During hand-pumped rail car races, a speed of 25.9 km/h has been achieved by tea

Question

During hand-pumped rail car races, a speed of 25.9 km/h has been achieved by teams of four people. A car that has a mass equal to 342 kg is moving at that speed toward a river when Carlos, the chief pumper, notices that the bridge ahead is out. All four people (each with a mass of 75.0 kg) simultaneously jump backward off the car with a velocity that has a horizontal component of 4.00 m/s relative to the car. The car proceeds off the bank and falls into the water a horizontal distance of 25.8 m from the bank.

1. How Long is the time of the fall of the rail car?

2. What is the horizontal component of the velocity of the pumpers when they hit the ground?

Explanation / Answer

Part A)

First 25.9 km/hr converts to 7.19 m/s

Then by conservation of momentum...

(M + m)v = Mv + mv

(342+ 4(75))(7.19) = (342)v + (4)(75)(-4)

v = 17.0 m/s

Then d = vt

25.8 = 17(t)

t = 1.52 sec

Part B)

Horizontal velocity of the pumpers is 7.19 - 4 = 3.19 m/s

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