How many bullets does an astronaut need to fire to return to the space shuttle?

  • Thread starter Thread starter lethal-octo
  • Start date Start date
  • Tags Tags
    Gun Momentum
Click For Summary

Homework Help Overview

The problem involves an astronaut drifting away from a space shuttle and firing bullets from a gun to return. The astronaut's mass, the mass of the gun, and the velocity of the bullets are provided, leading to a question about the minimum number of bullets needed to reverse the drift.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the application of momentum conservation principles, questioning whether the velocities are measured relative to the shuttle. There are attempts to calculate the number of bullets needed based on the given data.

Discussion Status

Some participants have provided calculations and raised questions about the assumptions made in the problem, such as the inclusion of bullet mass in the initial calculations and the effects of gas momentum on recoil. There is recognition of the complexity involved in accurately computing the recoil of the pistol.

Contextual Notes

Participants note the lack of information regarding the total initial mass of bullets and the potential impact of this on the calculations. The problem is derived from a practice exam, which may impose certain constraints on the assumptions that can be made.

lethal-octo
Messages
2
Reaction score
0

Homework Statement


An astronaut is drifting away from the space shuttle at a velocity of 1.1m/s. He fires his gun's 30g bullets to return to his ship. The mass of the astronaut and his spacesuit is 123kg, his gun has a mass of 12kg and the bullets leave the gun at a velocity at 250 m/s.
What is the approximate minimum number of bullets that he will need to fire to begin drifting back to the ship?

Homework Equations


mv=mv?

3. The attempt at a solution
mv=mv*(number of bullets)
(135)(1.1)=(0.03)(250)*n
148.5=7.5n
n=19.8
Therefore the astronaut will need to fire 20 bullets to start drifting backwards. Is this right?
 
Last edited:
Physics news on Phys.org
lethal-octo said:
mv=mv*(number of bullets)
148.5=7.5n

Are these velocities measured relative to the space shuttle?
 
Mister T said:
Are these velocities measured relative to the space shuttle?
I'm not really sure as this is a question from a practice exam, so I just plugged the variables in.
 
lethal-octo said:
Therefore the astronaut will need to fire 20 bullets to start drifting backwards. Is this right?
Yes. To be completely accurate you would need to take into account that the initial mass includes the bullets, and this reduces with each shot fired. But you are not told the total initial mass of bullets, and the effect would be tiny anyway.
 
Another point that is missed in idealized textbook problems is that the recoil of the pistol is equal and opposite not only to the momentum of the bullet, but to the momentum of the bullet plus the momentum of the expanding gases that propelled the bullet (at least the component in the same direction). This complicates accurate computation of pistol recoil considerably and renders simple formulas based on bullet mass and velocity a significant underestimate.

The attached photo shows stills from a high speed video taken in our lab to try and better quantify the contribution of the gases. The frames in view do a pretty good job showing the forward motion of the gases. Later frames (not shown) allow quantifying the subsequent rearward motion of the pistol. Bullet velocity was measured with an optical chronograph.
Sequence.png
 
haruspex said:
Yes. To be completely accurate you would need to take into account that the initial mass includes the bullets, and this reduces with each shot fired. But you are not told the total initial mass of bullets, and the effect would be tiny anyway.
Presumably, the mass of the gun (12 kg) includes the mass of the bullets with which it is loaded.
 
SteamKing said:
Presumably, the mass of the gun (12 kg) includes the mass of the bullets with which it is loaded.
Perhaps, but that is not at all clear.
 
If the speed of the astronaut were a significant fraction of the muzzle speed, you'd have to take into account that the velocity of the bullets with respect to the gun is not the same as the velocity of the bullets with respect to the shuttle. In this case it's insignificant.
 

Similar threads

How Does Firing a Machine Gun Affect the Movement of a Flatcar?
  • · Replies 2 ·
Replies
2
Views
3K
Astronaut and Wrench Momentum Problem
  • · Replies 2 ·
Replies
2
Views
15K