Another Physics Question

[crokett]

Remember in Knight Rider where at highway speeds Michael Knight would drive Kit up into the back of the big rig? In real life would he either

a) fail to stop and crash through the back of the tractor cab (unless he had really good brakes)
b) do exactly what was done on the TV show

[maxparot]

Remember that Kit was able to control himself so I'd say with a computer controlling the braking and tire pressure a safe stop may be possible dependant on the speed of the car when it hits the ramp with it's rear tires. Also a treadmill like device w/ speed sensors could be install in the semi so make it possible for a real human to safely enter the box. Without special equipment once the rear tires hit the ramp the car would most likely end up in front of the truck.

[cgallery]

Remember in Knight Rider where at highway speeds Michael Knight would drive Kit up into the back of the big rig? In real life would he either

a) fail to stop and crash through the back of the tractor cab (unless he had really good brakes)
b) do exactly what was done on the TV show

How was this typically done as a special effect? I imagine that both the truck and the car were braking. Or perhaps they ran the camera at high speed and the car/truck at low speed?

[TheRic]

It can be done. You have to know what you are doing, and timing is VERY important. If you have ever watched stunt shows they show how some things are done, and show things you didn't think were possible, but can be done.

I'm not sure how they do that scene. I'm making a few guesses here. The car is moving a little faster than the truck. The car is taken out of gear just before it hits the ramp. The momentum of the car carries it up the ramp, there is very little forward force left, so little brakes are required.
Also remember they have the whole semi to stop in, not all that extra stuff in the semi like they show. They also might shoot the same scene several times until they get it just right.

Think of it like when one person jumps from one vehicle to another. Everything is moving at the same speed, the person does not have to jump forward (other than to account for wind resistance) to compensate for the forward movement of the vehicles. OR you throw something from the back seat of a moving car to the front seat. You don't' have to throw it 60 MPH + to get it up front. Everything in the car is moving at the same speed.

Most chase scene in movies, and TV are done at about 35 MPH (at least they were about 10-15 years ago when I heard the statement). By constantly switching angles, and other things they can give the illusion that they are traveling faster.

[Kristofor]

My vote is for it being hard on the car's tires...

Lets say the truck is tooling along at 50mph and the car comes tooling along at 55mph.

As soon as the back wheels hit the ramp the car is now moving at only 5mph relative to the truck (plenty reasonable to stop in the length of the trailer) but the wheels are spining at 55MPH and the car either needs to accellerate another 50mph, the tires need to spin down that amount, or a combination of the two.

So I think they'd smoke the tires a bit on the ramp, then stop...

Kristofor.

[Zenaca]

Ever watch Fear Factor they do it all the time!

So i imagine that with a little practice it would not be hard to do it real smoooth.

[Perfidiajoe]

I would think that once the car (drive) tires hit the truck ramp, the truck is no longer moving relitive to the car, as the car tires are still turning 55 mph on the truck ramp, which is not moving(you could stand on the ramp w/ the truck going 50 mph). I believe they ran the film in slow motion in reverse, backed the car down the ramp, then played it at regular speed forward. Then it's a simple matter of putting in the background motion.

[onedash]

this is a good one. you would be traveling at say 55 when you hit the skids. so I guess you would be going 55 through the semi if you kept applying the same amount of power. if you used only enough power to get the back tires on the ramp you could then apply enough power to move at 2, 10 or 50 mph if you chose.

[cgallery]

Ever watch Fear Factor they do it all the time!

So i imagine that with a little practice it would not be hard to do it real smoooth.

But it seems like they are doing 35-40 MPH on Fear Factor when they attempt this. Or, are they going faster?

[gjat]

There are essentially two moving objects that have to be stopped. The Car and it's Wheels.

To have the Car pull up into the truck, it's only about stopping the differential energy between the car and the truck. If the truck was going 100 mph and the car 99, then it's a 1 mph differential. Stopping the car is theoretically easy.

The other moving object are wheels. First off, you'd have to take it out of gear to stop energy being added to the wheels spinning. Then you have to stop the wheels in relationship to the car and truck.

Some sort of rollers/conveyor on the ramp to the truck would could give the operator adequate time to slow the wheels. Remember, the relative difference between the car and truck is 1 mph. It's not hard to stop a car at 1 mph, but we have to stop the wheels.

Free spinning rollers would allow the brakes to stop the wheels because the momentum is limited to the wheels motion alone. We took it out of gear and are just trying to stop wheels going 99 mph without the momentum of the car transferring energy to the wheels.

Get the front wheels up on the ramp rollers. Apply the brakes to stop them relative to the truck. Let the 1 mph momentum of the car push the car up on to the truck. Brake to stop the 1 mph momentum of the car while the back tires are spinning on the ramp rollers to give them time slow to match the relative speed to the truck bed.

ABS brakes would be essential. If you 'lock up' the wheels causing a skid, that reduces the friction between the tires and truck bed and increase the stopping distance substantially.

[Richard in Smithville]

Michael Caine did it with three Mini's in the original Italian Job. Put them in the back of a coach. But then again, they were front wheel drive.

[LCHIEN]

it's doable. There will be a brief squeal of tires as the tire goes from road speed to nearly zero in the instant it touches the ramp and hangs on that point. The squeal because one part of the tire istrying to grip the road and the other the ramp at differential speed of whatever (vehicle speed minus truck speed). As soon as the drive wheels hit the ramp, he's got to slow from vehicle speed to near zero with just enough foward speed (relative to the ramp) to pull the vheicle fully in, then stop.