# Homework Help: Probably a very novice question

1. Apr 15, 2012

### Katchit

Hello everyone! I am new to the forum and it looks an excellent forum, and busy too, which is always good! I feel a bit of a novice for asking this, but i have never been that great at Maths or Physics, so bear with me please.

1. The problem statement, all variables and given/known data

Basically, i would like to know please, if i was dealing with a density which has a minus at the end of the cubic metres, how do i go about that?

I need to work out the density of an object with the calculation...

5 x 103 kg m-3

2. Relevant equations

3. The attempt at a solution

Im guessing that when there is 5 x 103 that means 5 x 30,000?

But it is just the m-3 which then confuses me.

Many thanks and again sorry for the novice post.

2. Apr 15, 2012

### MrWarlock616

the m-3 is just the unit; it means that for every cubic meter the mass is 5000 kg. Density is Mass/Volume.
The density given is $$5000 \frac{kg}{m^3}$$

3. Apr 15, 2012

### Katchit

So (again, sorry if it's a novice question), the - doesnt have any significance?

Do i not have to minus anything from the calculation or anything like that?

Thanks again.

4. Apr 15, 2012

### MrWarlock616

No, you don't have to subtract anything. Just use 5000 as the density. The minus indicates that cubic meter is in the denominator.

5. Apr 15, 2012

### Katchit

Ah right! Thank you again for the explanation and the advice!

Much appreciated.

6. Apr 15, 2012

### Katchit

Sorry, i still seem to be struggling with this, and not sure why, it is frustrating.

It is thought that the object was around 4.6 m across. Assume the body is a sphere, with a density of

5 x 103 kg m-3

i am required to work out the mass of the object.

This is really annoying me as i cant work it out! Thanks again for any help.

7. Apr 15, 2012

### MrWarlock616

Density is $\frac{Mass}{Volume}$. So the Mass to be found is

$Mass=(Density)(Volume)$

Volume of a sphere is $\frac{4}{3}\pi r^3$.

But the object is 4.6 meters across. Hence the diameter of the sphere is 4.6 meters.
So the radius $r=2.3$

Now use the formulae I gave you.

I love using LaTex

8. Apr 15, 2012

### Katchit

Thank you!

So...

The, if the radius is 2.3, the volume of the sphere =

50.965010421636

The density is 5000kg m3

Then the mass =

254825.0521081800 kg