thefatone
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Explain how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space.

i understand nothing all answers welcome

Kyx
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The-Spartan
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It is the change in momentum of the propellant.

Basically, the conservation of momentum states that since the propellant is accelerated and therefore has a momentum, there must be a force backwards. Due to this force of the propellant accelerating backwards, they push on the ship forwards (equal and opposite).

So imagine a spaceship in the vacuum of space, shooting gas out of the back of it (through chemical reactions). This gas will be accelerated out of the back of the spaceship, in a big cool looking stream. This gas will have had a change in acceleration from the chemical reaction giving the molecules energy. The gas molecules will then push against the spaceship as they have gained a momentum, and the conservation of momentum takes place, therefore impacting the spaceship with a change in velocity (m_1v_1=m_2v_2).
Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

\dfrac{m_1v_1}{m_2}=v_2
Where m_1 is the mass of the molecule of gas ejected, v_1 is the velocity of that molecule and m_2 is the mass of the spaceship.

for n particles with common mass m_1 and average velocity \bar v_1,

v_2=n\dfrac{m_1 \bar v_1}{m_2}

Below is a drawing of this effect using forces instead.
Spoiler:
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Kyx
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The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)
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thefatone
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(Original post by The-Spartan)
It is the change in momentum of the propellant.

Basically, the conservation of momentum states that since the propellant is accelerated and therefore has a momentum, there must be a force backwards. Due to this force of the propellant accelerating backwards, they push on the ship forwards (equal and opposite).

So imagine a spaceship in the vacuum of space, shooting gas out of the back of it (through chemical reactions). This gas will be accelerated out of the back of the spaceship, in a big cool looking stream. This gas will have had a change in acceleration from the chemical reaction giving the molecules energy. The gas molecules will then push against the spaceship as they have gained a momentum, and the conservation of momentum takes place, therefore impacting the spaceship with a change in velocity (m_1v_1=m_2v_2).
Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

\dfrac{m_1v_1}{m_2}=v_2
Where m_1 is the mass of the molecule of gas ejected, v_1 is the velocity of that molecule and m_2 is the mass of the spaceship.

for n particles with common mass m_1 and average velocity \bar v_1,

v_2=n\dfrac{m_1 \bar v_1}{m_2}

Below is a drawing of this effect using forces instead.
Spoiler:
Show
(Original post by Kyx)
The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)
ok thank you both of you very good explainations
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thefatone
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(Original post by Kyx)
The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)
is this bit supposed to mean something?
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The-Spartan
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(Original post by thefatone)
is this bit supposed to mean something?
Newtons third law The particles will be given a force by the controlled explosion, and that same force is applied to the spaceship by the particles leaving it.
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thefatone
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(Original post by The-Spartan)
Newotons third law The particles will be given a force by the controlled explosion, and that same force is applied to the spaceship by the particles leaving it.
so how is this linked with momentum?
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The-Spartan
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(Original post by thefatone)
so how is this linked with momentum?
Through Newtons Second law

F=ma

a=\frac{v-u}{t}

\rho = m(v-u) where \rho is momentum

so therefore \displaystyle F= \frac{m(v-u)}{t}

And so force is the rate of change of momentum A constant force is a constant rate of change of momentum.
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thefatone
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(Original post by The-Spartan)
Through Newtons Second law

F=ma

a=\frac{v-u}{t}

\rho = m(v-u) where \rho is momentum

so therefore \displaystyle F= \frac{m(v-u)}{t}

And so force is the rate of change of momentum A constant force is a constant rate of change of momentum.
this is the bit i was looking for i guess
but i know i'm being retarded right now but i'm not seeing the link between the bits in bold and the original question

how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space
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The-Spartan
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(Original post by thefatone)
this is the bit i was looking for i guess
but i know i'm being retarded right now but i'm not seeing the link between the bits in bold and the original question

how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space
Ok so rocket fuel is combusted to produce energy. The molecules of the rocket fuel are given an energy, a force is provided to them, and they shoot out the back (they are funnelled out of the back). This always happens even when the ship is in space.

The force can be calculated with E=Fd where d is the distance travelled by the molecule, and E is the work done on that molecule (this is the energy given to the molecule at the explosion)

Anyway this force acted on the molecules of propellant to move them a distance out of the ships back means that the molecules had a change in momentum. The rest of the explanation is in my first post
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thefatone
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(Original post by The-Spartan)
Ok so rocket fuel is combusted to produce energy. The molecules of the rocket fuel are given an energy, a force is provided to them, and they shoot out the back (they are funnelled out of the back). This always happens even when the ship is in space.

The force can be calculated with E=Fd where d is the distance travelled by the molecule, and E is the work done on that molecule (this is the energy given to the molecule at the explosion)

Anyway this force acted on the molecules of propellant to move them a distance out of the ships back means that the molecules had a change in momentum. The rest of the explanation is in my first post
hmmm so what i've gathered i could say the rocket motor provides a forward force which pushes the rocket forwards, and there's exhaust gases which are ejected. Newtons 3rd law applies here but i'm still not understanding the link between momentum and the question :/
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The-Spartan
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(Original post by thefatone)
hmmm so what i've gathered i could say the rocket motor provides a forward force which pushes the rocket forwards, and there's exhaust gases which are ejected. Newtons 3rd law applies here but i'm still not understanding the link between momentum and the question :/
Hi, sorry for the late response
The rocket has propellant in the back. This gets ignited and therefore the particles of rocket fuel gain an energy.
This gain in energy means they move round more, and due to the shape of the tailcone, they are forced out the back by the explosive force
This force therefore does work and changes the velocity of the molecules and so there is a change of momentum. (in the molecules of propellant)
This does not need to be in air, they will have a change in momentum even in space out of the back of the rocket.
This change in momentum on the molecules of the propellant causes a change in momentum on the ship (momentum is always conserved in a system, this is basically newtons 3rd law at the barebones).
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thefatone
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(Original post by The-Spartan)
Hi, sorry for the late response
The rocket has propellant in the back. This gets ignited and therefore the particles of rocket fuel gain an energy.
This gain in energy means they move round more, and due to the shape of the tailcone, they are forced out the back by the explosive force
This force therefore does work and changes the velocity of the molecules and so there is a change of momentum. (in the molecules of propellant)
This does not need to be in air, they will have a change in momentum even in space out of the back of the rocket.
This change in momentum on the molecules of the propellant causes a change in momentum on the ship (momentum is always conserved in a system, this is basically newtons 3rd law at the barebones).
this is the bit i don't understand
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Vikingninja
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(Original post by thefatone)
Explain how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space.

i understand nothing all answers welcome

Kyx
Are you asking how the propellant particles being blasted out of the back give momentum to the rocket?
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thefatone
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(Original post by Vikingninja)
Are you asking how the propellant particles being blasted out of the back give momentum to the rocket?
yes? i think so, i don't really understand *cries
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Vikingninja
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With Newton's 3rd law of motion there is an equal but opposite force. If you throw a bowling ball you will feel it pushing against you, you throw a bowling ball with force and it applies the force back. That requires no atmosphere like in space, just interaction between the two objects. The same happens with a space rocket, it throws out particles with a force (by reactions in the engine or wherever it happens) and when they're thrown out of the engine they are also pushing against the rocket like the bowling ball does against your hand.

I will add that the difference between this and say a propeller on a plane is that the propeller is using air in the atmosphere to push it. The fuel is used to turn the propeller which acts with the air.
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thefatone
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(Original post by Vikingninja)
With Newton's 3rd law of motion there is an equal but opposite force. If you throw a bowling ball you will feel it pushing against you, you throw a bowling ball with force and it applies the force back. That requires no atmosphere like in space, just interaction between the two objects. The same happens with a space rocket, it throws out particles with a force (by reactions in the engine or wherever it happens) and when they're thrown out of the engine they are also pushing against the rocket like the bowling ball does against your hand.

I will add that the difference between this and say a propeller on a plane is that the propeller is using air in the atmosphere to push it. The fuel is used to turn the propeller which acts with the air.
ok so my question is how does momentum link into this? (this pr0blem has been going on for 3 days now .-.)
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Vikingninja
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(Original post by thefatone)
ok so my question is how does momentum link into this? (this pr0blem has been going on for 3 days now .-.)
The force is equal to the rate of change of momentum. Force accelerate objects and in space would accelerate it no matter how weak. If the force applied stays constant then so does the rate of change of momentum. So momentum would increase by a constant amount over time.

EQUATION TIME.
/\ is that triangle showing a change in
f= ma

a= /\v/(/\t)

f=m/\v/(/\t)

mv= momentum
m/\v= change in momentum

f= change in momentum/ t
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thefatone
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(Original post by Vikingninja)
The force is equal to the rate of change of momentum. Force accelerate objects and in space would accelerate it no matter how weak. If the force applied stays constant then so does the rate of change of momentum. So momentum would increase by a constant amount over time.

EQUATION TIME.
/\ is that triangle showing a change in
f= ma

a= /\v/(/\t)

f=m/\v/(/\t)

mv= momentum
m/\v= change in momentum

f= change in momentum/ t
my mind is too feeble to understand i'm sure i'll understand this tomorrow since i've got a physics lesson tomorrow, a double too ^-^
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Vikingninja
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(Original post by thefatone)
my mind is too feeble to understand i'm sure i'll understand this tomorrow since i've got a physics lesson tomorrow, a double too ^-^
What exactly do you want to know when asking about how momentum links in?
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