Car Carriers- A hypothetical

  • Thread starter BK Bundles
  • Start date
  • #1
I totally lack the physics knowledge to reach a mathematical conclusion to this problem myself- but I have been wrestling with a scenario in my head for a long time that I hope someone a bit more erudite can answer- here's how it plays out.

Whenever I drive past one of those large car carrier trucks (that dealerships use to transport 10 or more cars), I notice the ramp they use for the top row of cars. My inner John Woo lights up and I start thinking-

I am going 65 mph, the car carrier in front of my is going 65 mph.
If the car carrier happened to be empty (no cars), and a latch were to break, causing the ramp they use to load cars onto the beds to hit the road and start shooting sparks-I have an opportunity for a great action sequence.

If I mash the accelerator to 80mph and drive up this ramp, with the car carrier still traveling at 65mph, what happens? Do I rocket off of the ramp with the velocity of my car + the moving car carrier, or do I roll along the top of the carrier until I drop off the front of the truck, at the same velocity as if I'd passed him on the left. I'm mixed up because this ramp becomes the "road", once I start traveling on it, but that "road" is moving at 65 mph.
Somebody please help or I'm just going to have to give it a shot to find out (only kidding).
 

Answers and Replies

  • #2
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Not a complete mathematical proof but some brain-thought and reasoning here:

If you go up the ramp at 66 or above miles per hour, you better hope that you decelerate fast enough to not crash into the end.

Because your wheels are moving at 66 (or more) mph PLUS at the velocity that the truck is also moving.

65mph + 66mph (or more) = 131mph+

So you therefore have to decelerate to 0 mph so you are going 65mph (relative to the speed of the truck).

I hope I wasn't confusing.
 
  • #3
2,685
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If you watch the original Italian Job, they answer this perfectly when trying to drive a mini into the back of a lorry.

Your speed relative to the truck on approach may only be +10mph, but once your driving wheels grip the truck (which then becomes the road) you begin accelerating again to the speed the wheels are turning at, so truck speed + car speed.

Once the whole car is on, you need to jump on the brakes to ensure you stop on the truck and don't take out the drivers cab.
 
  • #4
awesome guys, thanks for the repsonses.
 
  • #5
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If you watch the original Italian Job, they answer this perfectly when trying to drive a mini into the back of a lorry.

Your speed relative to the truck on approach may only be +10mph, but once your driving wheels grip the truck (which then becomes the road) you begin accelerating again to the speed the wheels are turning at, so truck speed + car speed.

Once the whole car is on, you need to jump on the brakes to ensure you stop on the truck and don't take out the drivers cab.
So are you saying that if the velocity of the truck is 50mph, and the velocity of the car is 60mph at the same direction then it will only have to decelerate 10mph on the truck?
 
  • #7
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So are you saying that if the velocity of the truck is 50mph, and the velocity of the car is 60mph at the same direction then it will only have to decelerate 10mph on the truck?
At no point did I say or imply that.

I said "Your speed relative to the truck on approach may only be +10mph". If you are travelling at 60mph and the truck is at 50mph (in the same direction), your relative speed is +10mph in relation to the truck.

Your initial speed once on the truck will be +10mph relative to the truck, but once the driving wheels are on the truck, you will accelerate at Y m/s^2. You simply have to stop this acceleration and bring the car to a halt on the truck. So it will be 10mph + (Y * t) (where Y is acceleration on truck and t is duration of acceleration). The easiest way is to remove the driving force from the wheels (foot off accelerator) and then ensure you stop as soon as possible and don't start rolling backwards (foot on brakes).

Note that if you take your foot off the gas as you hit the ramp (so you are decelerating before the driving wheels are on the truck, but there is enough speed to ensure you keep 'approaching' the truck to get onboard) you will have a negative Y value, reducing the speed you need to stop.

The wheels of the car will be going at 60mph on climbing into the truck and it will create a large amount of acceleration or wheel spin.
 
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  • #8
867
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You mean like this...
 
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  • #9
2,685
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Very interesting, I overlooked that (not good for an aero student). So, although my previous post stands, the acceleration value for Y is a lot smaller than I initially implied.
 
  • #10
berkeman
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I didn't watch the video, but if your car has a clutch, or if you slip the transmission into neutral, it could make stopping a lot easier.
 
  • #11
I had no idea they'd attempted something similar on MythBusters. It surprised me to see it in action, but that's the great thing about physics- it surprises me every time I learn something new. thanks for the posts guys, this is a good community.
 
  • #12
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You mean like this...
I've watched the video before. It confuses me how the car decelerates so smoothly and just doesn't go crashing in the front of the truck with its velocity. Maybe it is because you have to get the wheels in two at a time? 0_o

Or is there something I'm completely missing?
 
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  • #13
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I've watched the video before. It confuses me how the car decelerates so smoothly and just doesn't go crashing in the front of the truck with its velocity. Maybe it is because you have to get the wheels in two at a time? 0_o

Or is there something I'm completely missing?
If you listen to Jamie talking, he explains what is happening. The inertia of the vehicle resists any rapid changes in velocity that may occur and so the vehicle doesn't simply shoot forward. There is 'plenty' of time for you to respond and ensure that doesn't happen.
 
  • #14
russ_watters
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The inertia of the car is much much higher than the inertia of the wheels so the wheels decelerate much faster than the car accelerates. The opposit effect happens when a plane lands.
 
  • #15
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Now lets make this more interesting:
both vehicles have velocities of 0.6*c...
what is the velocity of the car that would be measured by an observer on the ground?
 
  • #16
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If you listen to Jamie talking, he explains what is happening. The inertia of the vehicle resists any rapid changes in velocity that may occur and so the vehicle doesn't simply shoot forward. There is 'plenty' of time for you to respond and ensure that doesn't happen.
The inertia of the car is much much higher than the inertia of the wheels so the wheels decelerate much faster than the car accelerates. The opposit effect happens when a plane lands.
I overlooked that.. amazing, this is why I love physics. <3

Thanks guys.
 
  • #17
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Yeah One can slow down, as one has the time and option... but if it were a full scale ramp bridge /TTT\ or /TTT| on top of a truck, would the car that went up with 65(ur desired unit of speed) on a truck/ramp with 60 (same unit - SU) wrt the road, have an effective speed of 125(SU) or the likes when it shoots off the top or touches down on the road... coz though the car has momentum the wheels are still spinning at the rate proportional to 65(SU).
I believe that if the truck brakes immediately after the car alights ( /TTT\ ), the car would experience a sudden drag, as I am not changing the accelerator position//engine speed, the vehicle body is at a higher speed, but the engine is being fed energy now by the car.. should literally tear the tyres and damage the engine and the torque//force carrying members as well. Something similar to me driving at 65 (SU) at a top gear and then without decelerating shifting to first gear...
Well just a thought...

And.. well let us forget about the suspensions if the car actually shot off the ramp... /TTT|
 
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  • #18
DaveC426913
Gold Member
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Yeah One can slow down, as one has the time and option... but if it were a full scale ramp bridge /TTT\ or /TTT| on top of a truck, would the car that went up with 65(ur desired unit of speed) on a truck/ramp with 60 (same unit - SU) wrt the road, have an effective speed of 125(SU) or the likes when it shoots off the top or touches down on the road... coz though the car has momentum the wheels are still spinning at the rate proportional to 65(SU).
I believe that if the truck brakes immediately after the car alights ( /TTT\ ), the car would experience a sudden drag, as I am not changing the accelerator position//engine speed, the vehicle body is at a higher speed, but the engine is being fed energy now by the car.. should literally tear the tyres and damage the engine and the torque//force carrying members as well. Something similar to me driving at 65 (SU) at a top gear and then without decelerating shifting to first gear...
Well just a thought...

And.. well let us forget about the suspensions if the car actually shot off the ramp... /TTT|
Just to reiterate.

The inertia of the vehicle is the primary factor. When it hits the ramp which is moving at 50mph, it matters little that the wheels might have been turning at 60mph - they slow down instantly - what matters is that the car is going 60mph. It goes up the ramp at 10mph relative to the ramp.

At best, it might pick up some speed if it can accelerate from 10mph over the 40ft length of the ramp. But that has absolutely nothing to do with how fast the car or ramp was moving.
 
  • #19
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Well I am talking of a case where the car does pick up the speed...
I was considering a case where the car on the truck//ramp is going 65 (SU) wrt the ramp on the road.. and the truck//ramp 60(SU) wrt the road.
Well on touch down, the driver would have to hit neutral gear in order to utilise any momentum, if gained, if the driver keeps the car in the same gear and accelerator position then the car would experience a drag.
 
  • #20
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n please do tell me how to start a thread of my own... I have a physics problem I'd like to discuss.
 
  • #21
russ_watters
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Yeah One can slow down...
The car doesn't have to slow down, it's barely moving with respect to the ramp!
... but if it were a full scale ramp bridge /TTT\ or /TTT| on top of a truck, would the car that went up with 65(ur desired unit of speed) on a truck/ramp with 60 (same unit - SU) wrt the road, have an effective speed of 125(SU) or the likes when it shoots off the top or touches down on the road... coz though the car has momentum the wheels are still spinning at the rate proportional to 65(SU).
None of that makes any sense. Did you read the thread?

Again, the car is barely moving with respect to the truck. If the truck were going backwards at 60 mph, then the car would be going up the ramp at 125 mph. But with the truck going 60mph wrt the road and the car going 65mph wrt the road, the car is only going 5mph wrt the truck and climbs the ramp at 5mph - a fast walking speed.
I believe that if the truck brakes immediately after the car alights ( /TTT\ ), the car would experience a sudden drag, as I am not changing the accelerator position//engine speed, the vehicle body is at a higher speed, but the engine is being fed energy now by the car.. should literally tear the tyres and damage the engine and the torque//force carrying members as well. Something similar to me driving at 65 (SU) at a top gear and then without decelerating shifting to first gear...
Well just a thought...
A truck doesn't brake anywhere near as well as a car does, so no, a truck could not brake anywhere near fast enough to make the car have trouble holding-on to the ramp.
 
  • #22
russ_watters
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Well I am talking of a case where the car does pick up the speed...
I was considering a case where the car on the truck//ramp is going 65 (SU) wrt the ramp on the road.. and the truck//ramp 60(SU) wrt the road.
So you mean if the car starts by driving on the road at 125 mph? That wouldn't be a good idea...
n please do tell me how to start a thread of my own... I have a physics problem I'd like to discuss.
Hit the "new topic" button in the forum view (where you see all the other topics).
 
  • #23
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A truck doesn't brake anywhere near as well as a car does, so no, a truck could not brake anywhere near fast enough to make the car have trouble holding-on to the ramp.
I am not talking about the truck braking and forcing the car to skid. I only spoke of a situation where the car is back from the ramp onto the road and at that instant when the car is free from the truck, the truck brakes...
But with the truck going 60mph wrt the road and the car going 65mph wrt the road, the car is only going 5mph wrt the truck and climbs the ramp at 5mph - a fast walking speed.
The scenario I was reffering to is different. Well if the ramp is long enough, and the car//bike or object on top of the ramp, accelerates and reaches a speed of 65 on the ramp. then when it comes down from the ramp on the other side, of a ramp structured as /TTT\, this was the condition that I had expressed my views on.. and not of the instant when the car climbs on the ramp, even then when u dont ease off the accelerator, the car will experience a thrust on the ramp, if u dont decelerate or change gears while going up the ramp. Kind of like when u lift he back wheels (not talking about front wheel drives and 4 wheel drives) accelerate and then let the car//bike back wheel down.
 
  • #24
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Hit the "new topic" button in the forum view (where you see all the other topics).
Thank you Russ... Thanks a lot.. But as dim-witted ;) as I am I think I needed ur help to find that...
Thank you again... Have a merry Christmas or anyways a wonderful year ending ..
;) Well not just Russ but everyone, everywhere.. tc..
 
  • #25
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:(
boohoo... Thank you everyone,.. but I guess as a newbie I still need to wait for the moderators or the system to provide me the rights to post...

Well got this message.... Can someone help me here...

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