Momentum = Mass x Velocity
Put the values where they belong and solve for Velocity.
In this case, since Mass is being multiplied by Velocity, to solve for be Velocity you would divide both sides by Velocity. The velocity will equal the momentum divided by the mass.
The formula is Ke = 1/2 m v^2
The two of them together have a Ke of mv^2. So you either increase m or v. That's what makes the problem difficult. He can do D or B. We have to choose.
A is no solution. The Ke goes down because Paul loses Ivan's mass.
C is out of the question 3 meters/sec is a big reduction from 5 m/s. So now what do we do about B and D?
The question is what does the third person add. The tandoms I've peddled only allow for 1 or 2 people to add to the motion. So the third person only adds mass. He does not have a v that he is contributing to. To say that he is going 5m/s is true, but he's not contributing anything to that motion.
I pick B, but it is one of those questions that the correctness of it is in the head of the proposer. Be prepared to get it wrong. Argue the point politely if you agree with me, but back off as soon as you have presented your case.
B <<<<====== answer.
Answer:
(we need the mass of the astronaut A)
Explanation:
We can solve this by using the conservation law of the linear momentum P. First we need to represent every mass as a particle. Also we can simplify this system of particles by considering only the astronaut A with an initial speed 
of 0 m/s and a mass 
and the IMAX camera with an initial speed 
of 7.5 m/s and a mass 
of 15.0 kg.
The law of conservation says that the linear momentum P (the sum of the products between all masses and its speeds) is constant in time. The equation for this is:
By the law of conservation we know that 
For 
(final linear momentum) we need to treat the collision as a plastic one (the two particles stick together after the encounter).
So:
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