Yacoubb's Physics 1+2 Thread

[Yacoubb]

Hey guys Will be posting some questions here.

A 2kg puck slides along an air table at a velocity of 2.0m/s north and collides with another puck of mass 1kg moving at 3.0m/s west. They stick together after the collision.

(A) What is the total initial momentum of the system?
(B) What is the total final momentum of this system?
(C) Calculate the final velocity of the pucks.

Do we need to use the law of conservation? I have no idea what to do. Help would be greatly appreciated!

[Nato]

for (a) you calculate the initial momentum on the first puck (so p=m1v1) and the initial momentum of the second much (p=m2v2).
then you add them together.

for (b) you have to find the total momentum. after their collision it says they'stick together'. so their mass would be 2kg+1kg=3kg. and their velocity would be v(we don't know this yet.

(C) now what does the conservation of momentum says: the initial momentum = final:

so from (A) you add to the two initial momenta together and equalise this to the final momentum, and you will then be able to solve for v.

[Yacoubb]

for (a) you calculate the initial momentum on the first puck (so p=m1v1) and the initial momentum of the second much (p=m2v2).
then you add them together.

for (b) you have to find the total momentum. after their collision it says they'stick together'. so their mass would be 2kg+1kg=3kg. and their velocity would be v(we don't know this yet.

(C) now what does the conservation of momentum says: the initial momentum = final:

so from (A) you add to the two initial momenta together and equalise this to the final momentum, and you will then be able to solve for v.

Thank you so much!

[RKTR]

the answer for B will be same as A

[Guest]

Just to clarify the answer for part (c).
m₁u₁+m₂u₂=m₁v₁+m₂v₂
Treating the mass of both pucks as one after the collision: m₁u₁+m₂u₂=(m₁+m₂)*v₂
So, (2*2) + (1*3) = 3v₂
v₂=7/3

[Yacoubb]

I am SO dropping Physics...

[psyxwar]

I am SO dropping Physics...

Hey it's not a particularly difficult subject. If you have a good teacher stick with it.

[Yacoubb]

Hey it's not a particularly difficult subject. If you have a good teacher stick with it.

Nerr I'm picking up Psychology 3+4 next year. I'm over Physics to be honest! It's easy - I just cannot stand how dry it is! Additionally, I have no incentive to study for it! Like I'm not even joking! lol

[psyxwar]

Nerr I'm picking up Psychology 3+4 next year. I'm over Physics to be honest! It's easy - I just cannot stand how dry it is! Additionally, I have no incentive to study for it! Like I'm not even joking! lol

Yeah I know what you mean. I was lost for words when we were taught how to calculate impulse from a force-time graph -- "count the squares under the curve!" Wasn't calculus invented a few centuries ago?

[Guest]

Yeah I know what you mean. I was lost for words when we were taught how to calculate impulse from a force-time graph -- "count the squares under the curve!" Wasn't calculus invented a few centuries ago?

Yeah Unit 1/2 Physics can get slightly "stupid" at times. My jaw dropped when they said we wouldn't be using Integration but the "counting-squares" method.

[BasicAcid]

Nerr I'm picking up Psychology 3+4 next year. I'm over Physics to be honest! It's easy - I just cannot stand how dry it is! Additionally, I have no incentive to study for it! Like I'm not even joking! lol

It's easier to get 40+ in physics if you know your stuff than psychology.

But that's subjective so do what you want, but you're missing out on an easy 40+.

[Yacoubb]

It's easier to get 40+ in physics if you know your stuff than psychology.

But that's subjective so do what you want, but you're missing out on an easy 40+.

Dunno.. still deciding. But the thing is, I feel like I've missed a lot. I mean, I barely listen in class and have no idea what I'm doing. Sometimes (e.g. vectors, velocity/speed/acceleration/displacement), I get it. My marks are also pretty good for Physics too- mid year, with literally no studying + a revision sheet, I managed an 84% (A). Can I catch up over the summer and still be able to prepare thoroughly for 3+4?

The thing is, I'd rather an easy 40+ over a more difficult one.

[Yacoubb]

Two identical 100g billiard balls collide when moving at 90^o to each other. Both were travelling at 5.0m/s. After the collision they both travel in the same direction at the same speed. Use the Law of conservation of momentum to find that speed.

m1v1 + m2v2 = v(m1+m2)
(100*5) + (100*5) = 200v
1000 = 200v
v = 5.0 m/s

Is that right?

[psyxwar]

I got something else, but then again I haven't touched my textbook in ages so take this with a grain of salt.

Two momentum vectors, let's say one is 0.1kg * 5.0m/s east and the other is that magnitude north.

We now have two vectors of 0.5Ns that are perpendicular to eachother.

Thus, by pythagoras: 0.5^2 + 0.5^2 = P_final^2

Final momentum is sqrt2/2

sqrt2/2 divided by 0.2kg yields 5sqrt(2)/2 m/s, or approximately 3.54 m/s.