Using a pulley block, a weight of 180 N is raised with a force 15 N. Then the mechanical advantage is

81) A screw jack has pitch of the screw p, length of lever l, then velocity ratio is given by

82) In which case the potential energy is decreases?

83) A pulley is used to lift a mass with a force of 900 N to height of 10 meters in a 90 seconds. Then the power required is

84) Which one of the following is useful friction?