Page 179 - Science Coursebook

Reference

It can help if you think about units. We want to know the mass (in g). We can find this
3
3
if we multiply the density (in g/cm ) by the volume (in cm ).
M = 13.5 g/cm × 4 cm = 54.0 g
3
3
(The cm units cancel out.)
3
Another method is to use a ‘formula triangle’. The three M
quantities in the equation are put into a triangle, as shown.
Mass M is at the top.

To find the equation which has volume V as its subject, D V
cover the V in the triangle.
M
You will see that this leaves .
D
mass
volume =
density M
M
V =
D
D V

Question
1 Use the density formula triangle to find the equation with mass M as its

subject.
2 The diagram below shows the formula triangle for pressure.

F

P A

a Use the pressure formula triangle to find the equation with area A
as its subject.
b Calculate the force on an area of 3 m when a pressure of 50 N/m 2
2
acts on it.
3 Here is the equation for the moment of a force:
moment = force × distance from pivot
a Rearrange the equation to make force the subject.
b Calculate the force which must be applied at a distance of 10 m
from a pivot to give a moment of 500 N m.

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