WJEC Physics for AS: Student Bk

87 1. (a) (i) Define mean speed. [1] (ii) A motorcyclist enters an 80 km/h restricted zone on a motorway. There is an average speed check on this 10 km section. After 6 km  the cyclist realises that he has been doing 90 km/h all the time. What must his top speed be for the remainder of the restricted section so that the mean speed is less than 80 km/h ? [3] (b) The combined mass of the motorcycle and rider is 350 kg . The graph shows the variation of velocity, v , with time, t , over a 10 s period. (i) Describe, without calculation, how the resultant force acting on the motorcycle varies over this 10 second interval. [3] (iI) Determine the resultant force acting on the motorcycle at 2.0 s . [mass of motorcycle and rider = 350 kg ]. [3] (c) (i) A force F acts on a body moving with a velocity v . F and v are in the same direction. Starting from the definition of power, show that the power, P , is given by P = Fv . [3] (ii) When the motorcycle in part (b) is travelling at the steady velocity shown in the last part of the graph, the useful power output by the engine is 2.45 kW . Calculate the driving force required to maintain this velocity. [1] (iii) Assuming that the resistive force is proportional to the velocity, calculate the power output of the engine at 2.0 s . [3] (d) At a later time the motorcycle in part (b) brakes to a halt in a distance of 25 m . Calculate the mean braking force. [3] Exam practice questions t / s 20 15 10 5 0 0 2 4 6 8 10 v / m s –1