## Problem discussions

sphy
Posts: 209
Joined: Sun Jan 30, 2011 7:23 am

### Problem discussions

A mirror (Area= A, Mass= M, perfectly reflecting) is suspended in a vertical plane by a weightless string. Light (Intensity=I) falls normally on the mirror and the mirror is deflected from the vertical by a very small angle $$\theta$$. Obtain an expression for $$\theta$$.

physicsworks
Posts: 80
Joined: Tue Oct 12, 2010 8:00 am

### Re: Problem discussions

For perfectly reflecting mirror the light pressure on it:
$$P=\frac{2I}{c}$$, where $$c$$ is the speed of light.
Hence, the force of the light on the mirror is
$$F=P \cdot A = \frac{2IA}{c}$$
For small angles $$\theta$$:
$$T \theta = F$$, where $$T$$ is the tension in the string
and
$$T = Mg$$.
From these two equations we get
$$\theta = \frac{2IA}{Mgc}$$

physicsworks
Posts: 80
Joined: Tue Oct 12, 2010 8:00 am

### Re: Problem discussions

Where did you get this problem? It doesn't look suitable for the PGRE preparation.

sphy
Posts: 209
Joined: Sun Jan 30, 2011 7:23 am

### Re: Problem discussions

physicsworks wrote:Where did you get this problem? It doesn't look suitable for the PGRE preparation.
Well, i was working out on some problems from previous entrance questions (India) where I got this.
Why are you saying it's not suitable for the PGRE questions.? Is it a silly question or some thing?

bfollinprm
Posts: 1203
Joined: Sat Nov 07, 2009 11:44 am

### Re: Problem discussions

sphy wrote:
physicsworks wrote:Where did you get this problem? It doesn't look suitable for the PGRE preparation.
Well, i was working out on some problems from previous entrance questions (India) where I got this.
Why are you saying it's not suitable for the PGRE questions.? Is it a silly question or some thing?
I dont think you'd be expected to know that P=2I/c for the PGRE. Maybe, but I doubt it. On second thought the I/c is perfectly reasonable, it's just the constant that I don't think they'd expect you to know (though it is a perfectly clear application of newton's third law, so....)

grae313
Posts: 2297
Joined: Tue May 29, 2007 8:46 pm

### Re: Problem discussions

I think it's a good question, and it's suitability for the GRE would probably depend on the multiple-choice answer selection and whether you can come up with a clever way to eliminate two or three options if you can't remember the formula.

physicsworks
Posts: 80
Joined: Tue Oct 12, 2010 8:00 am

### Re: Problem discussions

sphy wrote:Well, i was working out on some problems from previous entrance questions (India) where I got this. Why are you saying it's not suitable for the PGRE questions.? Is it a silly question or some thing?
Well...
bfollinprm wrote:I dont think you'd be expected to know that P=2I/c for the PGRE. Maybe, but I doubt it But I also partially agree with grae313. ETS can play a game called "guess dimensions, dude".