# Interview question

by D H
Tags: interview
 Sci Advisor P: 1,082 Great problem. I wish I'd known it when I was terrorizing graduate students in their physics PhD orals. Another great one, supposedly originating with Enrico Fermi, is: how far can a bird fly? Regards, Reilly Atkinson
 P: 15,294 African or European?
PF Patron
P: 867
 Quote by DaveC426913 OOHHHH!!! Upon turning on the jets, the craft accelerates to the right. Upon turning off the jets, the craft accelerates to the left BUT the craft is slightly less massive than before, causing it to have an slightly GREATER acceleration leftwards! Jeez. Why didn't you just say so! Izzat right? I hope that's right.
One thing i don't inderstand about this - in the first step (turning on the jets) the craft and the exhaust are accelerating to the right. but since there're no external forces how can the "system" have a positive momentum?
 Mentor P: 13,657 The "system" does not have a positive momentum. It has zero momentum. The "system" comprises the solid vehicle, the fuel in the tank, the exhaust, and the fuel flowing inside the vehicle from the tank to the thrusters. It is the leftward momentum of that fuel flow that counterbalances the rightward momentum of everything else.
 PF Patron P: 867 Ok, i hope i'm not being stupid here but i've thought about it awhile and am convinced that after the thrusters are turned off the craft will have moved to the right but will have no speed, and the exhaust will also have 0 speed, here's why: instead of fuel in the pipe think about someone throwing a ball (really 2 balls, one for each thruster from the place that the fual tank is to a "reflector" (a slanted wall) right under the thruster like this: (the ">" in the middle are the 2 reflectors) +=====v=========+ |. . . . . .. . . . . . . . | |. . . . . >-----O . . . | |. . . . . .. . . . . . . . | +=====^=========+ now lets see what happens when the person throws one ball at a reflector- first the craft is moving to the right and the ball to the left, then when the ball hits the reflector the craft and ball stop moving and the ball flies up - notice that i'm not using conservation of kinetic energy, i think that this is reasonable for liquid hitting a wall. so throwing one ball just moves the craft over a little to the right. now if the person is throwing a lot of balls one after the other at a constant rate then the craft will stop moving because as soon as one ball is thrown another one hits the reflector. but if the rate of the throws are accelerated then the craft will start moving to the right because more balls are being thrown then are hitting the reflector at any given time interval. but if the person stops throwing the balls then all the balls that are currently between the person and reflector eventually hit the reflector and the craft stops. so in the end the craft will haved moved to the right and the balls will all be flying straight up and to the left of the craft - each by a different amount. what is wrong with this reasoning? Thanks.
 Mentor P: 13,657 Your reasoning is correct only if at most one ball is in flight at any time between the thrower and the reflector. Now suppose the thrower releases balls in rapid-fire mode such that multiple balls are in flight between the thrower and reflector. I will assume the thrower fires balls at a constant rate and at a constant relative velocity and continues to do so for some time. The vehicle starts at rest with no balls in flight. The thrower starts firing balls. The vehicle's velocity changes with each thrown ball up to the time the first ball hits the reflector. When the first ball hits the reflector, it no longer stops the vehicle because there are N-1 balls in flight behind it. Instead, the collision merely reduces the vehicle's velocity a bit. The ball exits the vehicle with the vehicle's non-zero longitudinal velocity.
 PF Patron P: 867 Hmm... it seems like the real problem in my logic was the fact that the ball bounced straight up instead of up and to the right, if it actually bounces to the right then the craft would be deflected a little to the left and so in the end it's position would be moved to the right but it would have a velocity to the left. is that correct?
 Mentor P: 13,657 The balls bounce straight up/down---relative to the vehicle, that is. The have to, for two reasons: (1) They will miss the exit holes in your diagram1. (2) Anything else won't re-create the original problem. 1This ignores the problem of longitudinal acceleration during the flight from the reflector to the exit hole.
P: 15,294
 Quote by daniel_i_l but if the person stops throwing the balls then all the balls that are currently between the person and reflector eventually hit the reflector and the craft stops.
There's one piece you're forgetting here.

With a tank that's now half empty of balls, the craft is less massive than when you started. A ball throw with the same force will accelerate the craft more than at first. (a=F/m: as m decreases, a increases)

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