We can find the mass of the kangaroo from the Law of Conservation of Momentum. Let's choose the direrection toward the bank as the positive and apply the Law of Conservation of Momentum:
mkvk,i+mlogvlog,i=mkvk,f−mlogvlog,f,here, mk is the mass of kangaroo, mlog=250kg is the mass of the log, vk,i=0sm is the initial velocity of the kangaroo, vlog,i=0sm is the initial velocity of the log, vk,f=15sm is the final velocity of the kangaroo, vlog,f=3.8sm is the final velocity of the log.
Then, we get:
mkvk,f−mlogvlog,f=0.From this equation we can find the mass of the kangaroo:
mk=vk,fmlogvlog,f=15sm250kg⋅3.8sm=63kg.Answer:
mk=63kg.
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