Energy lost to heat

Question

In an attempt for a physics teacher to explain the conservation of energy to a student, the teacher goes down a 6.0 m high sliding board. At the bottom, he only travels 7.0 m/s. If the mass of the teacher if 70 kg, how much energy is missing?

Answer 1

At the bottom, the kinetic energy of the teacher should be

 

K = mgh = (70 kg)(9.8 m/s²)(6.0 m) = 4116 J

 

But he has a final speed v of 7.0 m/s, so

 

K' = (1/2)mv² = (1/2)(70.0)(7.0)² J = 1715 J

 

He lost K - K'.

 

K - K' = (4116 - 1715) J = 2401 J.