# Urgent!!! Physics density question (my exam is 2morrow!)

I've been doing a couple of past papers now and this question seems to come up very frequently. It talks about how to calculate the density of a sample of rock. They give you a list of apparatus, and we have to draw labelled diagrams.

A measuring cyclinder = to look at how much the rocks have displaced the water in the measuring cylinder so that we can read off the volume of the rock
An electronic balance = to measure the mass of the rock

Then, they give you these really weird pieces of apparatus that I have no idea what they're used for:

- spring balance?
- pivot?
- metre rule?

"A measuring cyclinder = to look at how much the rocks have displaced the water in the measuring cylinder so that we can read off the volume of the rock
An electronic balance = to measure the mass of the rock"
You now use density= mass/volume

"- spring balance?
- pivot?
- metre rule?"

I'd hang the meter rule on the spring until it is in equilibrium. Use the tread to hang the rock on one side of the rule and some weights on the other side.
Use principle of moments to find mass of rock. i.e. clockwise moment=anticlockwise moment

I hope that's right.
why would i want to find the turning effect of the rocks.. the question only asks about density.
why would i want to find the turning effect of the rocks.. the question only asks about density.

To find the mass of the rock? That's what I think anyway... it's probably wrong- sorry.
I think you hang the metre rule of the spring balance and take the reading...This is the weight of the ruler. (W1)
Then take the metre rule and balance it on the pivot to find it's centre of mass.(P1)
Hang the rock using the thread at one side of the rule and keep moving until the rule balances. Once it's balanced, take the new reading of the pivot(P2) and the where the rock is hanging.(R1)

To find the mass of the rock use moments- (W2 = weight of rock)
W2 * (P2-R1) = W1 * (P1-P2)

Rearrange and get the WEIGHT of the rock....To get the mass just divide it by 9.81...

I'm not sure though...hope this helps.
fisfos815
I think you hang the metre rule of the spring balance and take the reading...This is the weight of the ruler. (W1)
Then take the metre rule and balance it on the pivot to find it's centre of mass.(P1)
Hang the rock using the thread at one side of the rule and keep moving until the rule balances. Once it's balanced, take the new reading of the pivot(P2) and the where the rock is hanging.(R1)

To find the mass of the rock use moments- (W2 = weight of rock)
W2 * (P2-R1) = W1 * (P1-P2)

Rearrange and get the WEIGHT of the rock....To get the mass just divide it by 9.81...

I'm not sure though...hope this helps.

Yeah, that's what I meant...but explained much more clearly!
You can work out the weight (this is force downwards due to gravity) of the rock using moments on the ruler. By then suspending the rock in water the weight goes from mg (mass times 9.81) to mg - V (mass times 9.81 - volume of rock). This means you can work out both the volume and the mass. Easy.