Question

You just realized that your analogue wrist watch is always 5 seconds slower and hence always behind the real time.

Calculate the angular speed of your:

1. Second hand
2. Hour hand

A 30 kg skip attached to a steel rope on a crane is used to hoist bricks from the ground to the top of a construction site. The steel rope is wound onto a lifting drum with a radius of 20 cm and rotational frequency of 15 revolutions per minute. The lifting drum is situated on the top floor which is 230 m high.

1. How many minutes will it take to lift bricks ,three quarters up the height of the building.
2. The torque on the lifting drum to lift 20 bricks weighing 180 kg, Ignoring the weight of the steel rope.
3. Work done per minute in lifting the 20 bricks
4. Calculate the total work done to lift the 20 bricks from the bottom to the top of the building.
5. The power required on the load to lift the 20 bricks.
6. Calculate and comment on the efficiency of the lifting generator ,if the electric motor power of 1MW is needed to drive the loaded skip

Answer 1

a) seconds hand of a clock sweeps one rotation in 60 sec

angular speed = angle swept / time taken

                 = 2pi/60

                 = pi/30 rad/s

                = 0.1047 rad/s

b) hour hand of a clock sweeps one rotation in 12*60*60 sec

angular speed = angle swept / time taken

                 = 2pi/12*60*60 =

                 = 7.27*10^-5 rad/s