# Particle in a Box Question help??

## Homework Statement:

A ball of mass 3 g is trapped in a box with infinitely high walls. The width of the box is 8 cm.
a) Calculate the speed of the ball when it is in the ground state of the box
b)Estimate the quantum number n if the ball is moving with a constant speed of 2 m/s in the box

## Relevant Equations:

Ek= 1/2 (m)(v)^2
E=hf
E-n= (n^2*h^2)/ (8*m*L^2)
I am having difficulties understanding this concept. Particle in a box (as far as i understood) shows that the electrons or particles need a certain about of potential energy to escape the one-dimentional potential well. I think its pretty similar to the concept of the electons and ionising energies. (correct me if i am wrong).

so in question a) i tried to use the formula by just using the values i knew, L=0.08 m and m=3e-3 kg. I also took n=1.
As a result E=4.32e-30 J. When i equate this energy with KE. I get a small velocity of 3m/s. (The correct answer is 1.0e-30 m/s)
I am not sure how to find the right answer.
Also, because the ball is in a box with infinitely high walls, it should have an infinite potential energy thus inifinite KE. So how is it possible to find a value for this.

can someone explain/help?

thanks!

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PeroK
Homework Helper
Gold Member
As a result E=4.32e-30 J. When i equate this energy with KE. I get a small velocity of 3m/s. (The correct answer is 1.0e-30 m/s)
I am not sure how to find the right answer.
It's difficult to see how such a tiny energy could equate to ##3 m/s## for an object of ##3g##.

PeroK
Homework Helper
Gold Member
Also, because the ball is in a box with infinitely high walls, it should have an infinite potential energy
The potential is 0 inside the box.

oh right i used m as the electron but i had to use the mass of the ball.
this gives me v= 5.37e-14 m/s which is still not the correct answer.
Also, if the potential is 0 inside the box, E=KE.

PeroK
Homework Helper
Gold Member
oh right i used m as the electron but i had to use the mass of the ball.
this gives me v= 5.37e-14 m/s which is still not the correct answer.
Also, if the potential is 0 inside the box, E=KE.
You need to show your calculations. Although it looks like you've simply not squared Planck's constant.

yes you were right i made a mistake with my calculations.
then in this case my only question would be;
would it be possible to say that E(n)= PE+KE= hf?

PeroK