# Talk

## I need physics help (maybe early GCSE) - mass, density etc

(11 Posts)
winchester1 Sun 06-Sep-15 08:33:10

I'm studying this in a foreign language and can't work this out from the text.

Where F=ma and density=m/V are the m's in both equations the same or in the second equation is it F from the first so 'weight' as it would normally be called.

So if I'm told something weighs 2kg, and is 3m3 do I just do Density = 2/3 or do I need to do,
Density = F/(V*a) = 2/(3*9.82)= 0,07

Not sure I'm making sense but hopefully someone gets what I'm talking about.

timeforabrewnow Sun 06-Sep-15 08:37:44

m/s = metres per second

m/V = mass divided by volume

F = force

Not sure about the density question. Someone smarter can explain better!

AnyoneButAndre Sun 06-Sep-15 08:46:45

Oh bugger. I came in confident that I could sort it out and you've got me doubting myself now. My A level physics was a very long time ago. I'll hang around here and wait for someone who knows, otherwise I'd be forced to ask DH and nobody wants that.

lexiferluther Sun 06-Sep-15 09:25:19

If you have mass (m) and volume (V) then density is simply m/V - so in your example 2/3 kg/m^3. Using the units can help you check you've used the right values.

F is the force exerted on or by an object - in that equation mass is again represented by m. So force is mass (m) x acceleration (a). If you were trying to calculate density and had mass and volume you would not need to use this equation.

winchester1 Sun 06-Sep-15 09:51:00

I'm reading F=ma as if a person says they weigh 50kg (on earth) they have a mass of 50/9.82 so 5 ish - is that right?

But does density = weight / volume, where weight is simply what you see on the scales and I can just forget the whole F=ma stuff and assume that when an object is stated as 2kg they actually mean it has a mass of 2kg.
Hope so as that makes it really simple to do the homework questions.

lexiferluther Sun 06-Sep-15 11:33:11

Density is mass/volume - mass is a measure of how much matter there is in an object independent of gravity (expressed using kg as its SI unit) whereas weight is dependent on gravity and is expressed in Newtons (kg m / s^2). Your weight is your mass (e.g. 60kg) multiplied by the acceleration due to gravity (usually given as about 9.8 m/s^2).

Mass is what you see on the scales - just don't call it weight or it could get confusing.

NotDavidTennant Sun 06-Sep-15 11:47:13

One way to remember it is:

Astronauts are weightless but not massless.

Mass is the amount of 'stuff' in something, and doesn't change unless you add or remove some of that 'stuff'.

Weight is the force acting on mass due to gravity, and can change depending on the local gravity.

Density is the amount of 'stuff' in a given volume, so is mass/volume. Weight does not tell you the amount of 'stuff' so is not relevant to density.

winchester1 Sun 06-Sep-15 14:43:28

Ok I get it. So a person says the weigh 60kgs they mean mass and that remains the same on earth, moon etc but their force changes.

lexiferluther Sun 06-Sep-15 15:53:21

Yup - same mass no matter where they are and when people talk about the weight of something they, unless a physicist, generally mean mass.

cdtaylornats Mon 07-Sep-15 00:07:06

The equation
Density = F/(V*a)

if you replace F with ma you get

Density = ma/Va

cancel out the a from both parts and you are back to

Density = m/V

winchester1 Mon 07-Sep-15 09:11:38

Thanks for the help with this, the teacher gave me an improtu test today so it was good to understand this.

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