Force of declerating object losing mass

[Lukec29]

Homework Statement

A spacecraft traveling in free space, at a speed of 6 m s ^-1 has a mass of 10,100kg (including fuel)

A rocket engine on the spacecraft is fired up for 4 seconds, ejecting 100kg of fuel and slowing the spacecraft down to 0.02 m s ^-1

What is the force in Newtons, feing the 4 second burn of the spacecraft .

Homework Equations

The Attempt at a Solution

I really new to physics, and this is an example question in order to help us understand how to work these type
Of equations out.

I know that f = m a, however in this case I think the fact that the spacecraft is losing weight needs to be taken into acount.

The momentum before is 60,600 kg ^-1 s ^-1

As I said before I am really new to physics and being able to understand the reasoning for the formula is my main aim here.

Thank you in advance to any help that may be given here.

 

[NascentOxygen]

Hi Lukec29. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

It's probably good enough to use the average of its mass during that time in your calculations.

Do you know what the expected answer is?

 

[Lukec29]

Unfortunately not, my tutor is on holiday currently. I am more looking for the correct workings so I can understand the formula and put it to practice.

 

[HallsofIvy]

As long as the mass is decreasing "linearly", at a constant rate, then you can use the average mass as if it were the mass the entire time.
Initially, the mass, including fuel is 10,100 kg. At the end of the 4 seconds firing, the mass is 100kg less (I am assuming you did NOT mean "100 kg per second") so the average mass, over the 4 seconds is [10,100+ (10,100- 100)]/2 kg. Use that with "F= ma".

 

[PeroK]

What do you know about conservation of momentum?

 

[Lukec29]

Nothing at all, it's degree level but first year, very simple.

Would force in Newtons, from the average mass of before and after ejecting the fuel suffice