Brain Teaser

[TurboDewd]

To some of us the answer will be obvious, but Im surprised by the number of ppl Ive asked this who get it wrong.

Suppose there are 2 identical cars travelling on the same road in the same direction. one car is cruising steady at 100km/k, the 2nd car is stationary then accelerates at full throttle. As soon as the 2nd car reaches 100km/h its engine is turned off. At the same time the engine of the first car is turned off.

Which car comes to a halt sooner and why?

[KEN 24T]

Ok, I'm guessing that the 1st car, cruising @ 100 (and maintaining 100) will roll to a halt before the 2nd car that is still accelerating (past 100) when it's engine is turned off?

... now my head hurts ...

[Flukey]

  KEN 24T said:

Ok, I'm guessing that the 1st car, cruising @ 100 (and maintaining 100) will roll to a halt before the 2nd car that is still accelerating  (past 100) when it's engine is turned off?

that's what I would think too

[F6 RAPID]

They will both stop at the same time - all other things being equal, as they were both travelling at 100km/h.

You asked when they would stop, not distance apart??

[Honest Gaza]

Trick question ????

You said they were travelling in the same direction (but then suggest one is stationary)

From memory, Force = Mass x Acceleration....therefore, there will be greater Force in the 2nd Car, and as a result, will take longer to come to a halt.

[TurboDewd]

  Honest Gaza said:

Trick question ????

You said they were travelling in the same direction (but then suggest one is stationary)

From memory, Force = Mass x Acceleration....therefore, there will be greater Force in the 2nd Car, and as a result, will take longer to come to a halt.

<{POST_SNAPBACK}>

Its not a trick question. Well the direction they face isnt really important, lets just say the conditions they are experiencing are the same, no wind factors. They end up travelling in the same direction, yes one starts stationary.

Answer with explanation delivered shortly.

[4runner]

My guess is that they will both stop at the same time. This is assuming that the second car which was accelerating to 100km/h would then have zero acceleration at the point when both cars were switched off.

[F6 RAPID]

I thought I had already answered it - First I must say.

[Guest]

Were they allowled to coast after being turned off...

vik...that is the question

[TurboDewd]

F6 Rapid and 4runner are correct.

AT the moment the engines are switched off no more force is causing either vehicle to move. Newton's first law of motion applies:

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Each car had a force being applied to it (the engine) and that force stopped, they would then uniformly slow to a complete stop since the effects of air resistance and friction slow the cars down.