What happens when you shoot a ball from a cannon in the back of a moving truck?


#1

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#2

They should have fired one from a static truck sufficiently far up the road at the same time to show the two balls hitting the ground at the same time.


#3

That’s awesome how it just completely drops like that.

In other news, last Thursday I saw Adam and Jamie live. It was great.

Not so great was discovering the place I decided to park my car closed at 7pm, and I had to get a ride back home and to the parking garage the next morning. Then they had the balls to charge me $22 for over night parking. Bastards.


#4

I always love to see Myth Busters do this stuff!

But didn’t Galileo answer this question?


#5

Not with slow motion cameras he didn’t.


#6

The Church would probably have declared them heresy anyway :expressionless:


#7

Now what would happen if the truck approached the speed of light?


#8

88 miles per hour?


#9

At the Mythbusters Live show in Toronto, Jamie mentioned that he really wanted to prove the viability of a human launcher that would let you get off a bus while going full speed. For some reason the the insurance people were unhappy.


#10

I need to see it again, but with Knight Rider.


#11

I’ve actually wondered about this general (not specific, my wonderings aren’t as awesome as this) idea before. Brilliant way to demonstrate it.


#12

But what happens to the truck? Does it increase in speed? Suppose the truck was coasting at 50mph and the ball weighs the same as the truck (not those dinky li’l things you see hanging off the trailer hitch). If the truck increases in speed as the ball is accelerating to 50, would it still pop out at 0mph?
I know I’m changing some of the parameters but it’s still “What happens when you shoot a ball from a cannon etc etc.” If I’m in spacecraft A jettisoning an admittedly large can of garbage to get away from spacecraft B I don’t want to solve for the speed of the trash - it’s all about me!


#13

It’s much more awesome to see physics in action proving itself out here, as opposed to the airplane on a treadmill fracas.


#14

Sounds like a job for Super Dave!


#15

One thing that we’ve learned from cartoons is that if you step off a falling elevator right before it hits the ground, yer all good. :wink:


#16

I dunno… It isn’t really either unexpected or awesome. I’m yawning
a bit over this one!


#17

The overall momentum of the system has to be preserved, or to put it another way, “For every action there must be an equal and opposite reaction”.

Think of it as a separating force: the gun is applying a force to both objects which accelerates them both apart from each other. If the truck is much heavier than the ball, the force only accelerates the truck very slightly while the ball is accelerated to 50mph. If they’re the same mass, the force accelerates them equally.

(If you studied Physics at any point you’ll remember that F = m.a, so a = F / m)

Think of the system at a standstill (or in the reference frame of the truck; same thing). If you want to accelerate the cannon to 50mph, you will also accelerate the truck to 50mph the other direction.

From a point of view at the side of the track, the ball would be decelerated to 0mph while the truck would rocket off forwards at 100mph!


#18

It depends if he insulted the pope and several competent physicists. It shouldn’t depend on that, really.


#19

Well, unless you did it under high vacuum, air molecules in your way would start getting hit with enough force oxygen and nitrogen atoms would fuse with whatever nuclei in the front of your car they happened to hit, and you’d get something like a nuclear explosion (assuming you could somehow mitigate that problem, you’d probably end up slowed down eventually by the trail of near-vaccum something moving through air that quickly would leave.)

It would be like the first XKCD what-if, except bigger.


#20

Yeah, you would accelerate the truck forward slightly, but given that a soccer ball doesn’t weigh nearly as much as a truck, the force throwing the ball out the back exerts is very small, and think it’d too small to not be drowned out by the ‘noise’ of the slight vibrations the truck gets from not driving over a perfectly paved, really smooth road, although assuming you did have a road that nice, a good accelerometer might be able to pick it up.