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:

- Second hand
- 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.

- How many minutes will it take to lift bricks ,three quarters up the height of the building.
- The torque on the lifting drum to lift 20 bricks weighing 180 kg, Ignoring the weight of the steel rope.
- Work done per minute in lifting the 20 bricks
- Calculate the total work done to lift the 20 bricks from the bottom to the top of the building.
- The power required on the load to lift the 20 bricks.
- 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

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