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March 29, 2024, 10:11:05 pm

Author Topic: VCE Physics Question Thread!  (Read 603569 times)  Share 

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Ha_Nguyen

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Re: VCE Physics Question Thread!
« Reply #915 on: March 25, 2015, 08:50:21 pm »
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Hi, can someone help me with question 3????

if an object of mass m is moving uniformly (constant speed, v) in a circle of radius r:
1. state the equation that relates the centripetal force, F to m,v,r

F= mv^2/r

2. state the equation that relates the period for 1 revolution (T) to v,r
T= (2 pi r )/v

3. from the two previous equations, write an equation relating T to m&r?

Kel9901

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Re: VCE Physics Question Thread!
« Reply #916 on: March 27, 2015, 09:40:18 am »
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Hi, can someone help me with question 3????

if an object of mass m is moving uniformly (constant speed, v) in a circle of radius r:
1. state the equation that relates the centripetal force, F to m,v,r

F= mv^2/r

2. state the equation that relates the period for 1 revolution (T) to v,r
T= (2 pi r )/v

3. from the two previous equations, write an equation relating T to m&r?

I think you'll need F in the equation too... or does it mean T to a and r?

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RedCapsicum

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Re: VCE Physics Question Thread!
« Reply #917 on: April 02, 2015, 11:37:26 pm »
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Hi guys, can someone help me with this question:

The mass of Earth is 6.0*10^24 kg and the amass of the moon is 7.4*10^22 kg. The radius of the earth is 6.4*10^6 metres and the radius of the moon is 1.7*10^6 metres. The orbital radius of the moon around the Earth taken from the centre of the Moon and the centre of the Earth is 3.8*10^8 metres.

1) Calculate the gravitational force acting between the Earth and the moon
2) Calculate to two decimal places how long it takes for the moon to orbit the Earth in days
3) Calculate the orbital speed of the moon around the earth in km/hr

JackSonSmith

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Re: VCE Physics Question Thread!
« Reply #918 on: April 03, 2015, 08:10:02 pm »
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For electricity: does VIt equal energy or work?
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Re: VCE Physics Question Thread!
« Reply #919 on: April 03, 2015, 08:58:36 pm »
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Hi guys, can someone help me with this question:

The mass of Earth is 6.0*10^24 kg and the amass of the moon is 7.4*10^22 kg. The radius of the earth is 6.4*10^6 metres and the radius of the moon is 1.7*10^6 metres. The orbital radius of the moon around the Earth taken from the centre of the Moon and the centre of the Earth is 3.8*10^8 metres.

1) Calculate the gravitational force acting between the Earth and the moon
2) Calculate to two decimal places how long it takes for the moon to orbit the Earth in days
3) Calculate the orbital speed of the moon around the earth in km/hr

1) Use , and substitute in the given values.

2) which means that . Now you can substitute in the radius of the moons orbit around earth, the gravitational constant and the mass of earth. Remember to use the mass of the body which is being orbited around, not the mass of the body in orbit.

3) . Now you can substitute in the gravitational constant, the mass of the earth and the radius of the orbit.

So with these sorts of questions, firstly write down all the variables that you are given, and then write down the ones you are trying to find. Then take the circular motion formulas and rearrange them until you find the value you are looking for. Once you've got enough practice, these questions can be done quite quickly as long as you know what value you're looking for and what formula you need to find it.
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dankfrank420

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Re: VCE Physics Question Thread!
« Reply #920 on: April 03, 2015, 10:36:45 pm »
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For electricity: does VIt equal energy or work?

Aren't they the same thing in this context?

JackSonSmith

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Re: VCE Physics Question Thread!
« Reply #921 on: April 04, 2015, 03:12:41 pm »
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Aren't they the same thing in this context?

I was a little confused about the difference between energy and work
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Re: VCE Physics Question Thread!
« Reply #922 on: April 04, 2015, 05:32:24 pm »
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I was a little confused about the difference between energy and work

work is pretty much change in energy, in this case they are the same because the 'energy' means energy dissipated anyway.
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Re: VCE Physics Question Thread!
« Reply #923 on: April 09, 2015, 05:45:28 pm »
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Question about diodes.

There is a battery of 12v, a resistor of 150 ohms and a forward biased diode in circuit. The I-V characteristic of the diode says that the diode needs 0.6 v for current to flow.

Therefore, there is 11.4 volts across the resistor.

When working out the current flowing through the diode, why do we use 11.4/150 instead of 12/150? Since they're in series, shouldn't the current be constant across both the diode and the resistor? Why do we take away the 0.6v before we apply ohms law to find the current?

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Re: VCE Physics Question Thread!
« Reply #924 on: April 09, 2015, 09:30:59 pm »
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Question about diodes.

There is a battery of 12v, a resistor of 150 ohms and a forward biased diode in circuit. The I-V characteristic of the diode says that the diode needs 0.6 v for current to flow.

Therefore, there is 11.4 volts across the resistor.

When working out the current flowing through the diode, why do we use 11.4/150 instead of 12/150? Since they're in series, shouldn't the current be constant across both the diode and the resistor? Why do we take away the 0.6v before we apply ohms law to find the current?

The diode consumes 0.6V so what's left is 11.4V. This 11.4V is for the resistor. Since the current is the same for each component, you work out the current for the resistor first by doing VR/R=11.4/150=0.076A. Hence, since this is a series circuit, that same amount of current (0.076A) passes through the diode. Hope that made sense!

dankfrank420

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Re: VCE Physics Question Thread!
« Reply #925 on: April 09, 2015, 11:07:21 pm »
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Yes I understood.

However, why isn't the 0.6 v included? Isn't the current Vtotal/Rtotal?

So what you're saying is that the diode literally takes 0.6v away from the circuit for current to flow through it? But why isn't that voltage counted in the calculation of the total current?

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Re: VCE Physics Question Thread!
« Reply #926 on: April 09, 2015, 11:58:56 pm »
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Yes I understood.

However, why isn't the 0.6 v included? Isn't the current Vtotal/Rtotal?

So what you're saying is that the diode literally takes 0.6v away from the circuit for current to flow through it? But why isn't that voltage counted in the calculation of the total current?

Vtot/Rtot only works if every circuit component you have in your circuit is ohmic, or V/I is constant for every device you have.
Ohm's law isn't V=IR. Ohm's law is actually an assertion that V/I is a constant and devices that satisfy this assertion are ohmic, like normal resistors.

As you can quite clearly, diodes are NOT ohmic. Hence you can't just use normal voltage divider techniques to solve this question. You have to consider the voltage drop across each component separately.

This is what I don't like about VCE physics. Lots of concepts are not taught very well IMO.
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dankfrank420

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Re: VCE Physics Question Thread!
« Reply #927 on: April 10, 2015, 01:04:14 am »
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Vtot/Rtot only works if every circuit component you have in your circuit is ohmic, or V/I is constant for every device you have.
Ohm's law isn't V=IR. Ohm's law is actually an assertion that V/I is a constant and devices that satisfy this assertion are ohmic, like normal resistors.

As you can quite clearly, diodes are NOT ohmic. Hence you can't just use normal voltage divider techniques to solve this question. You have to consider the voltage drop across each component separately.


Thanks! Really cleared up some conceptual misconceptions (is that a phrase?) I had.

So moral of the story is don't treat a diode like another element in the circuit because it isn't ohmic.


Quote
This is what I don't like about VCE physics. Lots of concepts are not taught very well IMO.

I find this alot.

Most teachers simply teach you the way of getting the right answer instead of the concepts behind it. Most of the course can simply be ROTE learned or worst-still copied onto the cheat sheet.

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Re: VCE Physics Question Thread!
« Reply #928 on: April 11, 2015, 04:13:29 pm »
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Hii guys

Does anyone want some physics exams ? If yes, just inbox me your email and I'll send them !

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Re: VCE Physics Question Thread!
« Reply #929 on: April 11, 2015, 10:02:02 pm »
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For any of the detailed studies on the exam, is it possible to get 1 mark for a question if you showed workings? I'm asking because we were doing past detailed studies exams in class and our teacher and he was making us show all of our workings for the questions on the page and saying we may get method marks for the workings out if they are correct but we get an answer wrong. It kind of makes sense because they are all out of 2 but I'm still not sure and a bit confused. Even though it might just be good practice now to show workings, i don't want to waste time on the exam showing pointless steps.
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