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A 2 kg rock is released from rest at a height of 20 m. Ignore air resistance and determine the kinetic energy, gravitational potential energy, and total mechanical energy at each of the following heights: 20, 10, and 0 m.

Answer :

At 20 m kinetic energy is 0 J, Potential energy is 392.4 J and total energy is 392.4 J

At 10 m kinetic energy is 196 J, Potential energy is 196.2 J and total energy is 392.4 J

At 0 m kinetic energy is 392.4 J, Potential energy is 0 J and total energy is 392.4 J

Explanation:

Kinetic energy = 0.5 x Mass x Velocity²

KE = 0.5mv²

Potential energy = Mass x Acceleration due to gravity x Height

PE=mgh

Here m = 2 kg

          g = 9.81 m/s²

Substituting

         KE = 0.5 x 2 x v² = v²

         PE= 2 x 9.81 x h = 19.62h

At height 20 m:-

         v = 0 m/s

          h = 20 m

Substituting

         KE = v² = 0² = 0J

         PE= 19.62 x 20 = 392.4 J        

          Total energy = 0 + 392.4 = 392.4 J

          At 20 m kinetic energy is 0 J, Potential energy is 392.4 J and total energy is 392.4 J

At height 10 m:-

  We have equation of motion v²=u²+2as

                v = ?

                u = 0 m/s

                s = 20 - 10 = 10 m

                a = 9.81 m/s²

                v²=0²+2 x 9.81 x 10    

                v = 14 m/s

         v = 14 m/s

          h = 10 m

Substituting

         KE = v² = 14² = 196 J

         PE= 19.62 x 10 = 196.2 J        

          Total energy = 196 + 196.2 = 392.4 J

          At 10 m kinetic energy is 196 J, Potential energy is 196.2 J and total energy is 392.4 J

At height 0 m:-

  We have equation of motion v²=u²+2as

                v = ?

                u = 0 m/s

                s = 20 - 0 = 20 m

                a = 9.81 m/s²

                v²=0²+2 x 9.81 x 20    

                v = 19.81 m/s

         v = 19.81 m/s

          h = 0 m

Substituting

         KE = v² = 19.81² = 392.4 J

         PE= 19.62 x 0 = 0 J        

          Total energy = 392.4 + 0 = 392.4 J

          At 0 m kinetic energy is 392.4 J, Potential energy is 0 J and total energy is 392.4 J

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