Fault in calculator??

sudzs

Hi, first time posting here so not sure if I'm in the right place to post this....

Anyway, starting an OU course soon so have been brushing up on my leaving cert maths after many years! So from an OU maths booklet, I was using my new scientific calculator to work out the volume of a sphere (the Earth) No matter how I did it, I kept getting 1.1 x10 to the 24. I did the exact same calculations, same input order etc on the scientific calculator on my laptop and on another calculator both of which gave the correct answer of 1.1 x 10 to the 21.

I was struggling to re-learn how to use the calculator so too worn out to try any other calculations to see what relults I'd get. But do you think the calculator has a fault? Or am I doing some little thing wrong. It's a cheapo Sharp EL531x btw.

TheBody

It's very hard to say. Does the "faulty" calculator seem ok for other calculations? It may be that the other calculators are able to handle larger numbers and are therefore more accurate.

Yakuza

What units are you using? The fact that you're finding a difference of the order of 1000 suggests one might be in litres (dm³) and the other in m³.

Wiki refers to the volume of the earth as 1,083,210,000,000 km³ (~1.1 * 10^12 km³).
Now 1 km³ = 10^9 m³, so the volume of the earth in m³ is 1.1 * 10^21
But 1m³ = 1000 dm³ (1 dm³ = 1 litre), so the volume of the earth in dm³ is 1.1*10^24.

I would find it hard to believe a calculator would get something like that wrong, to be honest.

sudzs

Yakuza wrote: »

What units are you using? The fact that you're finding a difference of the order of 1000 suggests one might be in litres (dm³) and the other in m³.

Wiki refers to the volume of the earth as 1,083,210,000,000 km³ (~1.1 * 10^12 km³).
Now 1 km³ = 10^9 m³, so the volume of the earth in m³ is 1.1 * 10^21
But 1m³ = 1000 dm³ (1 dm³ = 1 litre), so the volume of the earth in dm³ is 1.1*10^24.

I would find it hard to believe a calculator would get something like that wrong, to be honest.

The unit is metres, the answer being metres cubed. The OU booklet gives the answer as 1.1 x 10^21 m³ But I'm just entering figures into the calculator, not units. and when I used the calculator on my laptop, inputting exactly the same figures in the same way, I got the right answer.

btw, how do you type the index as a small number... I copy and pasted yours there! ...so much to learn

henbane

The calculator is probably using a slightly different order of precedence, use more brackets.

Yakuza

Alt and 253 (on the numeric keypad) makes the ², Alt and 252 makes the ³

The fact that you're out by 1000 could be caused if you're entering a radius that 10 times too large (when you cube it, your answer will be 1000 times out).

Are you using the formula 4/3 * pi * r³ ? (Ignoring the fact the earth is squished sphere )
Are you converting the km to m:
Google gives the radius of the earth as 6378.1 km = 6378100m

4/3 * pi * 6378100³ is 1086832411937628837875.0037971403 (according to the Windows Calculator) or, as you say, ~1.08 * 10^21.
I'll whack those figures into my calculators at home (I have both an EL-531 and an EL-506 (having bought the second when I realised my EL-531 wasn't allowed in my exams:mad:)) but I suspect they'll work

sudzs

Alt and 253 (on the numeric keypad) makes the ², Alt and 252 makes the ³

Great! Thanks for that.

Are you using the formula 4/3 * pi * r³ ?
Are you converting the km to m:

Yes, using that formula and it's already in m. Radius given is 6.4 x 10^6 m (dammit, I can't get the little numbers to appear!!!)

I've tried it again on the Sharp and still getting the power 24 instead of 21 and still getting the right answer on the Windows calculator. :rolleyes:

Thanks again for the help.

Brussels Sprout

Can you write down exactly what buttons you are pressing in order please?

sudzs

Brussels Sprout wrote: »

Can you write down exactly what buttons you are pressing in order please?

Tried it again and managed to get the right answer. I was doing it wrong... of course! :rolleyes: At the end, (6.4 x 10^6 m)³ I was using exp 3 instead of the x³ key. duh! I need a crash course in how to use a scientific calculator! I gettit now though!

Thanks all!

Yakuza

Glad you got it sorted - it worked on both my calculators here

sudzs

Yakuza wrote: »

Glad you got it sorted - it worked on both my calculators here

Yes, but felt like a eejit for doing the wrong thing over and over, Homer style!!! And yet did it right on another calculator! :rolleyes:



btw, how do I type the symbol for pi? I found something line, Alt 227 but it gave me this instead... Ò and a transfer to my google sign in page at the same time!

Yakuza

I don't think there is one - use "charmap" if you're running windows and you'll be able to pick it from on of the unicode fonts that support it. I don't think they look that well - Π and π - so I didn't bother putting them in my post

TheBody

Do you know any latex? You get the [latex]\pi[/latex] symbol by wrapping latex tags around \pi

mallen

Came across this thread looking for anyone with a faulty calculator. ONE DAY before the LC Maths paper 2 found out my daughters calculator had the Cos and Tan buttons reversed and therefore they were giving the wrong answers. Son has the exact same calculator with the correct buttons. Also a Sharp brand, model EL-W531 Writeview.

Pherekydes

Have one here. Works fine?

MathsManiac

OP: Noting what you finally discovered your error to be, do you now know what the "EXP" key is for?

It's basically a shortcut for "by ten to the power of"

So you can either press: 6.4 [X] 10 [^] 6 or 6.4 [EXP] 6.

When you're using scientific notation, the [EXP] key is very handy. First, it's a single keystroke instead of four keystrokes. Second, it has super-high precedence, so 6.4[EXP]6 will never need brackets around it to be treated as a single number.

mallen

Yes, I am sure there are many that work fine, including my son's, but it is worrying that my daughter had been using hers for a year before a teacher checked it in a group grind class and found out it was faulty. Others of the same type could also be faulty and causing students to lose marks in exams.
This model was the one required by her school.

locum-motion

mallen wrote: »

Yes, I am sure there are many that work fine, including my son's, but it is worrying that my daughter had been using hers for a year before a teacher checked it in a group grind class and found out it was faulty. Others of the same type could also be faulty and causing students to lose marks in exams.
This model was the one required by her school.

What's even more worrying is the fact that your daughter went the whole way through her two years of LC prep with neither her nor her teacher copping on that she never once got the right answer in any problem that involved Cosines or Tangents.

mallen

I agree. Daughter is not the best at maths (hence the grinds). I would have thought the teacher would have noticed but she didn't.