Work done is change in kinetic energy. That is work done = K.E(final) -K.E(initial)
K.E = (mass*velocity*velocity)/ 2
Given the runner accelerates from rest hence initial kinetic energy is 0 since initial velocity is 0.
Final kinetic energy = (77kg*11.5m/s*11.5m/s)/ 2 = 5091.625 joules
Hence the work done by the 77kg runner to accelerate from rest to 11.5 m/s is 5.0916 Kilo joules.
