8 10 Concrete is a material used in buildings due to its high compressive strength. (a) A concrete post can be checked for internal cracks using a pulse-echo technique. A transducer that transmits and receives ultrasound pulses is positioned against the side of the post as shown. transducer 0.2 m A pulse hits a crack and is reflected and is then detected by the transducer. Deduce whether a crack at a depth of 0.2 m can be detected. time between pulses = 160 µs speed of sound in concrete = 3200 m s−1 (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (b) Another concrete post is reinforced with steel rods, to increase its tensile strength. A steel rod is under a tensile load of 130 N and extends by 4.0 × 10−4 m. The steel has not reached its elastic limit. Calculate the elastic strain energy in the steel rod. (2) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. Elastic strain energy = ....................................................... (Total for Question 10 = 5 marks)