A bobsled starts at the top of a track as human runners sprint from rest and the

Question

A bobsled starts at the top of a track as human runners sprint from rest and then jump into the sled. Assume they reach 40 km/h from rest after covering a distance of 50 m over flat ice. a. How much work do they do on themselves and the sled which they are pushing given the fact that there are two men of combined mass 185 kg and the sled with a mass of 200 kg. (If you haven't seen bobsledding, watch YouTube to understand better what's going on.) b. After this start, the team races down the track and descends vertically by 200 m. At the finish line the sled crosses with a speed of 55 m/s. How much energy was lost to drag and friction along the way down after the men were in the sled.

Explanation / Answer

(A) From work-energy theorem,

Total work done = change in KE

{ vf = 40 km/h = 40 x 1000 m / 3600 s = 11.11 m/s}

W = (185 + 200) (11.11^2) /2 - 0

W = 23760.7 J

(B) initial total energy = PE + KE = m v^2 /2 + m g h

= (185 + 200)(11.11^2)/2 + (185+200)(9.8)(200)

= 778360.7 J

final total energy = m v^2 /2 + 0

= (185 + 200) (55^2)/2

= 582312.5 J

energy lost = 778360.7 - 582312.5

= 196048.2 J

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