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Scenario 1. The driver of a vehicle braked all the way while driving up a hill, and then proceeded to accelerate all the way driving down the hill.

Scenario 2. The driver of a vehicle wanted to meet up with someone traveling in the same direction, with the same speed, but far ahead of him. So he started braking to reduce the distance between them. After a while, he started accelerating, to avoid overtaking his target.

Scenario 3. The driver of a vehicle arrived to his destination, braking as hard as possible, with loud creaking and rattling, but everyone around considered it perfectly normal. Some time later, the driver of a similar vehicle with similar cargo arrived to the same place, braking and halting very gently. Everyone around cursed him and he got nearly fired from his job. (mention of cargo removed as it proved to be disorienting)

What vehicles was I talking about?

Hint: nothing is meant figuratively. All three cases are quite normal for the respective vehicles, they don't merely depict exceptional conditions.

Further hints

  1. This is a very standard practice for a vehicle of this type, nothing unusual. It is a very common vehicle. If the drive is moving up a hill, it does not necessarily mean that the vehicle as a whole would decelerate by itself.

  2. It was correctly guessed that it's a spacecraft in orbit.

  3. Many people guessed train, but not why it is important for a very long train not to stop too gradually.

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    $\begingroup$ It's been some time without progress. It may be time to (a) identify which answers have correctly identified which vehicles (if any), and (b) to give a hint for what remains to be solved. $\endgroup$ – Rubio Nov 9 '17 at 10:07
  • $\begingroup$ Has a correct answer been given? If so, please don't forget to $\color{green}{\checkmark \small\text{Accept}}$ it. If not, some responses to the answerers to help steer them in the right direction would be helpful. $\endgroup$ – Rubio Apr 6 '18 at 10:42

13 Answers 13

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The second vehicle was a spacecraft in orbit.

Two spacecraft in the same orbit can never change the distance between them; a change in distance must be accomplished by changing orbits. So, by braking, the trailing spacecraft drops to a lower orbit, which increases his orbital velocity, closing the distance between the two. In order to avoid overtaking, he must then accelerate to rise back to a higher orbit and decrease his orbital velocity.

The two vehicles in the third example may be crash-test vehicles - the first driver crashed his vehicle, providing useful data for the test and the research; the second driver, by stopping “properly”, ruined the test - and if the tests are expensive to run, may have wasted a sizeable amount of money, enough to make it seriously worth considering discharging him.

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  • $\begingroup$ correct, but for it to be accepted, (besides the other two), a very short explanation might be helpful $\endgroup$ – vsz Aug 31 '17 at 19:29
  • $\begingroup$ Done! There's now a brief layman-compatible explanation. $\endgroup$ – Jeff Zeitlin Aug 31 '17 at 19:45
  • $\begingroup$ Interesting crash test vehicles that bring the human driver with them (I presume that's what "arrived to his destination"/"arrived to the same place" means, since it's implied that this is consequent with the crash) $\endgroup$ – Darren Ringer Sep 1 '17 at 18:11
  • $\begingroup$ @DarrenRinger - That's why I said 'may be' for that; I know that when thry'r looking at vehicle damage/occupant protection, it's standard to use mannikins - but I don't know that there aren't conditions under which 'live' drivers are needed... $\endgroup$ – Jeff Zeitlin Sep 2 '17 at 14:28
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I think these might be

Spacecraft entering orbit?

Braking up the hill would represent

Slowing down while ascending, to lower Apogee

And accelerating down the hill

Speeding up while descending, to raise Perigee

Going slower

Means a shorter, smaller orbit, and speeding up means a higher, slower orbit.

I'm not certain about the second driver,

He may be in trouble for wasting fuel?

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    $\begingroup$ your answer would perfectly fit the second vehicle, but not the first one. This is not lateral thinking, the hill is a physical hill. $\endgroup$ – vsz Aug 31 '17 at 15:10
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Number 1:

a mountain biker who wants to clear mud from his brakes will lightly hold them down for a period which happens to coincide with the next rise after hitting the puddle in the valley, and will also be inclined to ride at full blast down the next hill

Number 2:

The vehicles are tethered. Maybe they're canoes, maybe they're in orbit. By braking to a stop, the tether will transmit a yank to the forward one to accelerate it toward the braker. Eventually the braker will need to accelerate in the opposite direction to match speed.

Number 3:

Typical airplane landing. The second pilot may have overshot the runway into the taxiway or other hazard.

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Perhaps the vehicles are not literal vehicles, but instead they are

Subatomic particles in a large particle accelerator. The "driver" is an engineer or scientist controlling the machine.

1.

The Large Hadron Collider contains multiple circular particle accelerators, at different elevations. According to the layout maps, the particles begin in the ring at the highest elevations, and accelerate in each ring, progressively moving downward.

2.

The accelerators are circular. Suppose two particles A and B are moving at equal (angular) speed along a circular path, and particle A is "ahead" by some convention; if particle B slows down, it can reach the same trajectory as particle A (i.e., they "meet"). And if particle A slows down too much, its relative trajectory to B will be "too far ahead" (although the distance is negative), and so it would need to accelerate to catch up.

3.

The particles are intended to smash together at very high velocities, and they may explode violently upon impact. Operating the LHC is expensive; if the particles don't collide with enough speed, then the experiment may fail, and the scientist responsible could be blamed for wasting resources.

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Well for the first,

The hill is fully submersed in water (like an ocean, perhaps) and for this his actions are justified. The vehicle could be any buoy-like instrument fitted one.

for the second,

the vehicles are sports/ race /formula one cars in a friendly race, where a driver waits (by slowing down) for his friend who is already driving fast in a lap and once meets (in fact the other one crosses) he speeds to meet the overtaker's pace !

for the third

A train/motor driver avoiding a collision on a railway track (for which he was praised) whereas after sometime, an expected/schduled train arrived late (due to slow driving) for which driver was almost fired !

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  • $\begingroup$ The train cargo is either passengers or material being transported. I doubt that wrecking the cargo would be considered "normal"... $\endgroup$ – MikeQ Aug 31 '17 at 16:02
  • $\begingroup$ @MikeQ : Not unless you transport sand or gravel where it doesn't matter. $\endgroup$ – vsz Aug 31 '17 at 16:19
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The first scenario is:

A long train. When you're running on top of metal rails, going over the crest of a hill at speed is dangerous. When the engine is going up the hill, most of its carriages are likely still on level track behind it, and have quite a lot of momentum that isn't being slowed down by gravity; the train needs to brake (potentially quite severely) as it goes up the hill simply in order to reduce its speed to a safe level. The train would then accelerate again on its way down on the other side of the hill, as most of its carriages are still on the upward slope and the extra power is needed to counteract the gravity pulling them backward.

I concur with others, re: scenarios two and three;

Two: spacecraft in orbit, Three: crash test vehicles.

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  • $\begingroup$ True for #1. For #3, I thought it was obvious from my comments that it was a train as well, just the cause was never solved. $\endgroup$ – vsz Mar 17 '18 at 12:48
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The first is

a roller coaster.

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The first is

a trolley car - it brakes on the cable

Third

crash test

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For #1:

This is a roller-coaster, where the thrills are to come from downhill acceleration in a controlled environment.

For #2:

These are spacecraft, where the driver of the one "in front" is the one who wants to meet up, and "braking" here entails firing thrusters in the opposite direction. Acceleration (firing thrusters in the opposite direction) would then be needed to avoid overcompensating.

For #3:

This is a commuter train or tram, where loud braking is not uncommon, but a slow, gliding brake wastes precious time for commuters and possibly delays trains right behind on the line.

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  • $\begingroup$ There are other, more significant problems if a long train stops too slowly... $\endgroup$ – vsz Sep 1 '17 at 6:08
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I think Vehicle 1 is:

RollerCoaster - Brakes as it gets to the top of its course and accelerates downhill for added exhiliration

Vehicle 2:

Two cars on a racetrack (but not racing for some reason). They would always be going in the same direction - since the track is circular or something to that effect.

Vehicle 3:

Train coming into the station. A slower stop is a waste of time for passengers and the railroad itself. With regards to the cargo, cargo being secured on trains are not generally secured very well.

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  • $\begingroup$ 1: although they tend to go slowly up and quickly down, they don't really tend to brake upwards or apply power downwards. 2. There is a more important cause why stopping slowly might be a very bad idea. Especially in older times. $\endgroup$ – vsz Sep 1 '17 at 11:22
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Partial Answer for #1

A skier/snowboarder may often brake while going uphill, to avoid losing control (i.e. flying over the hill), and then accelerate going back down.

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For the first two, I like the already given answers of

Mountain biker cleaning brakes, and spacecraft in orbit. EDIT: reading some of the comments and finally the hints by OP, I think I need more training in the first answer.

For #3, I’d like to offer

a long freight train.

If you stop one gradually,

all the slack in the couplings between the cars will be stretched out, and getting the train moving again will be very difficult. Normally, the slack allows the locomotive to pull the cars into movement one by one, but with the whole train stretched out to maximum length, it’s going to be the locomotive(s) against 80 heavy freight cars at once.

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  • $\begingroup$ correct for #3. $\endgroup$ – vsz Mar 17 '18 at 21:29
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To finish the answer for #3, might it be:

Stopping a long train very gradually would mean that it would block nearby road crossings for much longer than necessary, and also it would be braking through road crossings farther away from the destination than otherwise. Therefore the drivers of the vehicles waiting at those crossings were the ones cursing. (Also, hence the mention of "especially in older times" in a comment, since nowadays grade crossings of railroad tracks are much less common than they used to be.)

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