An 80-Mg railroad engine A coasting at 6.5km/h strikes a 20-Mgflatcar C carrying

Question

An 80-Mg railroad engine A coasting at 6.5km/h strikes a 20-Mgflatcar C carrying a 30-Mg load B which can slide along the floorof the car (k=0.25). Knowing that theflatcar was at rest with its brakes released and that itautomatically coupled with the engine upon impact, determine thevelocity of the flatcar (a) immediately after impact. I can do part (b) For part (a) I use mAvA +kmBgt = (mA+mB + mc)v , but I am stuck there Please give me a hint. An 80-Mg railroad engine A coasting at 6.5km/h strikes a 20-Mgflatcar C carrying a 30-Mg load B which can slide along the floorof the car (k=0.25). Knowing that theflatcar was at rest with its brakes released and that itautomatically coupled with the engine upon impact, determine thevelocity of the flatcar (a) immediately after impact. I can do part (b) For part (a) I use mAvA +kmBgt = (mA+mB + mc)v , but I am stuck there Please give me a hint.

Explanation / Answer

F = mkN

First consider the load. Wehave F = mkN =0.20N. Since coupling occursin Dt    0: F Dt   0

0 +0 = mL ( vL )1

( vL )1 =0

We apply the principle of conservation oflinear momentum to the entire system.

mE vO =(mE + mC) v1 +mL ( vL )1

so 80*6.5=(80+20)v1

Therefore v1=5.2km/hr

b)Velocity after load has stoppedmoving:

  mE vO =(mE + mC + mL) v2

80*6.5=(80+30+20)v2

v2=4km/hr

        

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