Gilles

Kinetic energy = 1/2 * mv^(2) Then when v decreases, the kinetic energy decreases as well

Let A = the child and B = adult person. To balance this beam, the moment of the right hand side (B) must equal the moment of the left hand side (A). M(A) = M(B) m(A)*g*2.5 = m(B)*g*d(B) 62.5 = 70*d(B) => d(B) = 70/62.5 = 1.12m I hope this will help