Question

Two workmen are employed on a building project, as shown in figure. (i) Workman 1 drops a hammer, which falls to the ground. The hammer has a mass of 2.0 kg, and is dropped from a height of 4.8 m above the ground. (a) Calculate the change in gravitational potential energy of the hammer when it is dropped. (b) Describe the energy changes from the time the hammer leaves the hand of workman 1 until it is at rest on the ground. (ii) Workman 2 picks up the hammer and takes it back, up the ladder to workman 1. He climbs the first 3.0 m in 5.0 s. His total weight, including the hammer, is 520 N. Calculate the useful power which his legs are producing.

  • Work and energy CBSE Class 9
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Answer

(i) (a) Mass, m = 2.0 kg, height, h = 4.8 m

Change in gravitational potential energy

= mgh

=2 ร— 10 ร— 4.8 = 96 J

(b) Potential energy โŸถ Kinetic energy โŸถ Heat and/or sound energy

(ii) Weight, mg = 520 N, height, h = 3.0 m, Time, t = 5.0 s

useful power = {Work\ done \overย Time\ taken}

= {(๐‘š๐‘”)โ„Ž \over ๐‘ก} = {520 ร—3\over 5}

= 312 W

 

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