VCE Physics Question Thread!

[Theodric_Ironfist]

this helped alot, but im just confused about how the momentum can be conserved yet energy is not?

are clipping voltages in the study design?
mod edit: merged posts.

Momentum is conserved because its fundamentally a different quantity than kinetic energy. In an isolated system, which most of the questions are framed in, momentum is always conserved because the net force on all objects is zero (that's the definition of an isolated system). Newton's 2nd law in its original form states that
Thus since the net force is zero, in a collision the change in momentum is zero, so it's conserved (the change in momentum in this equation refers to the total momentum of the system, not the momentum of an object).

Kinetic energy isn't conserved because there are many forms of energy. Some of the original kinetic energy in your system will be present after a collision, however some of it is lost as sound, heat, etc. The TOTAL amount of energy in an isolated system is conserved, but the kinetic energy present at the beginning could transform into another form.

[minhalgill]

hi
just wondering about 'uncertainties' and 'uncetainty bars' ive checked a couple books and im still confused. does anyone know a good resource where i can learn from them or can someone please explain them?

thanks

[Bri MT]

hi
just wondering about 'uncertainties' and 'uncetainty bars' ive checked a couple books and im still confused. does anyone know a good resource where i can learn from them or can someone please explain them?

thanks

Hey,

Can you give me a quick description of what ideas you anyway have & I'll correct any misunderstandings/provide more info? 

[minhalgill]

Hey,

Can you give me a quick description of what ideas you anyway have & I'll correct any misunderstandings/provide more info?

my knowledge on uncetanties is very limitied. did not do this even once this year and my teacher had no idea what they are either, only discovered them now that im doing the 2017 vcaa exam and i see uncertainty bars.  i understand theyre a +- a certain value but thats pretty much it

also in question 6 c of the vcaa 2013, why cant the spring potential energy be 0 at the top?

[Bri MT]

my knowledge on uncetanties is very limitied. did not do this even once this year and my teacher had no idea what they are either, only discovered them now that im doing the 2017 vcaa exam and i see uncertainty bars.  i understand theyre a +- a certain value but thats pretty much it

also in question 6 c of the vcaa 2013, why cant the spring potential energy be 0 at the top?

I haven't brought up the question fully since I'm on my phone atm, but if you have a spring it has 0 elastic potential energy when its unstretched. However, if the spring changes from it's unstretched length (whether that's up or down) the elastic potential energy would increase. 

Let me know if you don't understand that explanation or that doesn't answer your question & if so I'll draw up a diagram for you to help. 

[minhalgill]

I haven't brought up the question fully since I'm on my phone atm, but if you have a spring it has 0 elastic potential energy when its unstretched.

can you not decide where to take the 0 of the spring potential energy? like for gravitational you can take the 0 for gravitational at the top or bottom, dont you do that for spring potential energy as well? because in the question it says 'they take the 0 of the elastic potential energy to be q'?

[Bri MT]

can you not decide where to take the 0 of the spring potential energy? like for gravitational you can take the 0 for gravitational at the top or bottom, dont you do that for spring potential energy as well? because in the question it says 'they take the 0 of the elastic potential energy to be q'?

You can't say that the elastic potential energy is zero there (the students incorrectly did this), we know that the elastic potential energy is zero when the spring is unstretched and that's at X

[minhalgill]

You can't say that the elastic potential energy is zero there (the students incorrectly did this), we know that the elastic potential energy is zero when the spring is unstretched and that's at X

i get it now! thanks!

also,
does anyone know how to do question 10? dont get why we wont double the answer we received in 9 to get 1.0?

[Bri MT]

i get it now! thanks!

also,
does anyone know how to do question 10? dont get why we wont double the answer we received in 9 to get 1.0?

There's an equation, f= nv/4l
170 = ((1)(340))/4l
340/170 = 4l
2= 4l
l = .5 m

Then we say "have n=2 (instead of 1) and see what the length is"
170= (2*340)/4l
(2*340)/170 = 4l
4=4l

l=1

[minhalgill]

There's an equation, f= nv/4l
170 = ((1)(340))/4l
340/170 = 4l
2= 4l
l = .5 m

Then we say "have n=2 (instead of 1) and see what the length is"
170= (2*340)/4l
(2*340)/170 = 4l
4=4l

l=1

yes, i got 1 as well, but the marking scheme says its 1.5?

[Bri MT]

yes, i got 1 as well, but the marking scheme says its 1.5?

The marking scheme says the answer is B, which is 1.0 m

[minhalgill]

for question 10, doesnt only the mass increase substantially? how does the speed increase slightly?

[Bri MT]

for question 10, doesnt only the mass increase substantially? how does the speed increase slightly?

If it's being accelerated to close to the speed of light its speed is increasing

[LOL_ABC]

No, speed of light is always constant remember that fact if you dont want to bomb the exam

[minhalgill]

For question 4, wont it be A because the observer has to be in the same frame of reference?