It happens a lot in film; a person driving a car drives into a truck while both are already moving along a road.
But surely when the car is on the ramp of the truck, it’s now moving on the truck at the same speed it was moving on the road? Say the car is going at 60mph (with the truck slightly slower); when the car gets onto the ramp, won’t it still be going at 60mph, and so drive straight into the front of the truck and damage it?
Mythbusters did a bit on this very thing. It was kind of cool to watch.
In fact, as soon as the drive wheels hit the ramp, the vehicle (more or less smoothly) slows to a crawl (relative to the ramp) or maintains a constant speed (relative to the road).
Inertia remains inherently the same. For the car to shoot to the front of the truck, it would need to build up 2X times inertia instantly as soon as the drive wheels hit the ramp. If the car is going 60 mph to the truck’s 58 mph, the car would have to instantly have the inertia equal to 120 mph to do what you suggest. That would require a great deal of instant energy that isn’t present at the time that the car hits the ramp.
To start with it’s going 60mph relative to the road and say 3 mph relative to the truck (the truck’s going at 57 mph.) Once on the truck, it’ll be going 3 mph still relative to the truck. You’d probably need to apply the brake as you get on the truck because the wheels will be spinning away madly which may cause the car to jump forward a bit.
Yeah, but what if the truck is on a treadmill?
ducking and running away
I am not sure you are right.
We are assuming the car to be one with a front wheel drive. Now picture this. The truck is going 58 mph. The car behind it is going 60 mph.
The speed of the car relative to the ground is 60 mph and relative to the truck is 2 mph.
The linear speed to the car is imparted to the car by the wheels that are rotating at a specific rpm, say X. This rpm of the wheel against the stationary road is giving the car the linear speed of 60 mph with respect to the road on which the wheels are running.
The car’s speed being higher than the truck’s, the car will approach close the distance between it self and the truck at a speed of 2 mph. As the wheels get on to the ramp, they will still be turning at the same rpm and will result in imparting the same linear speed of 60 mph being imparted to the car, but this time with respect, or relative, to the ramp.
Assuming the engine can be made to supply the required increase in power on account of the incline, the car will ride up the ramp and onto the back of the truck at a speed of 60 mph.
Dupe post removed.
Get the wheels in line, get the wheels in line with it and then slam yer brakes on or we’ll be in the cabin!
Not quite. As soon as the fast-spinning wheels leave the road and are on the truck, they will be severely slowed by the friction. Only the wheel assembly’s mass will tend to retain the rotational speed, not the car’s mass. In fact, the car’s mass would resist forward motion.
It sure does make a difference if it is front or back wheel drive, though. I wonder what would happen with all-wheel drive?
Sorry for the double-post, but SDMB is taking about 4 minutes to accept new posts this morning, if it doesn’t time out entirely.
The only change between my posts is the word “oppose” became “resist”.
Here’s the flaw in your logic. Just because the wheels are spinning at a speed that moves the car at 60mph, that doesn’t mean it quickly goes to 60mph relative to the ramp. Imagine you held a car in the air with a crane and got the wheels spinning at 60mph, then lowered the car onto the ground. It wouldn’t suddenly be moving at 60mph. The wheels would spin a little, they’d slow down a lot, and the car would move forward slowly (unless you kept giving it enough gas, in which case it would accelerate to 60 at about the same speed it could when taking off from a stoplight).
With an all-wheel drive car, you hope that the truck you just drove into is some kind of mobile mechanic station. 
I’m guessing if it doesn’t cause immediate damage, it puts way more strain on the drivetrain that it’s designed to handle and causes longer term problems.
Nothing of substance to add here, but does anyone else reading this thread have the theme from Knight Rider running through their mind while imagining KITT driving into the semi?
Forget the Mythbusters, long before them, Fear Factor has done this a brazillion times. With blindfolded drivers. It can be done.
With a modern electronically controlled AWD system, the control unit would see the rear wheels as spinning and channel the power away from them to the front wheels. You would have to lift your foot off the gas, but no harm no foul.
Now if it were an earlier mechanical system, the system would not be happy, but for the few seconds the speed difference lasted, I doubt any problems would occur. Now driving for an extended period of time with the fronts on the ramp and the rears on the ground would not be a good idea.
You’d have to let the car coast up the ramp, re-applying the power when all four wheels are on the ramp.
Or disengage the difflock.
I think I dealt with that in the last sentance of my post. The wheels would be spinning too fast so you need to ideally put it into neutral to take the engine out of the system and apply the brakes. When you did this would depend on whether it’s a front wheel or rear wheel drive.
Here’s the Mythbusters video where they drive a car into the back of a moving truck.
I was responding to this:
Once the wheels are off the road and on the truck, the car will be going at 60 mph relative to the truck.
Sorry if I did not make that clear but I was assuming theoretical conditions as I said:
.
You are quite right otherwise.
Wow. The engine’s increase in power would have to be able to instantly accelerate from 3 mph to 60 mph in that case. I’ve always been awed by people that question this. The rotational mass of a tire versus the mass of the rest of the car has to be some insane ratio that I could figure out if I bothered to look up car weights and tire/wheel weights and diameters.
You are, of course, correct. But they are still forgetting about friction, which is what amazes me. The tires can’t hold their grip. They spin. Upon hitting the ramp, rotational motion is effectively taken out of the equation.
It’s about power applied to the surface. The horsepower doesn’t matter. Imagine a top fuel drag racer. 8000 horses! You apply all that power and you don’t get a rocket, you’re just burning rubber as the tires uselessly spin against the surface. The same principle applies here.
We’ve all seen that stunt performed. If we listen closely, we can here the tires “bark” when they hit the ramp. Once the friction (or “grip”) of the tires is lost then there is no longer any acceleration applied to the mass of the car. So it coasts right into the truck.
The energy from the rotational motion of the wheels is irrelevant. The wheels “spin out” and the energy is lost as heat.