How much work is needed for a 77-kg runner to accelerate from rest to 11.5 m/s?

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.

    m₁v₁ + m₂v₂ = (m₁ + m₂)vmomentum is conserved.the left-hand side of the equation represents the momentum in the system before the collision.the right-hand side represents the momentum in the system after the collisionm₁ and v₁ represents the mass and the velocity of the physics blockm₂ and v₂ represents the mass and velocity of the ball of puttyv represents the final velocity of the combined mass.the question states the two masses stick together. they combined mass has its own velocity with the body mass the sum of the two objects' massestheir combined momentum is (m₁ + m₂)vto find the magitude of (m₁ + m₂)v, we need to add the vectors on the left-hand side of the equation: m₁v₁ + m₂v₂.note the diagram. i have positioned the vectors in accordance with their directions.the magnitude of the resultant can be calculated using the pythagorean theorem since this is a right triangle situation (north and west are perpendicular)|(m₁ + m₂)v| =  √[ (m₁v₁)² + (m₂v₂)² ]|(m₁ + m₂)v| =  √[ (1.5kg·8.0m/s)² + (0.40kg·20m/s)² ]|(m₁ + m₂)v| =  14.4 kg m/sfor two significant figures (assuming that 20m/s is two), this rounds to 14 kg m/s