## Homework Statement

Earth and Mars are both rocky planets, but Mars is half as far away again from the Sun as Earth, so radiation from the Sun is not as powerful at the surface of Mars as at the Earth’s surface.
Using the values in Table 2, calculate the amount of solar radiation intercepted by the surface of Mars. Give your answer in scientific notation, to the appropriate number of significant figures, and in the correct units.
Table 2 Values of solar constant (W m−2) and area facing the Sun (m2) for Mars.
Mars
Solar constant (W m−2)
5.20 × 102
Area facing the Sun (m2)
3.6 × 1013

## The Attempt at a Solution

5.20x10(p-o)2 / 3.6x10(p-o)13 = 1.444444444x10(p-o)15 -

1.4x10(p-o)15 W m

Can anyone tell me what i am doing wrong if my answer is wrong (which i am sure it is)

Cheers emz

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gneill
Mentor
Your result should be in watts (total amount of solar radiation incident on the surface of Mars). Look at the units of what you're given, and see what relationship will produce the desired resulting units.

To be honest i really dont know sorry i am really stuck. Are my calculations on the right track?

emz

gneill
Mentor
No. You've got problems with the units. You need to understand how units combine when you do the math. They will guide you to the correct way to solve problems like this.

You are given the solar constant at Mars in W/m2, and the surface area catching the radiation in m2. You want a result in watts. How do you combine the given information?

Joules?

Divide the area facing the sun by the solar constant?

6.9x10(p-o)14 W

?

Earth and Mars are both rocky planets, but Mars is half as far away again from the Sun as Earth, so radiation from the Sun is not as powerful at the surface of Mars as at the Earth’s surface.

gneill
Mentor
Joules?
Watts. You want watts. That's energy per unit time (Joules per second).

gneill
Mentor
Divide the area facing the sun by the solar constant?
Let's see how those units work out:

$$\frac{[m^2]}{[W/m^2]} = \;??$$

What does that yield?

Yeah thats what is in the question.

and W?

I just dont know I might just give up on this question sorry

Emz

I'm going to bridge dimensional analysis with our good old friend apples.

You have baskets which hold 30 apples each. The units are apples/basket.
You have 12 baskets, how many apples do you have?

Now you have 30 Watts/metre². How many Watts are incident on a sphere with a radius of 100 metres? Its the same process.

You seem to be flustered because you are overestimating the problem and underestimating your abilities. Also, Mars is not half the distance from the sun than the Earth.

gneill
Mentor
"Earth and Mars are both rocky planets, but Mars is half as far away again from the Sun as Earth, so radiation from the Sun is not as powerful at the surface of Mars as at the Earth’s surface."
Earth is at 1AU, Mars at about 1.5AU (okay, 1.523AU to be picky). So sure, the statement's okay.

Last edited:
ha solar constant x area facing the sun!

so 5.20x10(p-o)2 x 3.6x10(p-o)13 = 1.872x10(p-o)16

to appropriate significant figures -1.9x10(p-o)16 W

gneill
Mentor
Bravo!

By the way, you can use the x2 and x2 buttons on the edit page to produce "real" superscripts and subscripts.

cheers its a pain in the backside writing (p-o) all the time thank you both x

helpmz2011
You may not post any of the TMA questions from The Open University on a public forum. This is considered to be cheating.

Oi!

Stop cheating!

EW - S104 OU

helpemz2011

If you read the text book for S104 that you got you will find the method to do all the calculations you have asked about.

It is against the rules to post TMA questions to public forums and I suggest you di8scuss this within your tutor group forum in future.

EW