The force F is equal to ma, right?(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 ^^
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.
Momentum
Announcements  Posted on  

Four hours left to win £100 of Amazon vouchers!! Don't miss out! Take our short survey to enter  24102016 

 Follow
 21
 18042016 21:25

 Follow
 22
 19042016 01:03
fwiw a popular analogy is firing a gun.
if the gun at rest fires a cannonball of mass m_{1} at velocity v_{1} the cannonball has gained momentum m_{1}v_{1}
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 m_{1} = 5 kg and it's fired at v_{1} = +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. 
 Follow
 23
 19042016 16:15
(Original post by Vikingninja;64247495[b)
]What exactly do you want to know [/b]when asking about how momentum links in?
(Original post by TheSpartan)
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.
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 m_{1} at velocity v_{1} the cannonball has gained momentum m_{1}v_{1}
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 m_{1} = 5 kg and it's fired at v_{1} = +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.
i'm still super confused about everything
also i made a mistake i have a double physics tomorrow not today xD 
 Follow
 24
 19042016 16:21
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). 
 Follow
 25
 19042016 16:35
(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).
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 
 Follow
 26
 19042016 16:42
(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
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. 
 Follow
 27
 19042016 16:58
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.Last edited by morgan8002; 19042016 at 17:24. Reason: formatting 
 Follow
 28
 19042016 17:00
(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 force provided accelerates it so basically as v goes up so does momentum. 
 Follow
 29
 19042016 17:53
(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.
(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.
(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.
but i can't piece together all the info and make links to provide me with a single answer to the question 
 Follow
 30
 19042016 17:55
(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
So you need to know momentum for the question? 
 Follow
 31
 19042016 17:58
(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? 
 Follow
 32
 19042016 18:13
(Original post by thefatone)
i understand this but how is this linked to the question?
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. 
 Follow
 33
 19042016 18:28
(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. 
 Follow
 34
 19042016 19:04
(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
And everyone finds things difficult to comprehend now and again, dw about it. 
 Follow
 35
 19042016 19:22
thefatone
See if this helps

 Follow
 36
 19042016 19:24
(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
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. 
 Follow
 37
 19042016 20:17
(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.
(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.
except it takes up a different area 
 Follow
 38
 19042016 20:55
(Original post by thefatone)
i understand but where do the things gain momentum from?
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.Post rating:1
Write a reply…
Reply
Submit reply
Register
Thanks for posting! You just need to create an account in order to submit the post Already a member? Sign in
Oops, something wasn't right
please check the following:
Sign in
Not got an account? Sign up now
Updated: April 19, 2016
Share this discussion:
Tweet
TSR Support Team
We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.