Perfect Way to answer this... (Physics) (1 Viewer)

[Ajihood]

Can someone tell me the perfect way to answer this:

Explain why momentum is conserved in collisions in terms of Newton's Third Law of motion.

My answers seem fractured and not making much sense.

Thanks for any help. Its a 5 marks question too.

 

[Razzah]

This is by no means a 5 mark answer but what you need to get across is roughly like this.

The collisions in a said momentum scenario form a system in themselves. The impulse (change in momentum) of the objects in this system is equal and opposite and thus since there is no net force acting on the system (the two collisions balance themselves in accordance to Newton's Third Law which states that for every reaction there is an equal and opposite reaction) the total momentum of the system is constant.

Hmm.. Come to think of it there has to be a better way to phrase that.

 

[Ajihood]

This is by no means a 5 mark answer but what you need to get across is roughly like this.

The collisions in a said momentum scenario form a system in themselves. The impulse (change in momentum) of the objects in this system is equal and opposite and thus since there is no net force acting on the system (the two collisions balance themselves in accordance to Newton's Third Law which states that for every reaction there is an equal and opposite reaction) the total momentum of the system is constant.

Hmm.. Come to think of it there has to be a better way to phrase that.

THatsall i got too, and its five marks so there has to more to it.

 

[James747]

You can also mention the Law of Conservation of Energy, which is a result of Con. of Momentum

 

[sle3pe3bumz]

Well I would use the formula to show that also. Also maybe talk about why impulse is equal and opposite with mentioning of the velocities.

Although I've got to admit that momentum and impulse stuff is my weakest point in physics. I still can't seem to be comfortable with it.

 

[joyce89]

hey, I've studying for Prelim Physics Exam so i know the answer. How's about:
From Newton's third Law, in a collision, the force on object A is equal and opposite to the force on object B: Fa=-Fb. Because the time involved for each object is the same, that means the impulse on object A is equal and opposite to the impulse on object B: (Ft)a=-(Ft)b --> pa=-pb. this latter equation is an expression which represents the law of Conservation of momentum.

 

[ngogiathuan]

How can you conclude that pa=-pb from (Ft)a=-(Ft)b ?
They don't seem to have any relations.

 

[ngogiathuan]

I think we have to put in these manipulations to get full mark
(Note that m(a) means mass of car a and a(a) means acceleration of car a etc...)

From F(a)t = -F(b)t
m(a) a(a) t = -m(b) a(b) t
m(a) [v(a)-u(a)] = - m(b) [v(b) - u(b)]
swap it around we get
m(a) u(a) + m(b) u(b) = m(a) v(a) + m(b) v(b)
which means the momentum before collision = momentum after collision.

Or you can let impulse of car a = change of momentum of car a, impulse b= change of momentum b, put into equation,...
I did it in a complicated way
Hope it's correct
​

 

[iEdd]

How can you conclude that pa=-pb from (Ft)a=-(Ft)b ?
They don't seem to have any relations.

(Ft)a=-(Ft)b
÷ (Ft)] a=-b
x p] pa=-pb

 

[joyce89]

How can you conclude that pa=-pb from (Ft)a=-(Ft)b ?
They don't seem to have any relations.

well, from definition of Impulse: Impulse of a force is equal to the change in momentum it causes.