How to extract information from question given that u=0

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  • #1
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Homework Statement


An electron falls through a distance of 1.5 cm in a uniform electric field of magnitude 2.0 x https://www.physicsforums.com/callto:104 [Broken] N C-1. The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance . Compute the time of fall in each case



Homework Equations


s=ut +1/2 at^2

The Attempt at a Solution


In my book the following formula is used
t=√2s/a
here s is distance and a is acceleration
it is used when u is zero
u here means initial velocity
so my query is why should we take initial velocity to be zero as nothing such is mentioned in the question
 
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Answers and Replies

  • #2
RUber
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I think this is a standard assumption when you are given no information that the electron was already moving.
If I said, a ball falls 1.5cm under uniform gravity, how long does the ball fall for? You might intuitively assume that the initial downward velocity of the ball is 0.
 
  • #3
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initial downward velocity of the ball is 0.
And what about velocity in other directions?
 
  • #4
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why should we take initial velocity to be zero as nothing such is mentioned in the question
I think answer is hidden in my question only!as nothing such is mentioned in the question, we should assume that.
 

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