A child is standing on a walkway that is moving at 2 metres per second and decides to turn around and walk back to the start at 2 metres per second. Explain why the child cannot reach the start of the walkway at this speed.

Assuming that the walkway is moving in a positive direction, the child is walking in the opposite direction to it and is therefore walking in the negative direction. Both the child and the walkway have velocity, but their velocities are opposite since the direction they are moving in are directly opposite to eachother, however since they are moving at the same speed, the velocities will cancel out: -2+2=0. This result shows that in fact the child will not actually move and seem to be walking on the spot.