Dr. Stone and Matt meet in space

Classical Mechanics Level 2

Dr. Stone and Matt are the two astronauts in the movie "Gravity". Suppose that they are in a gravity-free space pushing each other out as above. If Matt is more massive than Dr. stone, which of the following statements is $\underline{\text{NOT}}$ correct?

a) The magnitude of the impulse is the same for both Dr. Stone and Matt.

b) The sum of the magnitudes of the change in momentum for the two persons is zero.

c) The magnitude of the velocity of Matt after the push is larger than that of Dr. Stone.

all of them b) c) a)

1 solution

Anish Puthuraya
Feb 17, 2014

Answer is $(c)$ .
Since mass of Matt is more than Stone, the velocity of Matt will be less than Stone, such that $mv$ remains constant (momentum is conserved).

Brilliant movie by the way...awesome and realistic effects.

But the option c says magnitude of velocity of Matt is larger than that of Dr Stone, which you clearly stated opposite in your answer.

Rohan Rao - 7 years, 3 months ago

The question asked the option that is NOT correct.

Anish Puthuraya - 7 years, 3 months ago

Oh yea, sorry.

Rohan Rao - 7 years, 3 months ago

@Rohan Rao – Happens to the best of us, it's ok.

faraz masroor - 7 years, 3 months ago

How i option b correct?

A Former Brilliant Member - 7 years, 3 months ago

If we consider both Matt and Stone as a system, then it is clear that there is no external force on this system.

According to Newton's Law,
$\displaystyle F_{ext} = \frac{\Delta P}{\Delta t}$

Thus, in this case, since $\displaystyle F_{net} = 0$ , we get,
$\displaystyle \Delta P = 0$

This means that the total change in Momentum of the system must be zero .

Now, the total change in Momentum is clearly the sum of the change in momentums of Matt and Stone..

thus, Option (B) is correct.

Anish Puthuraya - 7 years, 3 months ago

but they are push each other.. there are movement.

Hafizh Ahsan Permana - 7 years, 2 months ago

but hey are push each other.. there are movement.

Hafizh Ahsan Permana - 7 years, 2 months ago

They say the magnitude of the change in momentum Isn't it two times the change for one of them?

A Former Brilliant Member - 7 years, 1 month ago

@A Former Brilliant Member – You are correct...

Tanya Gupta - 6 years, 11 months ago

how about a option?

Hafizh Ahsan Permana - 7 years, 2 months ago

Dr. Stone and Matt meet in space

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