The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Dividing by area and volume?

So this has caused me to lose marks on exam questions. I've always assumed when given the area i divide by the area, not the area^2 as i've already given it. Yet my answer is right except the powers are just messed up. When dividing or multiplying the area or volume should i just multiply the indices by 2 (area) or 3(volume) and leave the rest?
Original post by WilliamSlim
So this has caused me to lose marks on exam questions. I've always assumed when given the area i divide by the area, not the area^2 as i've already given it. Yet my answer is right except the powers are just messed up. When dividing or multiplying the area or volume should i just multiply the indices by 2 (area) or 3(volume) and leave the rest?


I think you need to give a concrete example of what you mean to get an accurate answer.
I don't understand what you are asking. Sorry.
Reply 2
Avatar for WilliamSlim
WilliamSlim
OP
3
Original post by Stonebridge
I think you need to give a concrete example of what you mean to get an accurate answer.
I don't understand what you are asking. Sorry.

My abd, here

Q6 D (ii) 1 http://www.ocr.org.uk/Images/79221-question-paper-unit-g481-mechanics.pdf

I did 4.2 N/(0.2 X 10^-3) but in the mark scheme they did 4.2N/(0.2 X 10^-6)

Q2 (B) (ii) http://www.ocr.org.uk/Images/62266-question-paper-unit-g481-mechanics.pdf

I did 1.08 X 20^24/ (1.3 X 10^6) whereas in the mrk scheme they divided it by (1.3 X 10^6)^3

I guess what i'm saying is if i have to multiply or divide a value( even when given to me) that is the area or volume should i still square and cube it? I've never done this, yet i still get correct answers on questions where i have to find the area or volume sometimes.
I think there are two things causing a problem here.
1. Conversion of mm to m (or g to kg etc)
1m = 103 mm
so
1m2 = (103)2 mm2 = 106 mm2
1m3 = (103)3mm3 = 109 mm3

The inverse is similar

1mm = 10-3m
so
1mm2 = (10-3)2 m2 = 10-6 m2
and so on.

2.
If you have a value for r, say, which is 2.0 x 103 m
Then r3 = (2 x 103 m)3

which is 8 x 109 m3

Is this what you mean?
(edited 8 years ago)
Reply 4
Avatar for WilliamSlim
WilliamSlim
OP
3
Original post by Stonebridge
I think there are two things causing a problem here.
1. Conversion of mm to m (or g to kg etc)
1m = 103 mm
so
1m2 = (103)2 mm2 = 106 mm2
1m3 = (103)3mm3 = 109 mm3

The inverse is similar

1mm = 10-3m
so
1mm2 = (10-3)2 m2 = 10-6 m2
and so on.

2.
If you have a value for r, say, which is 2.0 x 103 m
Then r3 = (2 x 103 m)3

which is 8 x 109 m3

Is this what you mean?

Yeah thanks man. So is it when i am dealing with conversions of units into the SI form and dealing with area or volume i would have to do this?

Like on the second question it's volume which is m^3 but i don't cube the value because it's already in the proper units?
Original post by WilliamSlim
Yeah thanks man. So is it when i am dealing with conversions of units into the SI form and dealing with area or volume i would have to do this?

Like on the second question it's volume which is m^3 but i don't cube the value because it's already in the proper units?


That's right.

You calculate the volume from the formula (4/3)Pi r3

If r was in metres then that volume is in m3, which is the correct unit.
There is definitely no need to then cube the value of the volume.

Density is then mass / volume
The volume is what you just calculated and the mass is given.
Reply 6
Avatar for WilliamSlim
WilliamSlim
OP
3
Original post by Stonebridge
That's right.

You calculate the volume from the formula (4/3)Pi r3

If r was in metres then that volume is in m3, which is the correct unit.
There is definitely no need to then cube the value of the volume.

Density is then mass / volume
The volume is what you just calculated and the mass is given.

Ah okay thank you very much, glad i sorted out such a simple thing before the exam

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you