# Homework Help: 4 problems I need help with

1. Dec 6, 2007

### SharpTJ

Last test coming up and I need help with these last 4 problems please.....

(6.6e-34*2.57e15)-(3.80*1.6e10-19) to get 10.88 J what am i doing wrong here?

Number of Photons = Energy of pulse / Energy of one photon...so Ephoton is 4.01e19...how do I get energy of pulse?

I have no idea...I'm lost on this

need anser to #3 to solve this...

2. Dec 6, 2007

### CompuChip

Read the question again. Carefully. Might it be given?

You'll have to solve the given equation for the momentum, p. You are given the temperature T and the value of k. So only the mass is still unknown. What other information is given in this question that would allow you to determine (in the broadest sense: looking it up is also determining ) that mass?

3. Dec 6, 2007

### SharpTJ

Well, I though it was given also but when I 4.01319/5960 J...the computer still says it is the wrong answer

4. Dec 6, 2007

### Astronuc

Staff Emeritus
In

Prob 1. Please check the units and orders of magntiude. 6.6e-34*2.57e15 = 1.6962e-18 J and 3.80*1.6e-19 = 6.08e-19.

Prob 2. "how do I get energy of pulse?" - that is given in the problem statement "intense pulse of light having an energy of 5960 J". The pulse width is superfluous with respect to energy, but would be a factor in a calculation of power.

Prob 3. See - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

Think of how to relate momentum of a particle, p = mv, to kinetic energy KE or T = 1/2 mv2.

Prob 4. It appears one knows the answer.

This appears to be a homework problem. If so, I will move it to Introductory Physics HW forum.

5. Dec 7, 2007

### SharpTJ

1.) stupid mistake
2.) stupid mistake
3.) using p2=(3/2)(1.38e-23)(293)([2][8])=4.32e-20 with p=2.0797e-10 kg m/s can someone verify that it is correct...computer still says wrong but I don't know what else I am missing

6. Dec 7, 2007

### dynamicsolo

Last edited: Dec 7, 2007
7. Dec 7, 2007

### SharpTJ

i thought so too...that is sqr(4.85e-22) which is 2.20e-11 and still incorrect

Last edited: Dec 7, 2007
8. Dec 7, 2007

### dynamicsolo

You started from

KE = (3/2)kT ,

but KE is also

KE = (p^2)/2m .

so you should have

(p^2) = 3mkT = 3 · (4 · 1.67·10^-27 kg) · (1.38·10^-23 J/K) · (293 K) (kg·m/sec)^2

9. Dec 8, 2007

### SharpTJ

ah...got it thanks alot!!!!!!

WL=h/v -->6.6e-34/8.98e-34=7.35e-11 nm is that correct?

Last edited: Dec 8, 2007
10. Dec 8, 2007

### dynamicsolo

I am getting 9.00·10^-24 N·sec for the linear momentum of the helium atom.

The deBroglie wavelength is given by h/p , for which I am getting

(6.63·10^-34 J·sec) / (9.00·10^-24 N·sec) = 7.37·10^-11 m or 0.074 nm.

So I basically agree with your result, but not with the way you've presented it.