WoodYouLike said:
don't worry about the petrol
- Thread starter kendor
- Start date
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JohnnyT got it, here's the answer as copied:
One of the things that people find difficult is that you have not specified the distance, but in fact the answer is the same, whatever the distance. The answer is that there is no answer - there is no speed you can drive at that will make your average speed 40 miles per hour. The reason is that to make your average speed for the whole journey 40 miles per hour, you need to be at your destination NOW, and no speed will allow you to achieve this.
How does this come about? Well, suppose you started out at 2 o'clock, and your destination is exactly 40 miles away. For your average speed to be 40 miles per hour, you need to arrive at 3 o'clock. But you drove the first half of the journey, 20 miles in this case, at 20 miles per hour. Which has taken you an hour. So it is now 3 o'clock, and you have only covered half the distance. There is no speed that you can drive for the second half of the journey that will make the average speed 40 miles per hour, since you need to be at your destination at this moment. And this applies no matter what the distance of the journey. If you prefer, you can change everything to kilometres per hour, instead of miles per hour.
One of the things that people find difficult is that you have not specified the distance, but in fact the answer is the same, whatever the distance. The answer is that there is no answer - there is no speed you can drive at that will make your average speed 40 miles per hour. The reason is that to make your average speed for the whole journey 40 miles per hour, you need to be at your destination NOW, and no speed will allow you to achieve this.
How does this come about? Well, suppose you started out at 2 o'clock, and your destination is exactly 40 miles away. For your average speed to be 40 miles per hour, you need to arrive at 3 o'clock. But you drove the first half of the journey, 20 miles in this case, at 20 miles per hour. Which has taken you an hour. So it is now 3 o'clock, and you have only covered half the distance. There is no speed that you can drive for the second half of the journey that will make the average speed 40 miles per hour, since you need to be at your destination at this moment. And this applies no matter what the distance of the journey. If you prefer, you can change everything to kilometres per hour, instead of miles per hour.
Well, I do have an answer that would allow this... Kinda.
The observer is standing at the destination. He holds the reference clock. He observes the car setting out from it's starting point at T=0. He observes the car travelling at 20mph for the total allowed journey time.
HOWEVER. The driver of the car, upon reaching the halfway point, decides to drive the rest of the journey at superluminal speeds. By driving faster than light, he manages to arrive at the observer's location, i.e. the destination, BEFORE the observer observes the car reaching the halfway point.
My assumption is that the observer holds the reference clock, but the driver sets off BEFORE T=0, at such a time that the car appears to start moving at T=0, from the viewpoint of the observer.
The observer is standing at the destination. He holds the reference clock. He observes the car setting out from it's starting point at T=0. He observes the car travelling at 20mph for the total allowed journey time.
HOWEVER. The driver of the car, upon reaching the halfway point, decides to drive the rest of the journey at superluminal speeds. By driving faster than light, he manages to arrive at the observer's location, i.e. the destination, BEFORE the observer observes the car reaching the halfway point.
My assumption is that the observer holds the reference clock, but the driver sets off BEFORE T=0, at such a time that the car appears to start moving at T=0, from the viewpoint of the observer.
AdamW said:
Isn't that called 'The Picard Manoeuvre' ?
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