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# Momentum Watch

1. (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 ^-^
The force F is equal to ma, right?

Newtons second law. Force is proportional to the rate of change of momentum.

From acceleration: Acceleration is the rate of change of velocity.

Momentum is given by , mass times velocity

So force is the rate of change of momentum (from these simple equations)

Now the particles are given a force by the explosion:
This force changes the acceleration of the particles
Therefore changing the velocity
This gives them momentum
Momentum is converved etc etc

Sorry if im repeating myself im not sure how to explain it any other way.
2. fwiw a popular analogy is firing a gun.

if the gun at rest fires a cannonball of mass m1 at velocity v1 the cannonball has gained momentum m1v1

the story doesn't end there though because while the gun is pushing the cannonball out forward - the cannonball is also simultaneously pushing the heavier cannon backwards (Newton) and the interesting result of this is that momentum is conserved... the momentum of cannon+ball before firing equals the momentum of cannon+ball after firing.

e.g. if m1 = 5 kg and it's fired at v1 = +200 m/s giving it momentum of +1000 kg m/s

the momentum of the gun immediately after firing must be -1000 kg m/s - if the gun has a mass of 1000 kg it's new velocity is -1 m/s or 1 m/s in the opposite direction to the cannonball's direction.
note this is immediate - the gun doesn't need to wait for the cannonball to hit something and slow down before it can start moving.

same principle in a rocket except that the cannonballs are each the size of molecules and there are billions of them, so instead of one cannonball with a mass of 5kg you'd think about the momentum of 5kg of molecules being forced out of the nozzle of your rocket motor at a high velocity.
3. (Original post by Vikingninja;64247495[b)
how i provide an answer to the question with reference to momentum
(Original post by The-Spartan)
The force F is equal to ma, right?

Newtons second law. Force is proportional to the rate of change of momentum.

From acceleration: Acceleration is the rate of change of velocity.

Momentum is given by , mass times velocity

So force is the rate of change of momentum (from these simple equations)

Now the particles are given a force by the explosion:
This force changes the acceleration of the particles
Therefore changing the velocity
This gives them momentum
Momentum is converved etc etc

Sorry if im repeating myself im not sure how to explain it any other way.
i understand the maths bit
don't understand bold bit

(Original post by Joinedup)
fwiw a popular analogy is firing a gun.

if the gun at rest fires a cannonball of mass m1 at velocity v1 the cannonball has gained momentum m1v1

the story doesn't end there though because while the gun is pushing the cannonball out forward - the cannonball is also simultaneously pushing the heavier cannon backwards (Newton) and the interesting result of this is that momentum is conserved... the momentum of cannon+ball before firing equals the momentum of cannon+ball after firing.

e.g. if m1 = 5 kg and it's fired at v1 = +200 m/s giving it momentum of +1000 kg m/s

the momentum of the gun immediately after firing must be -1000 kg m/s - if the gun has a mass of 1000 kg it's new velocity is -1 m/s or 1 m/s in the opposite direction to the cannonball's direction.
note this is immediate - the gun doesn't need to wait for the cannonball to hit something and slow down before it can start moving.

same principle in a rocket except that the cannonballs are each the size of molecules and there are billions of them, so instead of one cannonball with a mass of 5kg you'd think about the momentum of 5kg of molecules being forced out of the nozzle of your rocket motor at a high velocity.
so how does this give me an answer to the question????
i'm still super confused about everything

also i made a mistake i have a double physics tomorrow not today xD
4. thefatone I'll try and put it into simpler terms.

Imagine a stationary rocket in outer space. Its total velocity is zero, so its total momentum is also zero (momentum = mass x velocity).
The key fact to remember is that the total momentum is constant. So no matter what happens it will always equal zero in this scenario.

Rocket engine fires; rocket fuel goes backward (with negative velocity therefore carrying negative momentum) and rocket goes forward (to balance momentum change).
5. (Original post by Alexion)
thefatone I'll try and put it into simpler terms.

Imagine a stationary rocket in outer space. Its total velocity is zero, so its total momentum is also zero (momentum = mass x velocity).
The key fact to remember is that the total momentum is constant. So no matter what happens it will always equal zero in this scenario.

Rocket engine fires; rocket fuel goes backward (with negative velocity therefore carrying negative momentum) and rocket goes forward (to balance momentum change).
ok it makes more sense but how does a rocket motor provide a propulsive force even in space? obviously there's something i'm not getting here and once i get that little thing everything will make sense but it just seems that i haven't got that thing yet :/

so far i understand
how newtons 3rd law is applied since rocket motor pushes rocket forward and exhaust gases push back
but i don't see where momentum fits into any of this :/
i understand everything you've said there but i don't see the link between the question and momentum
6. (Original post by thefatone)
ok it makes more sense but how does a rocket motor provide a propulsive force even in space? obviously there's something i'm not getting here and once i get that little thing everything will make sense but it just seems that i haven't got that thing yet :/

so far i understand
how newtons 3rd law is applied since rocket motor pushes rocket forward and exhaust gases push back
but i don't see where momentum fits into any of this :/
i understand everything you've said there but i don't see the link between the question and momentum
The explanation of the question hinges on the concept of conservation of momentum.

A rocket engine heats up gases. These gases are forced out of the back of the rocket with negative momentum. In order for momentum to be conserved, the rocket accelerates forward with a positive momentum i.e. in the other direction.
7. Newton's 3rd law: If an object A exerts a force on an object B, then B exerts an equal and opposite force, on A.

Newton's 2nd law: ie. force is the rate of change of linear momentum.

Let A and B be objects applying force on each other with no other forces on them.
Let be the change in momentum of A from time 0 to t and similarly for .

from Newton's second law, and .

Applying the fundamental theorem of calculus, and similarly .

.

So if an object gives another object an impulse, the first object receives the same impulse back.

The rocket is changing the velocity of the propellant ie. giving the propellant an impulse, so receives an impulse back.
8. (Original post by thefatone)
ok it makes more sense but how does a rocket motor provide a propulsive force even in space? obviously there's something i'm not getting here and once i get that little thing everything will make sense but it just seems that i haven't got that thing yet :/

so far i understand
how newtons 3rd law is applied since rocket motor pushes rocket forward and exhaust gases push back
but i don't see where momentum fits into any of this :/
i understand everything you've said there but i don't see the link between the question and momentum
Momentum is directly proportional to velocity as momentum= mv.
The force provided accelerates it so basically as v goes up so does momentum.
9. (Original post by Alexion)
The explanation of the question hinges on the concept of conservation of momentum.

A rocket engine heats up gases. These gases are forced out of the back of the rocket with negative momentum. In order for momentum to be conserved, the rocket accelerates forward with a positive momentum i.e. in the other direction.
well ffs then >.> i do this too much, sometimes i don't understand unless i fully understand the concept of what i'm being taught and that kills me to know this bc it tells me i'm retarded
(Original post by morgan8002)
Newton's 3rd law: If an object A exerts a force on an object B, then B exerts an equal and opposite force, on A.

Newton's 2nd law: ie. force is the rate of change of linear momentum.

Let A and B be objects applying force on each other with no other forces on them.
Let be the change in momentum of A from time 0 to t and similarly for .

from Newton's second law, and .

Applying the fundamental theorem of calculus, and similarly .

.

So if an object gives another object an impulse, the first object receives the same impulse back.

The rocket is changing the velocity of the propellant ie. giving the propellant an impulse, so receives an impulse back.
i understand this but how is this linked to the question?
(Original post by Vikingninja)
Momentum is directly proportional to velocity as momentum= mv.
The force provided accelerates it so basically as v goes up so does momentum.
i understand this

but i can't piece together all the info and make links to provide me with a single answer to the question
10. (Original post by thefatone)
well ffs then >.> i do this too much, sometimes i don't understand unless i fully understand the concept of what i'm being taught and that kills me to know this bc it tells me i'm retarded

i understand this but how is this linked to the question?

i understand this

but i can't piece together all the info and make links to provide me with a single answer to the question
"Explain how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space."

So you need to know momentum for the question?
11. (Original post by Vikingninja)
"Explain how it's possible for a space rocket motor to provide a propulsive force even when the rocket is in outer space."

So you need to know momentum for the question?
well yes? it's under the section Momentum test on the sheet i have so i'm assuming i'd need to refer to momentum to get full marks
12. (Original post by thefatone)
i understand this but how is this linked to the question?
The rocket is giving an impulse to the propellant, so receives an impulse in the opposite direction.

Unless you mean "How does the rocket give the impulse to the propellant?", in which case it depends on the type of rocket.
Chemical:
SRBs ignite a solid fuel/oxidiser mixture, which produces hot gas, which produces pressure in the combustion area, forcing the gas out. Liquid fuel rockets burn a liquid fuel/oxidiser mixture in a combustion chamber, which produces hot gas which is directed out.
Electrical:
Ion drives ionise gas, which is moved through an electric field to accelerate it and directed out of the rocket.
Nuclear:
Nuclear thermal rockets use nuclear energy to heat up a gas, which is directed out.
There are many other kinds.
13. (Original post by morgan8002)
The rocket is giving an impulse to the propellant, so receives an impulse in the opposite direction.

Unless you mean "How does the rocket give the impulse to the propellant?", in which case it depends on the type of rocket.
Chemical:
SRBs ignite a solid fuel/oxidiser mixture, which produces hot gas, which produces pressure in the combustion area, forcing the gas out. Liquid fuel rockets burn a liquid fuel/oxidiser mixture in a combustion chamber, which produces hot gas which is directed out.
Electrical:
Ion drives ionise gas, which is moved through an electric field to accelerate it and directed out of the rocket.
Nuclear:
Nuclear thermal rockets use nuclear energy to heat up a gas, which is directed out.
There are many other kinds.
i am more confused than before
14. (Original post by thefatone)
well ffs then >.> i do this too much, sometimes i don't understand unless i fully understand the concept of what i'm being taught and that kills me to know this bc it tells me i'm retarded
I'm gonna draw you a nice diagram brb

And everyone finds things difficult to comprehend now and again, dw about it.
15. thefatone

See if this helps

16. (Original post by thefatone)

so how does this give me an answer to the question????
i'm still super confused about everything

also i made a mistake i have a double physics tomorrow not today xD
are you happy about the gun being pushed backward when the canonball is pushed forward and how momentum is conserved?

so what's different if it fired out 5kg of gas instead of a 5kg metal ball?
5kg of gas is the same mass as 5kg of metal
gas going at 200 m/s is going at the same velocity as a ball going at 200 m/s
momentum is mass times velocity, but the mass doesn't have to be a solid object. liquid or gas also work because they also have mass.
17. (Original post by Alexion)
I'm gonna draw you a nice diagram brb

And everyone finds things difficult to comprehend now and again, dw about it.
problem is it takes me 20 times longer than everyone else to understand it >.>

(Original post by Alexion)
thefatone

See if this helps

i understand but where do the things gain momentum from?
(Original post by Joinedup)
are you happy about the gun being pushed backward when the canonball is pushed forward and how momentum is conserved?

so what's different if it fired out 5kg of gas instead of a 5kg metal ball?
5kg of gas is the same mass as 5kg of metal
gas going at 200 m/s is going at the same velocity as a ball going at 200 m/s
momentum is mass times velocity, but the mass doesn't have to be a solid object. liquid or gas also work because they also have mass.
yes i understand that bit
except it takes up a different area
18. (Original post by thefatone)
i understand but where do the things gain momentum from?
Combustion of the fuel causes gases to heat up. As they heat up, they gain energy; therefore velocity; therefore momentum.

As the gases are ejected out of the back of the rocket with their high momentum, the average overall momentum of the exhaust gas is backwards. The rocket therefore gains forward momentum (specifically from the gases colliding with the upper wall of the exhaust chamber) which balances the backward momentum of the exhaust gases.

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