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Two-car situation

  1. Mar 9, 2010 #1
    http://img18.imageshack.us/img18/3320/lalalacars.png [Broken]

    First of all sorry for the bad picture. But I hope you get the idea. I think we have all seen this situation in some movies.

    So basically a smaller car is leaning on a bigger truck with 2 wheels, while the other 2 wheels of the car are on the road.
    Now lets say the truck and the car are both driving at 90 km/h.
    The 2 wheels of the car that are on the road are going at the same speed as the wheels on the truck.
    But the road is moving at 90 km/h relative to the car, while the the truck is moving at 0 km/h relative to the car.

    How do the wheels act? How come the car keeps going at the same speed, like the 2 wheels were still on the ground?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 10, 2010 #2
    I've never actually seen this situation in films ... I've seen YouTube videos though where people are driving their cars on two wheels on the same side (it isn't leaning on another object).

    But I THINK the wheels on the car that are touching the truck should be thought of as ONE object. (in other words, truck+2 car wheels)

    So it'd be thought of as ONE object with 6 wheels. (not 8 because there are two rendered useless upon coming in contact with the truck)

    I'm most probably incorrect but I also find this an interesting question.
     
  4. Mar 10, 2010 #3

    A.T.

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    The left & right wheel of a car can rotate a different speeds. They have to do so when the car turns for example. The trick is possible if you have a "one side brake", that blocks the wheels leaned against the truck. The two opposite wheels can still roll freely and even drive the car, through to the differential gear:

    http://en.wikipedia.org/wiki/Differential_(mechanical_device)

    Without the "one side brake" it is might be still possible, but more difficult to balance for a longer time.
     
    Last edited: Mar 10, 2010
  5. Mar 10, 2010 #4
    Well what if the was straight and the truck and the car were going forwards ie. they are going straight, parallel to the two sides of the road (we'll assume that the sides of the road are PERFECTLY parallel)
     
  6. Mar 10, 2010 #5

    A.T.

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    I assumed both going straight during the stunt. The part about turning was just an example for different wheel speeds during normal operation.
     
  7. Mar 16, 2010 #6
    Assuming that both rear wheels of the lopsided car are where the movement comes from, in this case 90 km/h, I don't see how this could be plausible outside Hollywood films. After all, the wheel that is leaning on the truck would be rotating (for lack of a better word) at 90 km/h RELATIVE to the truck, which is also moving at 90 km/h. Therefore, relative to the road, (or any stationary object on Earth) that particular wheel would be rotating at 180 km/h, while the other wheel of the car is moving at 90 km/h. Therefore, the car would inevitably turn.

    I pity the fool who tries this at home.
     
  8. Mar 16, 2010 #7

    sophiecentaur

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    The differential on the slanting car should take care of things, in principle, I think. It would allow the driven wheel in contact with the road to go at 90mph and leave the one in contact with the car not rotating. Perhaps a brake, lightly applied to the stationary wheel could be applied - it would certainly make the stunt possible.
    Natch, the stunt driver would need to be pretty nifty with the accelerator control and the differential wouldn't last long.
    But the stunt would be always carried out much more slowly and there would be a strop, jointing the two together for the purposes of the film.
    The secret is that you just have to believe.
     
  9. Mar 16, 2010 #8
    After thinking about it a little more ... I imagine the "incorrectly placed" vehicle to suddenly jolt forward once the two wheels come in contact with the truck.

    (am I right to assume that the car was PLACED into that position? Or did it somehow fly into that position? Or did it accelerate at 91+ km/hr and then decelerate as it came into contact?)
     
  10. Mar 16, 2010 #9

    A.T.

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    Wrong, read post #3.

    Only if you push the gas pedal in that moment. But even then the differential gear will transmit most of the power to the wheel on the ground.

    Cars use ramps to get into "two wheel mode". Then while rolling they can lean onto the truck. Keeping their speed balanced there with the gas pedal would be tricky. But rolling & blocking only the leaned wheels would cause the truck to pull the car along at the same speed.
     
  11. Mar 16, 2010 #10
    So a car can have two different rear wheel speeds if you apply a "one side brake"? I'll take your word for it, but does that mean that one would have to calculate the brake so that the difference between the speed of two wheels would be 90 km/h?

    So for example, the wheel on the road would be 90 km/h, and the wheel on the truck would be 0 km/h?
     
  12. Mar 16, 2010 #11

    sophiecentaur

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    The transition between chasing and staying attached is dodgy. This is also true of the Hollywood scenario when a car catches up a pantechnicon with the rear ramp down and drives up inside whilst they're both on the move. As soon as the driving wheels of the car are on the ramp, the driver needs to back off the accelerator or it will zoom through and into the cab.
     
  13. Mar 16, 2010 #12

    A.T.

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    You don't have to take my word for it:
    http://en.wikipedia.org/wiki/Differential_(mechanical_device))
    What is there to calculate? You just block the wheels in the air completely.
    When both wheels are decelerated in a high gear, and you back off the accelerator, the engine could choke. I would rather build up speed, engage the clutch and come in rolling if the ramp is not to steep.
     
    Last edited: Mar 16, 2010
  14. Mar 16, 2010 #13

    sophiecentaur

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    "I would rather build up speed, engage the clutch and come in rolling if the ramp is not to steep"
    Yes - that makes a lot of practical sense. You'd look daft if you stalled and rolled back down.
    Take number 35 . . . .
     
  15. Mar 17, 2010 #14

    A.T.

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    After 40 takes, I would figure it out:
    - build up speed higher that the truck's
    - just before the driving wheels reach the ramp engage the clutch, and switch to lowest gear
    - as soon as the driving wheels roll onto the ramp, release the clutch and push the gas pedal gently.

    Now, if you just try to drive in, at high speed, high gear, no clutch and no velocity advantage over the truck, I doubt you can. The driving wheels will either slip on the ramp or stop to quickly and choke the engine. I don't think you would "zoom through and into the cab".
     
  16. Mar 17, 2010 #15
    There are some great differential animations on YouTube.

    This might be interesting: If one wheel is stopped the other wheel spins at twice it's normal speed. So for example if one wheel is on ice, the other stopped on pavement and your speedometer says 50 MPH, the spinning wheel is going 100 MPH.
     
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