2 Using Formulae
It is not only areas which can be worked out with simple formulae.
eg.1. Peter delivers newspapers on six days each week. How much will he be paid in a week if he is paid £2 per day
eg.2. What is the mass of a piece of copper which has a density of 8.96 and a volume of 24cm3?
eg1. P=rd eg2. M = dv
.......P=2x6 M = 8.96 x 24
...... P=£12 M = 215.04g
2A Work out the following using the formulae given. It is not necessary to know
what the formulae are for. The algebra tells us what to do, that is what makes
algebra so useful and powerful.
1. F=ma when m=12, a=9 7. a=bh when b=18, h=17
2. d=st when s=5, t=6 8. W=fs when f=7, s=15
3. C=pd when d=10 9. T=Fr when F=8.5, r=2
4. E=mg when m=20, g=9.8 10. M=mv when m=23, v=3.5
5. D=px when p=25, x=1.6 11. e=ul when u=2, l=0.003
6. v=at when a=5.5, t=15 12. v=mp when m=0.41, p=761
Always write down the formula first, then substitute the numbers, then do the calculation.
Some formulae have three variables. Eg. Volume of cuboid, V=length x width x height
Eg. Find the volume of a cuboid with length=10cm, width=7cm, height=5cm
Write down the formula V = lwh No symbols between the letters so ...
Substitute the numbers V = 10x7x5 numbers need to be multiplied
Do the calculation V =350cm³
2B Use the formula v=lwh or E=mgh, as appropiate in the following.
1. Work out the volume of a cuboid, length 12cm, width 9cm, height 6cm.
2. Work out the volume of a cd box, length 14cm, width 1cm, height 12cm.
3. Work out the volume of a cereal packet, l=19cm, w=8cm, h=28cm.
4. Work out the potential energy of a mass of 2kg which is 5m above the ground. (g=9.8 m/s)
5. Work out the potential energy of a mass of 5.5kg which is 12m above the ground. (g=9.8 m/s)