WJEC Physics for AS Level Student Book 2nd Edition

14 WJEC Physics for AS Level Example A light aircraft flies from Aberystwyth to Bangor and then to Flint in a time of two hours. Using the data on the previous page, calculate (a) the mean speed and (b) the mean velocity. Answer (a) Distance travelled = AB + BF = 91 + 65 = 156 km \ Mean speed = 156 km 2 h = 78 km h – 1 (b) Displacement AF ~ 112 km at 35.5° E of N \ Mean velocity = 112 km 2 h = 56 km h – 1 at 35.5° E of N . [Note: direction!] (c) Lists of scalar and vector quantities These lists include most of the scalar and vector quantities that you’ll encounter in AS/A level physics. The ones in italics are only in the full A level course. Scalars – density, mass, volume, area, distance, length, speed, work, energy (all forms), power, time, resistance, temperature, potential (or pd or voltage), electric charge, capacitance , activity , pressure, refractive index. Vectors – displacement, velocity, acceleration, force, momentum, electric field strength , magnetic field strength (or magnetic flux density ), gravitational field strength . (d) Adding more than two vectors We have seen how to add two vectors using either the parallelogram method (see Fig. 1.1.4) or the nose-to-tail method (Fig. 1.1.6). Fig. 1.1.7 shows how we can extend the latter way of addition to more than two vectors. An alternative method is given in Section 1.1.4. 1.1.4 Working with vectors (a) Subtracting vectors To calculate an acceleration we need first to find a change in velocity, Δ v . If the first velocity is v 1 and the second is v 2 , then Δ v = v 2 – v 1 . We know how to subtract scalars. How does it work for vectors? For numbers we know that instead of writing 53 – 45 we get the same answer ( 8 ) if we write (– 45 ) + 53 . Fig. 1.1.7 Adding more than two vectors b a c a+ b+ c Maths tip If you look at Figs 1.1.4 and 1.1.6 we seem to have used two different ways of adding vectors. In fact they give the same answer. Here is 1.1.6 drawn like 1.1.4: 91 km θ 65 km