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 Homework Statement:

Suppose a 15.0 g dart is fired into a wooden block that is mounted on wheels with a total
mass of 10.0 kg. The time required for the block to travel a distance of 45.0 cm is
measured after the dart lodges into the block. This can easily be accomplished with a
pair of photocells and an electric clock. If the coefficient of friction between the
wheels and floor is 0.1435 and the measured time to come to a stop is .80 seconds, what
is the muzzle velocity of the dart? (assume no drag force on the dart before it hits the
block).
 Relevant Equations:

x= vt + 1/2 at^2
m1v1=m2v2
This is a practice problem so I know that the answer is 750 m/.s. Not totally sure what to do with the friction or if any of my listed equations are relevant here but here goes what I've tried:
.45 kg = v(0.8 sec) + 1/2(9.8 *0.1435) * (0.8)^2
.45 kg = v(0.8) + 1/2(1.4063) * 0.64
.45=v(0.8)  0.450015
0.900016= V(0.8)
V does not equal 1.12 m/s
.45 kg = v(0.8 sec) + 1/2(9.8 *0.1435) * (0.8)^2
.45 kg = v(0.8) + 1/2(1.4063) * 0.64
.45=v(0.8)  0.450015
0.900016= V(0.8)
V does not equal 1.12 m/s