The Motor Boat

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Aqueous Id said:
That's relative to the road surface which is the same whether on a road or the deck of an aircraft carrier.
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All measures of the boat are relative to the embankment because that is what was measured in still water! The embankment is at a zero velocity!
 
Motor Daddy said:
NO it is not! Upstream the engine is operating at a lower RPM then it is when it travels downstream! There is more load on the crank due to the current. The engine is under a greater load, but it was already at wide open throttle, so it has to reduce RPM. You can't give it more throttle!
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No, the prop is turning at the same rate regardless of the current, so the RPM remains constant. This problem does not involve loading, so that's not a concern.
Yes, you can't give it more throttle, which is why the speed changes relative to shore.
 
Aqueous Id said:
No, the prop is turning at the same rate regardless of the current, so the RPM remains constant. This problem does not involve loading, so that's not a concern.
Yes, you can't give it more throttle, which is why the speed changes relative to shore.
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The "loading" is a hidden concept in this scenario, because the "still water" time was 1.25 hours, whereas the "loaded" (upstream) and "unloaded" (downstream) times were different than in still water.
 
Motor Daddy said:
All measures of the boat are relative to the embankment because that is what was measured in still water! The embankment is at a zero velocity!
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And the current is moving relative to the bank. The boat is moving in coordinates relative to the bank, but not physically. It's not clawing the bank. It's pushing water molecules. Physically, then, the boat is moving relative to the water. When the boat enters the 3.27 km/hr current, it slows relative to the bank - to a speed of 4.73 km/hr, but continues at 8 km/hr relative to the water. After 5 km, it turns around, still moving at 8 km/hr relative to the stream, but gaining 3.27 km/hr from the current, now moving at 11.27 km relative to the bank.
 
Motor Daddy said:
The "loading" is a hidden concept in this scenario, because the "still water" time was 1.25 hours, whereas the "loaded" (upstream) and "unloaded" (downstream) times were different than in still water.
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No loading would require us to have data not provided so we ignore loading in the problem. This is a simple problem in Galilean relativity, nothing more.
 
Aqueous Id said:
And the current is moving relative to the bank. The boat is moving in coordinates relative to the bank, but not physically. It's not clawing the bank. It's puhing water molecules. Physically, then, the boat is moving relative to the water. When the boat enters the 3.27 km/hr current, it slows relative to the bank - to a speed of 4.73 km/hr, but continues at 8 km/hr relative to the water. After 5 km, it turns around, still moving at 8 km/hr relative to the stream, but gaining 3.27 km/hr from the current, now moving at 11.27 km relative to the bank.
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The movement of the boat is measured compared to the embankment, not the water. The water could have traveled 100 MPH when the boat was traveling upstream in current, or the water could have been traveling 10 MPH while the boat was traveling upstream! You have no idea how fast the current was flowing compared to the embankment!
 
Motor Daddy, any analogy you draw between the boat and a car will be flawed, because cars travel on stationary roads, while the boat is traveling on flowing water. I'll refer to the road and the water as the "substrates" of motion for the car and boat, respectively. In both cases, as long as we neglect air resistance, the vehicle only interacts with the substrate, so its velocity relative to the substrate is the only thing that can affect how it behaves. In a car, the horsepower it takes to go faster depends only on how fast you're going relative to the road. In a boat, the load on the crank depends only on how fast you're going relative to the water.

This means that the speed of a vehicle relative to its substrate is entirely independent of the substrate's own movement, so it is correct to say that the total velocity of a vehicle is equal to its velocity on a still substrate plus the velocity of the substrate. This means that the boat is putting out the same engine power and moving at the same speed relative to the water going both ways, but the additional motion of the water itself changes the speed of the boat relative to the shore. In the case of the car on a highway, the velocity of the substrate is zero, so you can't observe this effect. If we use Aqueous Id's clever suggestion of driving around on an aircraft carrier, we could make the car on a moving road move identically to the boat in moving water.
 
Fednis48 said:
Motor Daddy, any analogy you draw between the boat and a car will be flawed, because cars travel on stationary roads, while the boat is traveling on flowing water. I'll refer to the road and the water as the "substrates" of motion for the car and boat, respectively. In both cases, as long as we neglect air resistance, the vehicle only interacts with the substrate, so its velocity relative to the substrate is the only thing that can affect how it behaves. In a car, the horsepower it takes to go faster depends only on how fast you're going relative to the road. In a boat, the load on the crank depends only on how fast you're going relative to the water.

This means that the speed of a vehicle relative to its substrate is entirely independent of the substrate's own movement, so it is correct to say that the total velocity of a vehicle is equal to its velocity on a still substrate plus the velocity of the substrate. This means that the boat is putting out the same engine power and moving at the same speed relative to the water going both ways, but the additional motion of the water itself changes the speed of the boat relative to the shore. In the case of the car on a highway, the velocity of the substrate is zero, so you can't observe this effect. If we use Aqueous Id's clever suggestion of driving around on an aircraft carrier, we could make the car on a moving road move identically to the boat in moving water.
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In this scenario the measurements were conducted according to the embankment, not the substrate!
 
Motor Daddy said:
The movement of the boat is measured compared to the embankment, not the water. The water could have traveled 100 MPH when the boat was traveling upstream, or it could have been traveling 10 MPH! You have no idea how fast the current was flowing compared to the embankment!
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The 8 km movement of the boat is referenced to still water which tells us that as long as the water is not moving, the speed relative to the bank is 8 km/hr.

When the water moves relative to the bank, it carries the boat with it. Thus, with no power, the boat will move relative to the bank, at the speed of the current. In the original statement of the problem we knew the current was moving at 3 km/hr, which gave a distance in each leg of 5.15625 km + 5.15625 km = 10.3125 km total distance traveled (post #10). This version of the problem changed that to 5 km/hr, increasing the current to 3.27 km/hr (post #13).
 
Motor Daddy said:
NO it is not! Upstream the engine is operating at a lower RPM then it is when it travels downstream! There is more load on the crank due to the current. The engine is under a greater load, but it was already at wide open throttle, so it has to reduce RPM. You can't give it more throttle!
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No information is given to make any determination of loading. It's not germane.
 
Aqueous Id said:
The 8 km movement of the boat is referenced to still water which tells us that as long as the water is not moving, the speed relative to the bank is 8 km/hr.

When the water moves relative to the bank, it carries the boat with it. Thus, with no power, the boat will move relative to the bank, at the speed of the current. In the original statement of the problem we knew the current was moving at 3 km/hr, which gave a distance in each leg of 5.15625 km + 5.15625 km = 10.3125 km total distance traveled (post #10). This version of the problem changed that to 5 km/hr, increasing the current to 3.27 km/hr (post #13).
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Are you saying that if the water moves at 10km/hr compared to the embankment that the boat is traveling with the water at 10km/hr in the same direction as the water compared to the embankment?
 
Motor Daddy said:
In this scenario the measurements were conducted according to the embankment, not the substrate!
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That's true, but we also know that the boat doesn't interact with the embankment. The fact that the vehicle only interacts with the substrate has nothing to do with how we took our measurements; it's a simple fact of how boats work. So before we even look at the boat, we can confidently say that its maximum speed relative to the water does not depend on the speed of the water itself. Once we know that, we can measure the boat's max speed in still water with buoys if you're really that bothered by the clocks on the embankment.
 
Motor Daddy said:
Are you saying that if the water moves at 10km/hr compared to the embankment that the boat is traveling with the water at 10km/hr in the same direction as the water compared to the embankment?
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Yes. This is correct. (As long as the boat is idling.)
 
Motor Daddy said:
Are you saying that if the water moves at 10km/hr compared to the embankment that the boat is traveling with the water at 10km/hr in the same direction as the water compared to the embankment?
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Speed of the boat relative to the bank = speed of the water relative to the bank + speed of the boat relative to the water
 
Fednis48 said:
That's true, but we also know that the boat doesn't interact with the embankment. The fact that the vehicle only interacts with the substrate has nothing to do with how we took our measurements; it's a simple fact of how boats work. So before we even look at the boat, we can confidently say that its maximum speed relative to the water does not depend on the speed of the water itself. Once we know that, we can measure the boat's max speed in still water with buoys if you're really that bothered by the clocks on the embankment.
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The max speed of the boat in water is unknowable unless you test it! You can claim that a 1,000 HP boat will travel 100 MPH in 100 MPH current, but until you MEASURE it you are just blowing smoke, and I can guarantee you your math answer is DIFFERENT than the actual measured speed!
 
Aqueous Id said:
Speed of the boat relative to the bank = speed of the water relative to the bank + speed of the boat relative to the water
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Try again! The boat's speed is not relative to the water, it's relative to the embankment. The boat's speed relative to the water is called closing speed.
 
Motor Daddy said:
Try again! The boat's speed is not relative to the water it's relative to the embankment. The boat's speed relative to the water is called closing speed.
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The boat speed is relative to both.

In this problem the boat speed is relative to the water and the water speed is relative to the bank. As long at the water is still, the boat speed relative to the water = the boat speed relative to the bank. But when the boat enters a current, that scenario changes. Now the speed relative to the water is still 8 km/hr but the boat speed relative to the bank is -3.27 km/hr ± 8 km/hr , depending on the direction of the boat.
 
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