Energy can be neither created nor destroyed. What is energy?. ability to do work. what is work ? work = force x displacement. Consider a body of mass 'm' moving at a uniform velocity 'v'.work done here is zero(no acceleration).So energy is zero. DONT SAY KINETIC ENERGY is there.please check the derivation of kinetic energy. Since work = F*d and F = m*a then work = m*a*d. From kinematics we know that d = 1/2 * a*t2 and t = v/a. so d = (1/2* a *v2)/ a2 = (1/2* v2)/ a so work = m* a * (1/2* v2)/ a = 1/2 * m * v2. in this derivation please note that t = v/a, is only applicable when initial velocity is zero. here initial and final velocity are v ,and kinetic energy will be zero. now come back to the point.what will happen ,if this body hits another body at rest.?(mass is irrelevant-body at rest have no energy stored,isn't it?). After collision, both of the bodies have energy(since there is an acceleration). from where does this energy comes from.?