1. Give the formula for calculating moments in Physics:
Moment = Force x Perpendicular Distance
(N/m) (N) (m)
(N/cm) (N) (cm)
Y8 – TERM 2 REVISION
Explain what is meant by density and write the formula to calculate density:
Density is the mass per unit volume of a substance.
Density = Mass/Volume
(kg m-3) (kg) (m-3)
(g cm-3) (g) (cm-3)
Describe how you could find the centre of mass of this shape:
Hang the shape from a pin.
Attach a plumb line (a mass on a string).
Use a pencil to draw a straight line along the plumb line.
Remove the pin and rotate the shape.
Repeat the procedure from this new position.
The centre of mass is where the two pencil-drawn lines cross.
What is meant by centre of mass?
The point on an object through which the weight appears to act.
Explain, in terms of density, why cork floats on water:
Cork floats on water because it has a lower denisty than the water.
Explain why the bus on the right would topple:
The bus on the right would topple because the line of action of the weight does not fall within the base of the bus. The bus on the left is more stable because the line of action does fall within the base of the bus.
How could it be made more stable?
The bus would be more stable if it had a lower centre of mass or a wider base.
Explain why regular coke sinks and diet coke floats:
Regular coke has a density higher than the density of water and diet coke has a density lower than the density of water. The difference is due to the sugar in regular coke.
Calculate the mass of 60 cm3 of mercury. The density of mercury is kg m cm3 = m-3
Mass = Density x Volume
Mass = x
Mass = kg or 816 g
Find and mark the centre of mass on each of the following shapes: 1. 2. 3.
Calculate the density of ethanol cm3 of ethanol has a mass of 78.9 g.
Density = Mass/Volume
Density = 78.9/100 = g/cm3

2. Describe, in terms of energy, particle movement and particle arrangement, the difference between solids, liquids and gases:
Solids have low energy, the particles vibrate and are close together. They have a fixed shape and volume.
Liquids have more energy than solids, the particles rotate around each other and are close together. They have a fixed volumed but can change shape to fit the container they are in.
Gases have a lot of energy, the particles move at high speed and a far apart. They can change shape and volume.
Gases will always fill the container they are in and they can be compressed.
Explain what is happening at each stage:
a: Solid is increasing in temperature.
b: Solid changing into liquid without changing temperature.
c: Liquid increasing in temperature.
d: Liquid changing into gas without changing temperature.
e: Gas increasing in temperature.
Describe some factors to increase the rate of evaporation of a liquid:
Liquids will evaporated quicker when:
The temperature is higher
They have a bigger surface area exposed
There is a breeze present to carry away evaporated particles.
Describe some factors to increase the rate of condensation of a gas:
The rate of condensation will increase when:
The temperature of the gas is decreases.
High levels of water vapour are present in the air.
Describe an experiment to find the density of a regular shape:
Find the mass of the object using a balance.
Measure the length, width and breadth of the object using a ruler.
Calculate the volume - length x width x breadth
Calculate denisty  Density = Mass/Volume
Explain what is meant by mechanical advantage:
Mechanical advantage is a measure of how much a force is magnified when using a simple machine.
MA>1: Force is magnified
MA=1: Force is equal but directional advantage is gained.
MA<1: Force is reduced but the speed or distance through which the force acts is increased.