Physics

[pnoybballin]

For a particle that enters the hole in the upper plate with a vertical speed of 5 m/s and falls straight though without collision, ultimately striking the bottom plate at 5 m/s, what is true?

A. Magnitude of Felectric > Magnitude of Fgravitational
B. Magnitude of Felectric = Magnitude of Fgravitational
C. Magnitude of Felectric < Magnitude of Fgravitational
D. Felectric is perpendicular to Fgravitational

The answer is B. Could someone please explain why this is the case? Thanks a lot!

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[bjb305]

For a particle that enters the hole in the upper plate with a vertical speed of 5 m/s and falls straight though without collision, ultimately striking the bottom plate at 5 m/s, what is true?

A. Magnitude of Felectric > Magnitude of Fgravitational
B. Magnitude of Felectric = Magnitude of Fgravitational
C. Magnitude of Felectric < Magnitude of Fgravitational
D. Felectric is perpendicular to Fgravitational

The answer is B. Could someone please explain why this is the case? Thanks a lot!

think of newton's laws... if there is a net force, there will be acceleration
in this case, the particle started AND ended at 5m/s.. therefore you know that there was NO acceleration and hence NO NET force. this makes B true because the magnitude of both forces are equal and opposite and thus cancelling out to give you no net force

 

[pnoybballin]

think of newton's laws... if there is a net force, there will be acceleration
in this case, the particle started AND ended at 5m/s.. therefore you know that there was NO acceleration and hence NO NET force. this makes B true because the magnitude of both forces are equal and opposite and thus cancelling out to give you no net force

Thanks for the response. I understand why there is no acceleration and thus not net force. But how can the Mag. Felectric = Mag. Fgravitational? Aren't they in different directions? Well I know you can break them down into their components so are we specifically looking at the y direction?

 

[FutureScaresMe]

This is basically the Millikan oil drop experiment. When F_up = F_down, then there is zero acceleration and the oil drop (or "particle" in this question), would either be suspended in air, or fall with a constant velocity.

Fup = Eq
Fdown = mg

so from this experiment, Millikan was able to calculate q, but that's beyond this question...