# Uncertainty principle

1. Nov 4, 2009

### zoner7

1. The problem statement, all variables and given/known data
A mosquito of mass .15mg is found to be flying at a speed of 50 cm/s within an uncertainly of .5 mm/s.

a) How precisely can it's position be known?

2. Relevant equations

Delta p * Delta x = h bar / 2
p = mv

3. The attempt at a solution

I look at problem originally and simply solved for momentum using the given velocity 50 cm/s. Then i plugged that into the delta x * delta p = h bar / 2.
I got the answer 7.033 X 10^-31

Then I realized that I never used the variable describing the uncertainly of the fly's velocity. Clearly this is relevant to the solution of the problem. I think that I calculated the acutal momentum, as opposed to the uncertaintly of momentum, which the above equation calls for. How am I supposed to differentiate between the actual velocity and the uncertaintly of velocity?

Does the problem want me to give a range of positions? I could solve for the uncertainly of momentum using 50 cm/s + or - .5 mm/s. Could I then could find a max and a min position using the uncertainly equation? What exactly should I be solving for?

Thank you in advance!

2. Nov 4, 2009

### Jasso

You found momentum p, which is not what the equation uses. It uses delta p.

Given that $p=mv$ , then $\Delta p=m\Delta v$.

The problem just wants to know delta x.

Last edited: Nov 4, 2009