How much force is required to stop a space ship?

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Homework Help Overview

The discussion revolves around calculating the force required to stop a spaceship that is accelerating at 1000 m/s². Participants explore the implications of Newton's laws and the relationship between force, mass, and acceleration in this context.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants attempt to apply Newton's second law to determine the stopping force, questioning the relationship between acceleration and force. Some express confusion regarding the definitions of force and velocity, while others highlight the need for additional information, such as time or current speed, to solve the problem accurately.

Discussion Status

The discussion is ongoing, with various interpretations being explored. Some participants have offered guidance on checking units and clarifying the definitions of terms involved. There is recognition of the need for assumptions regarding time and current speed to arrive at a more concrete answer.

Contextual Notes

Participants note that the problem lacks specific details, such as the time required to stop the spaceship and the implications of stopping in space. There are also concerns about the feasibility of the given acceleration value in practical scenarios.

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Homework Statement


A spaceship is accelerating at ##1000# m/(sec per sec). How much force is required from the backthrusters to completely stop the spaceship?


Homework Equations





The Attempt at a Solution


Let's assume the spaceship has mass ##m## and no mass is lost when firing the backthrusters.
Using Newtons second, we have
$$
\sum F = 1000m.
$$
The only force would be the velocity from the backthrusters correct?
So we have
$$
-v_0x = 1000m.
$$
Is this correct? How do I find the force needed to stop the ship?
 
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Dustinsfl said:

Homework Statement


A spaceship is accelerating at ##1000# m/(sec per sec). How much force is required from the backthrusters to completely stop the spaceship?


Homework Equations





The Attempt at a Solution


Let's assume the spaceship has mass ##m## and no mass is lost when firing the backthrusters.
Using Newtons second, we have
$$
\sum F = 1000m.
$$
The only force would be the velocity from the backthrusters correct?
So we have
$$
-v_0x = 1000m.
$$
Is this correct? How do I find the force needed to stop the ship?

I'd like to point out that an object in motion will remain in motion unless a constant acceleration is applied to it. That being said,

The only force would be the velocity from the backthrusters correct?

-v_0x = 1000m.

No, velocity would not be the force you're looking. Here's two tips:
1. Check your units, as they do help verify you're doing something right if your calculations look funny
2. Newton's Laws of Motion.
 
majormaaz said:
I'd like to point out that an object in motion will remain in motion unless a constant acceleration is applied to it. That being said,



No, velocity would not be the force you're looking. Here's two tips:
1. Check your units, as they do help verify you're doing something right if your calculations look funny
2. Newton's Laws of Motion.

So the force on the LHS needs to be acceleration then?
 
Dustinsfl said:
So the force on the LHS needs to be acceleration then?

Force has units of kg * m/s2. Acceleration has units of m/s2.
You're almost there. :)
 
The question doesn't make complete sense since if it is accelerating then it must be moving as well. And therefore the stopping force would need to be slightly larger than the force causing it to accelerate in which case you would also need to know the time required to bring it to a an instantaneous stop.

And 1000 m/s^2 is 100g's which is a huge acceleration. I doubt any practical rocket ship could withstand this amount of acceleration. It would just crumple or tear apart.

Otherwise I thought the mass assumption by the OP was very insightful.
 
paisiello2 said:
The question doesn't make complete sense since if it is accelerating then it must be moving as well. And therefore the stopping force would need to be slightly larger than the force causing it to accelerate in which case you would also need to know the time required to bring it to a an instantaneous stop.

And 1000 m/s^2 is 100g's which is a huge acceleration. I doubt any practical rocket ship could withstand this amount of acceleration. It would just crumple or tear apart.

Otherwise I thought the mass assumption by the OP was very insightful.

What other information do we need to assume to solve the problem then?
 
Dustinsfl said:
What other information do we need to assume to solve the problem then?
The question fails to specify the time span in which you are required to competely stop the ship. (Whatever 'stop' means in space, anyway.)

It would seem better to have been asked for the force needed to exactly cancel the gravitational acceleration.

Do you have the answer yet?
 
NascentOxygen said:
The question fails to specify the time span in which you are required to competely stop the ship. (Whatever 'stop' means in space, anyway.)

It would seem better to have been asked for the force needed <i>to exactly cancel</i> the acceleration.

Do you have the answer yet?

No I don't have an answer. So we can make our own assumptions.
 
Dustinsfl said:
No I don't have an answer. So we can make our own assumptions.

:smile: No, I don't think you should decide on an arbitrary time to halt the ship.

For starters, you are not told its current speed...

Probably the best you can do is to state that any force exceeding xxx Newtons will eventually bring the ship's speed to zero if it remains in the grip of that field. So, find xxx.
 
  • #10
NascentOxygen said:
:smile: No, I don't think you should decide on an arbitrary time to halt the ship.

For starters, you are not told its current speed...

Probably the best you can do is to state that any force exceeding xxx Newtons will eventually bring the ship's speed to zero if it remains in the grip of that field. So, find xxx.

How do I find xxx? This problem comes from the Nuclear Propulsion Study Guide for the Navy.
 
  • #11
I think you kind of stated the answer already in your first try. Your trying to find the force Fstop to stop the ship.

Fstop = ?
 
  • #12
paisiello2 said:
I think you kind of stated the answer already in your first try. Your trying to find the force Fstop to stop the ship.

Fstop = ?

So that is just a 1000 then?
 
  • #13
No, not quite, that is the acceleration remember. Again you actually already stated the answer in the OP.
 
  • #14
paisiello2 said:
No, not quite, that is the acceleration remember. Again you actually already stated the answer in the OP.

1000m
 
  • #15
You got it.

My suggestion in any problem you do is to always include the units. Even if none are given you should assume some appropriate unit. This a good way to make sure you have understood the problem correctly and also catch any mistakes you make in the arithmetic.
 
  • #16
NascentOxygen said:
:smile: No, I don't think you should decide on an arbitrary time to halt the ship.

For starters, you are not told its current speed...

Probably the best you can do is to state that any force exceeding xxx Newtons will eventually bring the ship's speed to zero if it remains in the grip of that field. So, find xxx.

Would the answer change if we assumed the current speed is ##v_0##?
 
  • #17
Dustinsfl said:
Would the answer change if we assumed the current speed is ##v_0##?
Yes, as now you need a larger force with enough time to slow the ship from v0 to zero.
 
  • #18
paisiello2 said:
My suggestion in any problem you do is to always include the units.
+1

I read your answer "1000m" as 1000 metres, and puzzled how you managed to express a force in metres. :frown:

Had you written "1000m Newtons" I would have recognized it without hesitation.
 
  • #19
Dustinsfl said:
Would the answer change if we assumed the current speed is ##v_0##?
No problem if you wish to do that, but you'll also need to nominate a time, ##t_s##, or a distance, ##d_s##, in which to bring the ship to a standstill.
 

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