# Momentum Watch

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#2

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 ().

Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

Where is the mass of the molecule of gas ejected, is the velocity of that molecule and is the mass of the spaceship.

for particles with common mass and average velocity ,

Below is a drawing of this effect using forces instead.

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 ().

Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

Where is the mass of the molecule of gas ejected, is the velocity of that molecule and is the mass of the spaceship.

for particles with common mass and average velocity ,

Below is a drawing of this effect using forces instead.

Spoiler:

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#3

The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)

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(Original post by

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 ().

Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

Where is the mass of the molecule of gas ejected, is the velocity of that molecule and is the mass of the spaceship.

for particles with common mass and average velocity ,

Below is a drawing of this effect using forces instead.

**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 ().

Now the molecules have a mass and velocity going backwards. The spaceship has a mass, and therefore

Where is the mass of the molecule of gas ejected, is the velocity of that molecule and is the mass of the spaceship.

for particles with common mass and average velocity ,

Below is a drawing of this effect using forces instead.

Spoiler:

Show

(Original post by

The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)

**Kyx**)The rocket forces the propellant out. The propellant forces the rocket forward (equal and opposite forces)

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**Kyx**)

The rocket forces the propellant out. The propellant forces the rocket forward

**(equal and opposite forces)**

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#6

(Original post by

is this bit supposed to mean something?

**thefatone**)is this bit supposed to mean something?

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(Original post by

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.

**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.

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#8

(Original post by

so how is this linked with momentum?

**thefatone**)so how is this linked with momentum?

where is momentum

so therefore

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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(Original post by

Through Newtons Second law

where is momentum

so therefore

And so

**The-Spartan**)Through Newtons Second law

where is momentum

so therefore

And so

**force is the rate of change of momentum**A**constant force is a constant rate of change of momentum**.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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#10

(Original post by

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

**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

The force can be calculated with 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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(Original post by

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 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

**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 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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#12

(Original post by

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 :/

**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 :/

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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(Original post by

Hi, sorry for the late response

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).

**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).

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#14

(Original post by

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

**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

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(Original post by

Are you asking how the propellant particles being blasted out of the back give momentum to the rocket?

**Vikingninja**)Are you asking how the propellant particles being blasted out of the back give momentum to the rocket?

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#16

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.

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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(Original post by

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.

**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.

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#18

(Original post by

ok so my question is how does momentum link into this? (this pr0blem has been going on for 3 days now .-.)

**thefatone**)ok so my question is how does momentum link into this? (this pr0blem has been going on for 3 days now .-.)

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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(Original post by

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

**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

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#20

(Original post by

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 ^-^

**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 ^-^

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