Driving a car on the roof of a truck.

[aLLie04]

With limited knowledge of physics, my friends and I have been discussing this question and cannot come to an agreement.

If a truck is driving on a highway (at 60 mph let's say), and a car is on top of the truck, how fast does the car need to be moving to not fall off the back or to drive off the front?
Does it need to be driving the speed of the truck? Or does it just need to be traveling fast enough to break through the wind (and how fast would this be)? Can it stay parked, or will it fall off?

 

[rcgldr]

If a truck is driving on a highway (at 60 mph let's say), and a car is on top of the truck, how fast does the car need to be moving to not fall off the back or to drive off the front? Does it need to be driving the speed of the truck? Or does it just need to be traveling fast enough to break through the wind (and how fast would this be)? Can it stay parked, or will it fall off?

The car needs to be parked. It's speed relative to the truck is zero, otherwise if there's a difference in velocity, the car goes off the truck.

 

[aLLie04]

So the wind doesn't have an effect on it when its parked?

 

[arildno]

So the wind doesn't have an effect on it when its parked?

Sure it does.
But if you have any relative velocity with respect to the truck, your car is eventually going to fall off.

Thus, you must hope that when standing still with respect to the truck, the friction between the car's tyres and the truck roof is sufficient to witstand the windy assault.

 

[Danger]

If it were in neutral, and not tied down, it could conceivable be blown off of the back. In reality, anyone transporting a car uses restraints. Check out a car-hauler the next time you see one parked.

 

[aLLie04]

If you were to be driving on top of the truck, would the speeds become cumulative? I mean, in reference to the ground would the car be going the speed of the truck plus its own speed? I understand that in reference to the truck it would only be going the speed the speedometer says, but what happens when you change the reference point to the ground?

Thanks.

 

[aLLie04]

Thats what I was thinking, but in theory could you have the car sit there without blowing off the back while its parked (oppose to in neutral). In my mind it seems that the wind would eventually cause it to creep its way off the back because its not restrained by anything.

 

[Danger]

That entirely depends upon the wind speed (assuming that the road isn't very bumpy) and the aerodynamics of the car. A good tornado can toss a car half a mile or more. It wouldn't likely happen under any normal driving conditions. A reasonably streamlined car sitting on a truck going 100 kph on a still day wouldn't likely notice at all.

 

[russ_watters]

If you were to be driving on top of the truck, would the speeds become cumulative? I mean, in reference to the ground would the car be going the speed of the truck plus its own speed? I understand that in reference to the truck it would only be going the speed the speedometer says, but what happens when you change the reference point to the ground?

Thanks.

Speeds are additive, just like your instincts tell you.