- #1

EthanB

- 44

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A train with known mass collides with a stationary train and the two link together. If 27% of the initial kinetic energy is dissipated, what is the mass of the stationary train? We are given no velocities.

M1 = mass of moving train (known)

M2 = mass of stationary train

Vi = initial velocity of moving train

Vf = final velocity of the 2-train system

Ki = initial kinetic energy

Kf = final kinetic energy

Wnc = work done by non-conservative forces

Pi = initial momentum

Pf = final momentum

**Using conservation of momentum:**

Pi = Pf

(M1)(Vi) = (M1+M2)(Vf)

**Using conservation of energy:**

Ki + Wnc = Kf

(1/2)(M1)(Vi)^2 - (.27)(1/2)(M1)(Vi)^2 = (1/2)(M1+M2)(Vf)^2

(.73)(M1)(Vi)^2 = (M1+M2)(Vf)^2

...2 equations, 3 unknowns. What am I forgetting?