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# Higher Physics Prelim. watch

1. (Original post by megara)
Doing Scholar...
A 2500 kg train carriage travelling at 10 m s-1 runs into the back of a stationary carriage of mass 4000 kg. If the carriages couple together, what is their velocity just after impact?
How do you work this out?
You need to make use of the conversation of linear momentum.

The momentum before a collision, should be the same as afterwards.

Calculate the initial momentum of the system (momentum of carriage added to the momentum of the STATIONARY carriage). The momentum afterwards must be the same value as the initial momentum. You can then say that final momentum = mv = initial momentum. Note that it says the carriages "couple together". This means you can consider them as one big mass. See if that helps

In the exam you should try and say "as momentum is conserved in a system provided there are no external forces" or something similar, I think you sometimes get marks for it!

Another point to remember when dealing with momentum questions like this is the direction of velocity. It's very important to watch out for your negative/positive signs!!
2. (Original post by CallumFR)
You need to make use of the conversation of linear momentum.

The momentum before a collision, should be the same as afterwards.

Calculate the initial momentum of the system (momentum of carriage added to the momentum of the STATIONARY carriage). The momentum afterwards must be the same value as the initial momentum. You can then say that final momentum = mv = initial momentum. Note that it says the carriages "couple together". This means you can consider them as one big mass. See if that helps

In the exam you should try and say "as momentum is conversed in a system provided there are no external forces" or something similar, I think you sometimes get marks for it!

Another point to remember when dealing with momentum questions like this is the direction of velocity. It's very important to watch out for your negative/positive signs!!
Okay - so could I do something like

m1u1+m2u2 = (m1+m2) v?

Ill make sure to say that! Thankyou<3
3. (Original post by megara)
Okay - so could I do something like

m1u1+m2u2 = (m1+m2) v?

Ill make sure to say that! Thankyou<3
Yep, that's right
4. (Original post by CallumFR)
Yep, that's right
Ahh seriously?
5. (Original post by megara)
Ahh seriously?
Just to clarify, in that little sentence I suggested you write, it should say "conserved". I noticed that in your quote it was conversed (i.e. it was quoted before I made the edit to my original post), so I just wanted to make sure you knew I'd edited it
6. (Original post by CallumFR)
Just to clarify, in that little sentence I suggested you write, it should say "conserved". I noticed that in your quote it was conversed (i.e. it was quoted before I made the edit to my original post), so I just wanted to make sure you knew I'd edited it
Yeah i guessed that, hehe. I love English. This is why I shouldnt be doing Physics
7. Anyone want to tutor me a little on impulse? I get most of it, but some bits i'm 0_0-ing at.
8. (Original post by megara)
Anyone want to tutor me a little on impulse? I get most of it, but some bits i'm 0_0-ing at.
Impulse is simply a change in momentum.

Ft = mv - mu

An idea you need to appreciate is that a small force, for a long time can result in the same IMPULSE as a large force for a short amount of time.

If you have a graph of force against time, the area under the graph represents the impulse.

You might get asked a question about impulse which involves explaining why car airbags help to save lives. The overall IMPULSE is the same, however, as they increase the time for your deceleration to occur, a smaller force is experience by the people in the car. You might also get asked about footballs being kicked, or golf balls being hit, something along those lines

That's all I can think of to tell you off the top of my head
9. Still struggling with Physics I see?

(Original post by megara)
Anyone want to tutor me a little on impulse? I get most of it, but some bits i'm 0_0-ing at.
What don't you understand. It's just ONE formula you have to know.

For pretty much every impulse question, you'll just be using that equation. I don't understand what's so difficult.

(Original post by megara)
Could anyone give help on how to rearrange formulae? I think thats my main buggery-uppy bit.
DON'T re-arrange then if you can't. Just use the formula as it's given to you in the databook, and plug the numbers in. For example, if you were trying to find the time given a velocity of 0.5ms-1 and a displacement of 4m, you don't need to re-arrange it. See below.

(From databook)
(Sub values in without rearranging formula)
(Then just use basic algebra to solve the rest)

All done with no actual re-arranging of the formula itself. Just sub the values the formula as it's given, and use algebra to solve.
10. (Original post by CallumFR)
Impulse is simply a change in momentum.

Ft = mv - mu

An idea you need to appreciate is that a small force, for a long time can result in the same IMPULSE as a large force for a short amount of time.

If you have a graph of force against time, the area under the graph represents the impulse.

You might get asked a question about impulse which involves explaining why car airbags help to save lives. The overall IMPULSE is the same, however, as they increase the time for your deceleration to occur, a smaller force is experience by the people in the car. You might also get asked about footballs being kicked, or golf balls being hit, something along those lines

That's all I can think of you tell you off the top of my head
Thank you so much<3!
This seems weird, but our teacher hinted there would be a lot of unit comparison, like 1 N = 1 J s^-1. Could you give me all of these things? n__n
11. (Original post by innerhollow)
Still struggling with Physics I see?

What don't you understand. It's just ONE formula you have to know.

For pretty much every impulse question, you'll just be using that equation. I don't understand what's so difficult.

DON'T re-arrange then if you can't. Just use the formula as it's given to you in the databook, and plug the numbers in. For example, if you were trying to find the time given a velocity of 0.5ms-1 and a displacement of 4m, you don't need to re-arrange it. See below.

(From databook)
(Sub values in without rearranging formula)
(Then just use basic algebra to solve the rest)

All done with no actual re-arranging of the formula itself. Just sub the values the formula as it's given, and use algebra to solve.
Okay! That helps a lot. Thank you!
12. (Original post by megara)
Thank you so much<3!
This seems weird, but our teacher hinted there would be a lot of unit comparison, like 1 N = 1 J s^-1. Could you give me all of these things? n__n
Ehm... not really sure

Ns and kgms^-1 can both be units for momentum/impulse... that might be useful to know.

In electronics, F (farad) = CV^-1, V (volts) = JC^-1

there's some

EDIT: Thought of another one! Pa (pascal) = N m^-2
13. When a cannon fires a 10 kg cannonball at 30 m s-1, its recoil velocity is 6.0 m s-1. What is the recoil velocity when a 15 kg cannonball is fired at 24 m s-1?
This. I know you have to find the mass of the cannon, but how? >< Sorry, Im useless at this.
14. (Original post by CallumFR)
Ehm... not really sure

Ns and kgms^-1 can both be units for momentum/impulse... that might be useful to know.

In electronics, F (farad) = CV^-1, V (volts) = JC^-1

there's some

EDIT: Thought of another one! Pa (pascal) = N m^-2
-nodnod-

I need to memorise these. Great XD
15. (Original post by megara)
This seems weird, but our teacher hinted there would be a lot of unit comparison, like 1 N = 1 J s^-1. Could you give me all of these things? n__n
I suck at physics, but this kind of thing is easily worked out by doing dimensional analysis 'backwards'. What I mean is that, you have equations where you know the units involved on both sides. So you can equate these (try to avoid ones with constants like G, since these indicate that the units don't work 'nicely'!).

Newton is a measure of force.

W = Fd
F = W / d

16. (Original post by megara)
When a cannon fires a 10 kg cannonball at 30 m s-1, its recoil velocity is 6.0 m s-1. What is the recoil velocity when a 15 kg cannonball is fired at 24 m s-1?
This. I know you have to find the mass of the cannon, but how? >< Sorry, Im useless at this.
It's back to conservation of linear momentum. The cannon and cannonball are obviously at rest to begin with. Do the cannonball and cannon act as one big mass afterwards?
17. (Original post by TheUnbeliever)
I suck at physics, but this kind of thing is easily worked out by doing dimensional analysis 'backwards'. What I mean is that, you have equations where you know the units involved on both sides. So you can equate these (try to avoid ones with constants like G, since these indicate that the units don't work 'nicely'!).

Newton is a measure of force.

W = Fd
F = W / d

You have just saved my brain from implosion. Wow.
18. (Original post by CallumFR)
It's back to conservation of linear momentum. The cannon and cannonball are obviously at rest to begin with. Do the cannonball and cannon act as one big mass afterwards?

Youd be a good tutor, by the way.
19. (Original post by megara)
Thank you so much<3!
This seems weird, but our teacher hinted there would be a lot of unit comparison, like 1 N = 1 J s^-1. Could you give me all of these things? n__n
You can resolve dimensions using any physical relationship you know is valid. Since we know that two things equalling each other must have the same units then you can express units as analogues of each other even though they may sound "weird".

For example you know that F = ma, which implies that the unit for F, Newtons, must be a product of the units for m (kilograms) and a (ms-2). Therefore N = kgms-2. But we also know that F=W/d, which by a similar process gives us N = Jm-1. Equating the two dimensional equations for force we can rearrange to obtain J as J = kgm2s-2.

Hmm, but what can we do with this? Well we also know that Pressure=F/A, so dimensionally Pa = Nm-2 = kgm-1s-2. Ah, this is very similar to the units in J, in fact we can substitute Pa into J and obtain J = Pam3. What does this tell us? It tells us that 1 Joule of energy can also be defined as the work of 1 Pascal of pressure done over 1 m3 of space

Sorry for inundating you with Physics.
20. http://www.bbc.co.uk/scotland/learni...uiz/resistors/

Questions 3, 4 & 5. I cant do this for some reason? *useless*

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