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September 21, 2019, 08:54:47 pm

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Bri MT

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Physics misunderstandings
« on: November 23, 2018, 11:52:30 pm »
+8
To help foster understanding, this thread will contain misunderstandings and it's your job to either:

- state a misunderstanding (use italics to reduce confusion)
- explain what the truth actually is (at a VCE level)

For example:
User1: a book on a table is stationary because there are no forces acting on it
User2: There are forces acting on the book, but because they are equal in magnitude and opposite in direction they cancel out, which results in no net force and no acceleration (this is why it stays stationary). Specifically, these forces are weight (force due to gravity) and the normal force. If we remove the normal force by sliding the book off the table, its weight will result in the book falling.

Some misunderstandings to start us off:
- the moon doesn't crash into the earth because there is no gravity in space
- if I unplug an extension cord quickly from a powerpoint and plug it into itself it'll store electricity forever
- in circular motion, an object is pushed away from the center of a circle

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Re: Physics misunderstandings
« Reply #1 on: December 16, 2018, 10:16:49 am »
+4
the moon doesn't crash into the earth because there is no gravity in space
The net force on the moon, which supplies its centripetal force, can be approximated by the gravitational force by the Earth on the moon. This force is directed towards the centre of the Earth, which is also the centre of the moon's orbit. Thus, the moon is in uniform centripetal motion around the Earth.
2017: Mathematical Methods [48] | Chinese (SLA) [41]
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lzxnl

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Re: Physics misunderstandings
« Reply #2 on: December 20, 2018, 07:49:43 pm »
+5
the moon doesn't crash into the earth because there is no gravity in space
The net force on the moon, which supplies its centripetal force, can be approximated by the gravitational force by the Earth on the moon. This force is directed towards the centre of the Earth, which is also the centre of the moon's orbit. Thus, the moon is in uniform centripetal motion around the Earth.

I'm going to be incredibly pedantic about this one: (don't take this personally; university makes people pedantic because you lose marks otherwise)

The moon doesn't crash into the Earth not just because the centripetal force is supplied by the Earth, BUT also because the moon's perihelion (closest point to Earth) is more than one Moon radius away from the surface of the Earth. This sounds like a trivial point, but for obvious reasons, if the moon came too close to the Earth, then the Earth's gravitational force would no longer stop the Moon from hitting the Earth. Indeed, gravity would then accelerate the Moon towards the Earth and actually increases the explosiveness of any collision.

Also, just because the force is directed radially inwards, does NOT mean there is uniform circular motion. There is a direction AND a force requirement; if the force magnitude isn't right, the Moon's distance from the Earth will change.

If I unplug an extension cord quickly from a powerpoint and plug it into itself it'll store electricity forever
The extension cord doesn't store energy. It just transmits energy; the energy comes from the wall, so if you unplug it from the energy source, you're not getting any electricity.

In circular motion, an object is pushed away from the center of a circle
It may feel like it, but in circular motion, objects are pulled towards the center of the circle. Imagine you're travelling in a car about to turn in a clockwise circle, and you're currently at the 9 o'clock position. You're going up, but you're being pulled to the right to move clockwise. Where does the feeling of a force going to the left come from?

Well, as the car turns, you don't turn immediately. Newton's first law says that before any force acts on you, you still want to move up. Yet the car turns right. Therefore, from your perspective, you're moving to the left inside the car and get squished against the left door of the car. Normally, when doors push on us, we interpret this as being pushed onto the door, so being squished against the left door of the car feels like something from the right is pushing you into the door. Except, of course, nothing is actually pushing you.

Here are some of mine:
If you're on a plane, you can put a wind-turbine on the plane to get free energy from the relative motion of the air against the plane.

If you jump out of a plane, you're going to be flung backwards really quickly because you're no longer flying on the plane.

If you hear two balls hitting each other, the lost energy is mostly attributed to sound and heat. (I am referring to one of these not being important)

You get heavier as you move faster. This is a pet peeve of mine.
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2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

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Re: Physics misunderstandings
« Reply #3 on: December 22, 2018, 10:20:41 pm »
+1
I'm going to be incredibly pedantic about this one: (don't take this personally; university makes people pedantic because you lose marks otherwise)

The moon doesn't crash into the Earth not just because the centripetal force is supplied by the Earth, BUT also because the moon's perihelion (closest point to Earth) is more than one Moon radius away from the surface of the Earth. This sounds like a trivial point, but for obvious reasons, if the moon came too close to the Earth, then the Earth's gravitational force would no longer stop the Moon from hitting the Earth. Indeed, gravity would then accelerate the Moon towards the Earth and actually increases the explosiveness of any collision.

Also, just because the force is directed radially inwards, does NOT mean there is uniform circular motion. There is a direction AND a force requirement; if the force magnitude isn't right, the Moon's distance from the Earth will change.

If I unplug an extension cord quickly from a powerpoint and plug it into itself it'll store electricity forever
The extension cord doesn't store energy. It just transmits energy; the energy comes from the wall, so if you unplug it from the energy source, you're not getting any electricity.

In circular motion, an object is pushed away from the center of a circle
It may feel like it, but in circular motion, objects are pulled towards the center of the circle. Imagine you're travelling in a car about to turn in a clockwise circle, and you're currently at the 9 o'clock position. You're going up, but you're being pulled to the right to move clockwise. Where does the feeling of a force going to the left come from?

Well, as the car turns, you don't turn immediately. Newton's first law says that before any force acts on you, you still want to move up. Yet the car turns right. Therefore, from your perspective, you're moving to the left inside the car and get squished against the left door of the car. Normally, when doors push on us, we interpret this as being pushed onto the door, so being squished against the left door of the car feels like something from the right is pushing you into the door. Except, of course, nothing is actually pushing you.

Here are some of mine:
If you're on a plane, you can put a wind-turbine on the plane to get free energy from the relative motion of the air against the plane.

If you jump out of a plane, you're going to be flung backwards really quickly because you're no longer flying on the plane.

If you hear two balls hitting each other, the lost energy is mostly attributed to sound and heat. (I am referring to one of these not being important)

You get heavier as you move faster. This is a pet peeve of mine.

hahahaha thought we are keeping things at a VCE physics level... but nice, very interesting!
2017: Mathematical Methods [48] | Chinese (SLA) [41]
2018: English (EAL) [50] | Chemistry [50] | Physics [50] | Specialist Mathematics [48]
ATAR: 99.95
UMAT: 100
2018 Australian Physics Olympiad Summer School participant

2019: BMedSci/MD @ Monash
Looking to tutor in 2019!

Bri MT

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Re: Physics misunderstandings
« Reply #4 on: December 23, 2018, 06:07:41 am »
0
hahahaha thought we are keeping things at a VCE physics level... but nice, very interesting!

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Re: Physics misunderstandings
« Reply #5 on: December 27, 2018, 10:31:02 am »
+1
Sorry, I hope you aren't offended. I'm just saying we don't really take the apsis in an orbit into account in answering typical vce questions, although it is very important. Please do more explanations. They're quite a fun read.
2017: Mathematical Methods [48] | Chinese (SLA) [41]
2018: English (EAL) [50] | Chemistry [50] | Physics [50] | Specialist Mathematics [48]
ATAR: 99.95
UMAT: 100
2018 Australian Physics Olympiad Summer School participant

2019: BMedSci/MD @ Monash
Looking to tutor in 2019!

Bri MT

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Re: Physics misunderstandings
« Reply #6 on: December 28, 2018, 12:18:55 pm »
0
Sorry, I hope you aren't offended. I'm just saying we don't really take the apsis in an orbit into account in answering typical vce questions, although it is very important. Please do more explanations. They're quite a fun read.

I'm definitely not offended at all  and although I can't speak for lzxnl (who wrote those explanations) I doubt they would be either. Although terms like perihelion aren't really VCE level, imo all the concepts are at VCE level, so I was curious about what you thought was outside the scope of the study design.

There'll be more explanations coming in the future (although I can't guarantee they'll be at lxznl's level of quality), but for now I'll put out another misunderstanding:

If a bike rider is going around a banked track in a perfect circle,  they're experiencing a net force diagonally down and towards the center
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lzxnl

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Re: Physics misunderstandings
« Reply #7 on: December 28, 2018, 12:44:54 pm »
0
Sorry, I hope you aren't offended. I'm just saying we don't really take the apsis in an orbit into account in answering typical vce questions, although it is very important. Please do more explanations. They're quite a fun read.
I'm not offended as such; I just thought it was surprising and slightly amusing that someone thought my explanation was beyond the scope of the VCE course. I mean, technically in VCE we do indeed only consider circular orbits, but it's not beyond the average VCE student to think about what happens in an elliptical orbit too. If anything, you're offending the typical VCE student than you are offending me

I'm definitely not offended at all  and although I can't speak for lzxnl (who wrote those explanations) I doubt they would be either. Although terms like perihelion aren't really VCE level, imo all the concepts are at VCE level, so I was curious about what you thought was outside the scope of the study design.

There'll be more explanations coming in the future (although I can't guarantee they'll be at lxznl's level of quality), but for now I'll put out another misunderstanding:

If a bike rider is going around a banked track in a perfect circle,  they're experiencing a net force diagonally down and towards the center
You're also assuming the bike rider is moving at a constant speed, otherwise there is a force component tangential to the motion too.
2012
Mathematical Methods (50) Chinese SL (45~52)

2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

2014-2016: University of Melbourne, Bachelor of Science, Diploma in Mathematical Sciences (Applied Maths)

2017-2018: Master of Science (Applied Mathematics)

2019-:

Accepting students for  VCE tutoring in Maths Methods, Specialist Maths and Physics! PM for more details