Driving a car on to the back of a moving truck

Okay, and odd title, but let me continue.

I recently read a 15+ page argument on another board about what would happen in the following scenario...

You are traveling 62mph(~100kph). There is another vehicle (let's say, a car carrier truck as pictured below, with a ramp to the roadway :p ) moving at another, slightly slower speed.


http://www.midamericawrecker.com/images/trucks/000002.jpg



If the car were to drive up a ramp into the back of the truck, what would happen?
A. The car would drive up and be able to hit the breaks to stop on the truck.
B. The car would traveling at 62mph on the back of truck and thus unable to stop before flying off the other end.
C. your theory.

I've made up my mind, and am frankly astonished at the amount of conversation this derived on another board. However, I believe the PF crowd to be slightly of higher intelligence, and thus don't expect such a large deviation in answers

There's no need to invoke relativity for 62 mph, so Gallilean relativity is sufficient. Your speed relative to the truck is just the difference of your speed and its speed relative to the road (say 10 mph) so you could drive up and stop with no problem, provided the truck doesn't accelerate some way while you are doing this manoevre.

Well, the front tires are spinning so quickly when they hit the truck that you could run into some problems...

If you replaced cars with people in the question, then A would definitely be the answer. Motion is all relative, and relative to the ramp, the person would not have such a great speed.

Originally posted by TonyG
The car won't retain its 62 mph speed on the bed of the truck, because the car will lose a lot of its energy in making the transition from the static road, to the bed of truck that's moving away from the road at 60 mph.

The first part of this sentence is correct, everything after "because" is wrong. An object traveling at speed has the same amount of kinetic energy whether it is traveling under its own power or whether it is being carried by another vehicle. The only energy lost in the case of a car is the extra energy of the spinning wheels. In a typical car, this constitutes less than 1% of the total energy.

I think in the real world the car would most likely just stall (depending on it's characterstics)

My assumptions
1. the truck and car have mass and are at constant velocity before the touch down
2. the car does not have infinite torque
3. all velocoties discussed below are absolute (ie not relative)
4. The car accelerates until the moment the driver senses acceleration (ie when the drive wheels touch down), and then he hits the brakes
5. The truck driver does not depress the clutch or release throttle at any point

I see two main things happening once the cars drive wheels hit the ramp:
1. the truck will be decelerated
2. the car will accelerate
3. losses occur (wheelspin, engine damage etc.)

the amount by which 1. and 2. occur is dependant on the truck and cars characteristics (engine power, weight, aerodynamics etc), but you couldn't simply add the two velocities together to see the car suddenly go from 100 kph to 200 kph; this would require infinite torque, or zero mass etc. and this of course is not reality

Anyway, real world scenario would not see the car off the end of the ramp in my opinion, BUT...

IF the car had a huge amount of torque, and the ramp angle was low, and the truck was very light, and the cars engine did not expode when subjected to the loading etc. etc. then yeah, maybe, but not in the real world IMO.

my first post by the way,

Martin

Originally posted by mighty2000
...
The fact that the vehicle's velocity is so much greater than the truck's velocity could conclude that the truck is basically static, or insignificantly moving...

You did not read the question carefully enough. The case considered is where the truck is moving slightly slower than the car.

The only things that should prevent the car from going up smoothly would be the gap between the truckbed ramp and the gound and the tires of the car spinning out, similar to when street racers press the brakes while also pressing the accelerator to spin the wheels while not moving. I would think that the tirespin effect should be negligible, if you are expecting to just cruise onto the truck, without turning or pressing the gas.

I can't imagine it being problem free; the way I look at it its the equivelent of driving at 100kph, then suddenly the tarmac is hauled from beneath the car in the direction of travel, in zero time, simulating the tyres touching the moving ramp (apart from the change in co-efficient of friction)

the fact the the ramp is probably made of metal helps towards absorbing the forces less dramatically, I just wonder if the stuff we see on knight rider, and in the movies (italian job etc), is quite as it looks or has been helped a little, I'd be interested to know

omg this is so simple i can't understand why no ones said it before.

drive towards the truck at say, 10 mph faster than the truck is going. put it in nutrual. roll onto the truck, apply breaks. no danage to engine at all.

This has been done as a driving stunt in a movie. I believe the title was "Self-presrvation Society". Lining up a car's tires with the type of ramp used on a car-carrying truck is very difficult, and the cars approached the truck at a speed that was only slightly faster than the truck's speed (relative to the road, the speed on the speedometer). I'm not certain if that was done due to the need for precission in lining up the front tires, or for dramatic effect. A bit of both, I suspect.

At any rate, the cars were rear-wheel drive (c. '65 Jaguire XKE's; reeeaaally nice ), and the results were rather a combination of A snd B. At such slow closing speeds ( <1 kph relative to the truck), stepping no the clutch and coasting was not an option, so power was still being sent to the rear tires when they made contact with the ramp. Jags are so light and pwoerfull that each vehicle lept foward when the drive wheels hit the ramp, and the breaks needed to be applied quickly and with considerable force. I'm sure this breaking force was accentuated for film, but nevertheless a real need.

Physics of driving car onto moving truck

I can't believe how much incorrect speculation, false information and general confusion there is on the web regarding driving a car onto a truck while both are moving at high speed. Not that I understand the physics behind it, which is why I am posting here and hoping that since this is the Physics Forum, that there is someone here who can CORRECTLY explain the physics behind how it works. I have not found any complete or correct explanations in searching the web. (I did find a lot of people saying it can't be done, and one plausible but incomplete explanation of how it is done.)

Bear in mind it has been clearly proven that the Knight Rider like scenario of driving into the truck and backing off the truck at speed both work perfectly. See the MythBusters episode of earlier this year, where they did it at 55 mph. There was no tire squealing or excessive crazy braking or crazy skidding when backing off the ramp.

The most sensible sounding explanation I've found so far regarding driving onto the truck, is that the fast spinning tire slows dramatically once it hits the ramp due to the car being in high gear and not having enough torque to keep spinning the wheel. But the logical follow-on question is why doesn't the car stall then?

I've found no explanation of how backing off the trailer works, intuitively it seems like backing off while truck is going 55mph and your back wheels hitting the pavement would really buck the car out of control, but in practice it seemed very smooth indeed.

So, anyone? We've seen it done. Can someone explain how it works? Pretty please?

thx

Perhaps to back off the truck, they don't go in reverse but coast off in neutral? I haven't seen that mythbusters episode, though...

russ_watters said:

Perhaps to back off the truck, they don't go in reverse but coast off in neutral? I haven't seen that mythbusters episode, though...

That's certainly the way that I'd do it if I were the driver. Going the other way, I'd certainly get off of the throttle as soon as the driving wheels got onto the ramp. I also never saw that episode.
As for seeing the Knight Rider do it repeatedly with no complications whatsoever... anyone ever hear of 'editing'? We've also all seen the General Lee jump 50' gaps repeatedly with no complications whatsoever—except that they needed 20 cars in constant rotation to maintain that illusion.

Ok, I've watched the video now and the rain clearly helped things. The powered wheels can be seen spinning as it goes up the ramp, perhaps 6 feet. They don't say anything about it, but I can't see how the car would not stall as the RPM drops going up the ramp (have you ever tried driving at 5mph in 5th gear?).

Well, if anyone is enough of a Philistine to use an automatic transmission, stalling won't be a problem.

Danger said:

Well, if anyone is enough of a Philistine to use an automatic transmission, stalling won't be a problem.

i would consider someone who uses a manual a philistine, in the strict sense of the word

Same principles but could someone help with this. If a car is driving at say 60kph and then drives up the ramp of a truck moving at 50kph what speed is the car then moving at. Is it 110kph or is it not just moving at 60kph on the truck and 50kph relative to the static road.

The car won't stall if you know how to drive. Just press the clutch pedal. That's what it's for.

If you're driving an automatic, then it won't stall even if you don't know how to drive, because automatics don't stall when you stop the wheels.