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Author Topic: Yacoubb's Physics 3/4 Thread  (Read 12768 times)  Share 

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Yacoubb

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Yacoubb's Physics 3/4 Thread
« on: December 10, 2013, 11:26:16 pm »
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Okay, so I'm going to have PLENTY of questions for Physics next year.

First up:
* Law of Conservation of Momentum - does that state that the momentum of a system prior to a collision is equal to the momentum of a system after a collision.
* Law of Conservation of Energy - does this state that energy can only be transferred or transformed, but not created nor destroyed.

Plus, how much of 1/2 is relevant to 3/4? I've gone through the electricity and motion chapters of the Jacaranda Physics 1 textbook, made some notes. I just want to know how much of units 1 and 2 I should know for 3/4.

Thanks! :D

Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #1 on: December 10, 2013, 11:42:43 pm »
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Hi, I did Units 1/2 for Physics this year as well (in conjunction with some extension stuff), so my answers may not be entirely correct, but from my understanding:

- Law of Conversation of Momentum: yes. That law states that the sum of the initial momentums must equal the sum of the final momentums in a closed system. (which, in VCE, basically just means total initial momentum=total final momentum).
- Law of Conservation of Energy - again, same idea. So, yes, energy can only be transferred or transformed, but not created nor destroyed.

Motion and electricity are vital for Units 3/4 Physics and form at least 50% of the course. Anything else outside of this is irrelevant (e.g. at my school, Unit 1 was radioactivity, which is completely irrelevant to the Year 12 course).

Hope this helps!  :)

Thanks! :)

Yeah, I was going to add in the bit on closed systems. Probably good to throw that in just in case. Thank you!

lzxnl

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Re: Yacoubb's Physics 3/4 Thread
« Reply #2 on: December 11, 2013, 11:07:21 pm »
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A "closed" or "isolated" system is one which has no external forces acting upon it. Just a definition to keep in mind.

Hmm. Aside from motion and electricity, most of 1/2 physics didn't seem that useful IMO for 3/4. The physics course allows for rather flexible time management, so you don't need to worry about not catching up in class (:

I'm going to be picky with the conservation of energy and say that really, it's mass-energy; from relativity, mass can be converted into energy and vice-versa. If you're doing the relativity detailed study it may be important (does anyone even do that one?)
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Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #3 on: December 12, 2013, 12:04:14 am »
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A "closed" or "isolated" system is one which has no external forces acting upon it. Just a definition to keep in mind.

Hmm. Aside from motion and electricity, most of 1/2 physics didn't seem that useful IMO for 3/4. The physics course allows for rather flexible time management, so you don't need to worry about not catching up in class (:

I'm going to be picky with the conservation of energy and say that really, it's mass-energy; from relativity, mass can be converted into energy and vice-versa. If you're doing the relativity detailed study it may be important (does anyone even do that one?)

Thanks for that! So quick recap; a cloaed system is one where no external forces are acting upon it. So, the initial momentum prior to a collision is equal to the final momentum after a collision, in a closed system (i.e. one with no external forces acting on it).

The detailed study we're doing is Sound.

lzxnl

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Re: Yacoubb's Physics 3/4 Thread
« Reply #4 on: December 12, 2013, 09:03:44 am »
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Thanks for that! So quick recap; a cloaed system is one where no external forces are acting upon it. So, the initial momentum prior to a collision is equal to the final momentum after a collision, in a closed system (i.e. one with no external forces acting on it).

The detailed study we're doing is Sound.

Yeah that's right
For a collision though, note that the momenta are only equal iMMEDIATELY before and after the collision, as VCE collisions aren't quite isolated systems (cars running at each other have friction too)

For sound
You'll need to know microphone types and operating principles too. Enjoy xP
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Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #5 on: December 23, 2013, 12:41:02 pm »
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I've come across this question and I'd just like someone to please clarify if this is correct.

When a stationary car is hit from behind by another vehicle at moderate speed, headrests behind the occupants reduce the likelihood of injury. Explain in terms of Newton's laws how they do this.

Newton's First Law of Motion:
Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest prevents this jolt, and thus prevents injury.

Newton's Second Law of Motion:
Newton's Second Law of Motion states that the force is directly proportional to the product of the mass and acceleration. But I have no idea how this relates to the question.

Newton's Third Law of Motion:
Newton's Third Law of Motion states that every action has an opposite and equal reaction. The stationary car is hit, and the person jolts forwards. The opposite and equal reaction would be for the passenger to jolt backwards. The headrest limits how far back the passenger jolts backwards, and so prevents any injuries.


Could someone please go over this and help me. I'd also like to be assisted in how I could apply Newton's Laws to questions (i.e. how I should go about explaining it on exams/SACs).

Thanks

alchemy

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Re: Yacoubb's Physics 3/4 Thread
« Reply #6 on: December 23, 2013, 01:11:54 pm »
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When a stationary car is hit from behind by another vehicle at moderate speed, headrests behind the occupants reduce the likelihood of injury. Explain in terms of Newton's laws how they do this.

Newton's Second Law of Motion:
Newton's Second Law of Motion states that the force is directly proportional to the product of the mass and acceleration. But I have no idea how this relates to the question.


If you think about it, this relates to Newton's First law in this instance. In explaining Newton's First law you mentioned
"...an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result." The Second law serves as a reference for making this inference by specifying the 'non-zero' force as being "directly proportional to the product of mass and acceleration."
That's all I can understand from it. Remember that not all questions require you to quote and refer to all of Newton's laws in explaining your reasoning. However, in this question it was probably a good idea to do so.

Anyway, I don't do Physics so it would be great if someone could expand on what I've said. Hope that helped (:


psyxwar

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Re: Yacoubb's Physics 3/4 Thread
« Reply #7 on: December 23, 2013, 01:22:03 pm »
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Newton's first law is just a special case of the second law: when the net force is zero (if net force is zero, the acceleration is also zero and by extension velocity must be constant).
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Re: Yacoubb's Physics 3/4 Thread
« Reply #8 on: December 23, 2013, 04:24:47 pm »
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I've come across this question and I'd just like someone to please clarify if this is correct.

When a stationary car is hit from behind by another vehicle at moderate speed, headrests behind the occupants reduce the likelihood of injury. Explain in terms of Newton's laws how they do this.

Newton's First Law of Motion:
Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest prevents this jolt, and thus prevents injury.

Newton's Second Law of Motion:
Newton's Second Law of Motion states that the force is directly proportional to the product of the mass and acceleration. But I have no idea how this relates to the question.

Newton's Third Law of Motion:
Newton's Third Law of Motion states that every action has an opposite and equal reaction. The stationary car is hit, and the person jolts forwards. The opposite and equal reaction would be for the passenger to jolt backwards. The headrest limits how far back the passenger jolts backwards, and so prevents any injuries.


Could someone please go over this and help me. I'd also like to be assisted in how I could apply Newton's Laws to questions (i.e. how I should go about explaining it on exams/SACs).

Thanks

Correct me if I'm wrong here (which I likely am); but I don't think it's necessary to restate the law and their properties. Especially in exam conditions -- do you? Otherwise, I would've just used key terms (as my former physics teacher would suggest) to demonstrate your understanding. For example,  I would have wrote Newton's First Law's explanation would be something like this:
    "Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest acts against the movement of the head as it is in a state of inertia.Thus it prevents injury. "

In another light, Newton's Second Law pertains to the behaviour of objects having a netforce when force is unbalanced. In this case, it's all about the net force. Thus, the 'head rest provides support against the movement of the head as it now has a force. Headrests absorb the force. (or something similar)
I actually have no idea what I'm saying or talking about.

alchemy

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Re: Yacoubb's Physics 3/4 Thread
« Reply #9 on: December 23, 2013, 04:31:46 pm »
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Correct me if I'm wrong here (which I likely am); but I don't think it's necessary to restate the law and their properties. Especially in exam conditions -- do you? Otherwise, I would've just used key terms (as my former physics teacher would suggest) to demonstrate your understanding. For example,  I would have wrote Newton's First Law's explanation would be something like this:
    "Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest acts against the movement of the head as it is in a state of inertia.Thus it prevents injury. "


Well, that's not wrong by the look of it. It's just that I've also heard it's a good idea to mention key words such as "Newton's _ Law" as assessors can be quite pedantic.

Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #10 on: December 23, 2013, 06:14:57 pm »
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If a ball is being dropped, would we say that the normal reaction force is less than the weight force? And when the ball is thrown, normal reaction force is greater than the weight component.

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Re: Yacoubb's Physics 3/4 Thread
« Reply #11 on: December 23, 2013, 07:04:17 pm »
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If a ball is being dropped, would we say that the normal reaction force is less than the weight force? And when the ball is thrown, normal reaction force is greater than the weight component.
Do you mean while the ball is in flight? There is no normal force while the ball is in flight. The only forces acting on the ball are gravity and air resistance (although air resistance is usually ignored in VCE physics).
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Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #12 on: December 23, 2013, 07:06:55 pm »
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Do you mean while the ball is in flight? There is no normal force while the ball is in flight. The only forces acting on the ball are gravity and air resistance (although air resistance is usually ignored in VCE physics).

Ah fair enough, thanks. So whilst the ball is in flight, gravity and weight components act. The force of gravity is greater than the force of weight when the ball is being dropped, and the force of weight > gravity when the ball is thrown up. Thank you :)

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Re: Yacoubb's Physics 3/4 Thread
« Reply #13 on: December 23, 2013, 07:10:09 pm »
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Ah fair enough, thanks. So whilst the ball is in flight, gravity and weight components act. The force of gravity is greater than the force of weight when the ball is being dropped, and the force of weight > gravity when the ball is thrown up. Thank you :)
Weight is the force of gravity. It is a constant downward force, both when you drop or throw the ball.
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Yacoubb

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Re: Yacoubb's Physics 3/4 Thread
« Reply #14 on: December 23, 2013, 07:14:01 pm »
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Weight is the force of gravity. It is a constant downward force, both when you drop or throw the ball.

Sorry I'm so confused :s. So the downwards force is weight (mass x gravitational field strength). What is the upwards force acting on the ball?

Also, does anyone know how I can obtain worked solutions for the Jacaranda Physics 2 textbook?