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Driving a car onto a semi while both are in motion

  1. Jun 19, 2012 #1
    A debate I have long had with my father-in-law. Can someone please set the record straight on what would happen given the following scenario:

    Semi truck travelling at 55 MPH with a ramp down on the trailer. A car accelerates to 60 MPH and drives up the ramp and into the back of the trailer.

    a) Does the car move across the trailer surface at 60 MPH and thus zooming to the front once in contact.

    b) The car continues gaining on the front of the trailer at 5 MPH, thus pulling in slowly.

    c) Something else entirely.


    Thanks!
    Rex
     
  2. jcsd
  3. Jun 19, 2012 #2

    Danger

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    Welcome to PF, Rexmaster.
    Answer "b" is close, but not exact. Mythbusters performed the test a couple of years ago. Exiting the truck was equally painless.
     
  4. Jun 19, 2012 #3

    K^2

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    Last edited by a moderator: Sep 25, 2014
  5. Jun 19, 2012 #4
    Without any experiments I would say that the car will move at 55+60=115MPH, relative to the ground, and at 5MPH relative to the truck. Is that right?
     
  6. Jun 19, 2012 #5

    DaveC426913

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    That is self-contradictory. If the car is moving at 5mph wrt the truck, how can it be moving at 115mph wrt the ground, if the truck is only moving at 55mph?

    Keep analyzing.
     
  7. Jun 19, 2012 #6

    Nugatory

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    Much is going to depend on what the driver of the car does with the throttle as the car drives up onto the ramp.

    If the driver doesn't lift off as the driving wheels leave the road for the ramp, then the engine will start accelerating the car relative to the truck. This will lead to the car ramming the cab of the truck from behind. If the truck's bed were long enough (several hundred feet) the car could accelerate from 5 mph to 60 mph on the bed of the truck, for a combined speed of 115 mph relative to the ground, before it smashed into the cab at 60 mph.

    If the driver lifts off completely, the car will be coasting at 5 mph relative to the ramp. That might not be enough to get it to roll all the way up the ramp.
     
  8. Jun 19, 2012 #7

    DaveC426913

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    Actually, what will happen is the engine will stall.

    What Mythbusters showed is that the wheels of a car have very little angular momentum. The moment they are on ramp, they immediately slow to match the speed of the surface underneath them. So there's no lurch forward from that.

    But you're talking about something different. You're talking about keeping the engine at high enough revs that it should immediately accelerate as-if-from-rest. Except that the wheels are virtually stopped while the engine is revving high. If you're ever been in a car when it comes to a very abrupt halt, most often what happens is the engine just stalls.
     
  9. Jun 20, 2012 #8
    Yes, on a motorcycle for instance, the effect is noticed more. When the front wheel hits the ramp it is like applying the front brake. Once the rear wheel hits the motorcycle starts to accelerate.
     
  10. Jun 20, 2012 #9

    Nugatory

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    With a stick-shift, feathering the clutch... It won't stall, that's what the clutch does. On the other hand the throttle setting that maintains a 60 mph cruise on the highway as we approach the ramp isn't going to be enough to accelerate us up the ramp... So I still believe that the original question was incompletely specified and the result will depend on what the driver does with the throttle (and clutch, which I should have mentioned as well).
     
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