This intent of this forum is to provide guidance and help for students who are preparing for a science fair. There is the possibility a student could come across this post, and consider the material within as a potential topic for a science fair contest, so I'd like to add a supplemental opinion to ghariman's response.
ghariman wrote: The potato in your hand will be travelling at the same velocity and direction as you and the truck in which you are sitting in. At the instant you throw it out of your hand it will still be at the same velocity and direction you throw it at.
I agree.
So let's take an example to clarify things.
Say the truck, you and the potato are travelling north at 60mph. And the truck has a wake (which I think goes to the south) of 5mph.
And say you managed to throw the potato with your hands and get 100mph.
I believe it may be confusing to say "The wake will travel south at 5 mph." A more accurate statement would be that the air behind the truck has been sufficiently beat up (from the fast moving truck that recently passed through it), and that an object in that air, moving at the speed of the truck, would feel less deceleration due to that body of air, than if it were not behind the truck. This is like race cars or bicyclists drafting (following close behind one another).
And to make things simple say the person you are trying to 'chunk-down' is very close to your hands and so the distance doesn't matter as the potato will eventually slow down and hit the ground.
A good assumption, but in my approach, perhaps we simply assume that we're so good at throwing potatoes, that we know how to compensate for all the forces that will act on the potato after it leaves our hands; we can put it right-on-target with a similar flight path everytime. Further, we'll ignore gravity--pretend we throw the potato horizontal, and it doesn't gain or lose velocity in the vertical direction.
Scenario 1:
As the unfortunate lad is in front of the truck the total velocity and direction of the potato = 60mph + 100mph - 5 = 155mph to the north.
First, we'll assume the person we're aiming at is directly in the path of the truck's travel, so we are throwing the potato straight ahead. The potato's velocity, after it leaves the thrower's hands, will not be affected by the turbulent wake behind the truck. Instead, it the potato will travel at 160 mph due north. In this case, the potato will be slowed by the full frictional force af a 160 mph headwind, and will thus slow fairly rapidly, it's final speed depending on how long the potato is aloft. Assuming a short time-of-flight, this is the scenario in which our potato is traveling the fastest when it hits the target.
Scenario 3 (number 2 is complicated):
The potato will travel = 60mph - 100mph - 5mph = -45mph (that is 45mph to the south).
In scenario 1, we assumed we could through the potato at a speed of 100 mph. In scenario 3, then, when the potato leaves our hands, it will leave a truck traveling at 60 mph to the north at a speed of 100 mph, to the south, and have a net speed of 40 mph to the south. Here the effect of the "wake" will be pronounced; the potato traveling to the south will be subject to an airstream that has a net velocity to the north (pulled by the truck that just passed through). So, the potato will travel at 40 mph, and be exposed to a somewhat-greater-than-40 mph frictional headwind. Assuming a short time-of-flight, this scenario gives the slowest speed with which the potato will hit the target.
Scenario 2:
Now you're throwing the potato perpendicular to both the truck (north) and the wake (south). To first order the only component will be 100mph from your hand to the west/east. However in reality there should be an effect from the truck and wake on the potato in that it won't travel exactly perpendicular but it will sway (here more to the north, so north-west).
In this case, the potato has two components of motion as it leaves the thrower's hands: along the direction of the truck, at 60 mph, and perpindicular to the truck, at 100 mph. It will be subject to the effects of friction in both directions; but since the total speed of the potato is around 117 mph (sqrt(60^2+100^2)) it will face less of a decelerating force than the potato in scenario 1, but more than that from scenario 3. It will likely (again--under the assumption of the a similar time-of-flight with the other scenarios) strike the target with a total speed less than that of the first potato, but more than that of the third potato.
Anyway I think in the first case the potato will hit the lad faster than anycase. Then again I may wrong as I am not an expert in this.
Besides why throw the potato at a man on the side of the street if you could instead boil, mash it and make a good meal out of it.
Hope this helps.
Agreed. A tomato might carry the same message, without the same potential for injury.
I also am also not in the field of fluid mechanics, but I don't believe I've made any gross oversights. However, I believe the original response may have been somewhat misleading in stating the velocity of the potato would change by +/-5 mph due to a wake that purportedly travels both ahead, and behind, the truck. Certainly, the wake will have a net effect on the velocity of the potato, but only when the potato is thrown behind the truck, and--with our throwing speed greater than the speed of the truck--this wake would slow the potato more than if the wake were ignored.
Adam