Which of the following materials does not serve as a safety device in electrical circuits?

A body of mass m moving round a circle of radius r with a uniform speed v experiences a centripetal force F. The work done by the centripetal force on it is

The inner diameter of a small test tube can be measured accurately using a

An aeroplane lands on a runway at a speed of $180kmh^{-1}$ and is brought to a stop uniformly in $30$ seconds. What distance does it cover on the runway before coming to rest?