# Physics 30 question about conservation of momentum

## Homework Statement

A space person is motionless a distance of 500m away from the safety of the spacecraft. The person has exactly 11.32min of air left and the person's mass is 103.2kg, including equipment. The person throws a phaser at a velocity of 50.2km/h away from the space craft in order to get back. I f the mass of the phaser is 5.3 kg, how many minutes of extra air does the person have when they get back to the spacecraft？(Ans: 0.28min)

I attempted to solve it , but get the answer of 0.25min.

T

## Homework Equations

Conservation and momentum.
P=mv
Ft=mv
Impulse momentum theory

## The Attempt at a Solution

① m1v1+m2v2=m1v1'+m2v2'
v1'=42.97m/min

②t=500/(836.9m/min+42.97m/min) ＝0.57min

③d=v1't=42.97m/min *0.57min=24.42m
④d(remaining)=500m-24m=475.6m
⑤t=d/v=475.6m/ (42.97m/min)=11.07min
⑥t(remaining)=11.32-11.07=0.25min

I try to solve the question, but got the wrong answer. Could anyone help me out？ Thank you so much.

haruspex
Homework Helper
Gold Member
m1v1+m2v2=m1v1'+m2v2'
What values are you using for m1 and m2 here?
②t=500/(836.9m/min+42.97m/min) ＝0.57min

verty
Homework Helper
I would leave it in km/h till the latest possible moment (unless you've been taught differently).

What values are you using for m1 and m2 here?

d=500m
t1=11.32min
m1=103.2kg
m2=5.3kg
v2'=50.2km/h=836.7m/min

I would leave it in km/h till the latest possible moment (unless you've been taught differently).
If you would like to solve it with the unit km/h , please feel free to do that.

So do you think I am on the right track of solving this？

verty
Homework Helper
It's up to you what way you want to solve it. By the way, I got a different answer: 0.31. I don't think 0.28 is correct.

PS. Your way of doing it is confusing to me so I won't comment anymore.

haruspex
Homework Helper
Gold Member
m1=103.2kg
That is the mass of the astronaut including all equipment.
I still do not see any explanation for your equations.

Also, why do you add the two speeds?

Last edited:
haruspex
Homework Helper
Gold Member
I got a different answer: 0.31. I don't think 0.28 is correct.
I get 0.28. Check you did not get -0.31. If you did, see the bold text in my post #8.

I get 0.28. Check you did not get -0.31. If you did, see the bold text in my post #8.

That is the mass of the astronaut including all equipment.
I still do not see any explanation for your equations.

Also, why do you add the two speeds?

Does it mean that the phaser is also included to have the total mass of 103.2kg？

For the reason why I added the two speeds together.....I think I had been misled by another question I just done

Thank you guys, i get the right answer.