# Physics HUP help

A ball of mass 50g moves with a speed of 30 m/s. If its speed is measured to an accuracy of 0.1%, what is the minimum uncertainty in its position? What does this answer indicate about the relevance of quantum mechanics to macroscopic objects?

I am completely confused. My prof just gave us HUP but didn't explain how to use it.

I know: (delta p)(delta x) > h /2, where h is actually h bar i just don't know how to type it.

I think i need to sove for delta x, but i have tried similar problems that i have the answers to and i'm not even close.

please, if anyone can help me understand this i would greatly appreciate it.
Thanks.

Indeed solving for $\Delta x$ is correct. Why don't you show us the work you've done.

honestly i haven't figured out where to start.
i can get momentum by multiplying the mass and the speed. i wanted to multilpy that by 0.1% to get a delta p value and then solve for delta x. this is the method i used for other questions, and i did not get the right answer.

also i am not sure what is significant about the fact that it asks for the minimum uncertainty.

That sounds right. Post your calculations.

Since you know the uncertainty in momentum, and $\Delta p \Delta x \geq \frac{\hbar}{2}$, the minimum uncertainty in position is achieved when there is equality.

p=0.05kg * 30 m/s = 1.5 kgm/s.

1.5 kgm/s * 0.1% =0.0015 kgm/s

(delta x) > h/(2Delta p) where h is h bar ie) h/2pi

delta x > 6.626*10^-34/ (4 *pi* 0.0015)

delta x > 3.52*10^-32

like i said, i tried this method with another question, but i gave the kinetic energy and a 5% accuracy in the momentum. for that, i calculated the velocity from 1/2mv^2 and then multiplied by the mass. then i multiplied that value by the 5% and used the same method above for determining delta x, and i was way off. i'm not too sure about this whole HUP thing.

thanks

Yes, that looks right.

Can you post the other question (that your answer was wrong for) in its entirety (preferably verbatim)?

i just realized that i kept repeatedly using the same wrong number to figure out the other question. now i get it exactly right. thanks for your input on the my original question. guess i'm not as lost as i thought.
thanks.

Good :)

I really would like to be able to make one word replies. asdf