aware of what a current is, and that you know the difference between upstream and downstream so I can only conclude that you are no longer arguing honestly, if indeed you ever were.

    Last chance:

    The speed of the current is 3.27km/hr.
    The reading on the speedometer is 8km/hr.
    How long does it take the boat to go 5km up the river?

    Try using the same logic you correctly used in this post:
     
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