A block moves with uniform circular motion because a cord tied to the block is anchored at the centre of a circle. Is the power of the force exerted on the block by the cord positive, negative or zero?

One often comes across the following kind of statement concerning circular motion. A particle moving uniformly along a circle experiences a force directed towards the centre and an equal and opposite force directed away from the centre. The two forces together keep the particle in equilibrium. Explain, what is wrong with this statement?

A block of mass 15 kg is placed on a long trolley. The coefficient of static friction between the block and the trolley is 0.18. The trolley accelerates from rest with 0.5 m s-^2 for 20 s and then moves with uniform velocity. Discuss the motion of the block as viewed by (a) a stationary observer on the ground, (b) an observer moving with the trolley.