How to solve multiple choice questions?

[meisterluv]

Hello I was wondering if some one can give me advice on how to approach these types of problems. I do horrible on these types of questions but do well on all other, any strategy helps.

Homework Statement

This is an example problem:

Oxygen molecules are 16 times more massive than hydrogen molecules. At a given temperature, how do their average molecular speeds compare? The oxygen molecules are moving
A. at 1/4 the speed.
B. 16 times faster.
C. at 1/16 the speed.
D. 4 times faster.

Homework Equations

I know that

$$v = \sqrt{\frac{3KT}{m}}$$

The Attempt at a Solution

My thought process would be to do this for oxygen:

$$v = \sqrt{\frac{3KT}{16m}}$$

and this for hydrogen

$$v = \sqrt{\frac{3KT}{m}}$$

comparing the two equations, I see that oxygen has the value 16, which is the square of 4. Thus I would think that A. would be the correct choice.

Questions like these are what get me most of the times. Thanks!

 

[MostlyHarmless]

When you're faced with a multiple choice problem, it's best to ignore the choices at first and work it as if it were open ended. If you start trying to look at the choices you may try to construct your answer to fit a particular choice which can be misleading.

 

[berkeman]

Hello I was wondering if some one can give me advice on how to approach these types of problems. I do horrible on these types of questions but do well on all other, any strategy helps.

Homework Statement

This is an example problem:

Oxygen molecules are 16 times more massive than hydrogen molecules. At a given temperature, how do their average molecular speeds compare? The oxygen molecules are moving
A. at 1/4 the speed.
B. 16 times faster.
C. at 1/16 the speed.
D. 4 times faster.

Homework Equations

I know that

$$v = \sqrt{\frac{3KT}{m}}$$

The Attempt at a Solution

My thought process would be to do this for oxygen:

$$v = \sqrt{\frac{3KT}{16m}}$$

and this for hydrogen

$$v = \sqrt{\frac{3KT}{m}}$$

comparing the two equations, I see that oxygen has the value 16, which is the square of 4. Thus I would think that A. would be the correct choice.

Questions like these are what get me most of the times. Thanks!

Welcome to the PF.

I agree with MostlyHarmless that it's best to estimate or figure out the answer on your own, and then look at the multiple choice selections.

In this problem, I just thought to myself: KE = 1/2 * m * v^2, so 16x mass means v^2 goes down by 16 to keep the same KE. So v goes down by 4 to keep the equation balanced.

 

[lightgrav]

You can save time and effort and avoid some confusion if you only compare the things that are different
... ignore 3, k, T
so oxygen's v ~ √1/16 = 1/4 as fast as hydrogen's

 

[meisterluv]

Thanks. does anyone have a good site for practicing multiple choice concept questions?