VCE Physics Question Thread!

[NerdyPi]

Hi guys

I have a question regarding air resistance in projectile motion;

If you were to describe the air resistance (to quote the VCAA study design "a qualitative description of the effects of air resistance"), would it be correct to say that the force of air resistance in the vertical direction is increasing as the object accelerates downwards (assuming this is before it reaches terminal velocity), but that the air resistance in the horizontal direction is constant, as there is no acceleration?
 If not, could someone please clear by my misunderstanding...
Thanks

[Syndicate]

Hi guys

I have a question regarding air resistance in projectile motion;

If you were to describe the air resistance (to quote the VCAA study design "a qualitative description of the effects of air resistance"), would it be correct to say that the force of air resistance in the vertical direction is increasing as the object accelerates downwards (assuming this is before it reaches terminal velocity), but that the air resistance in the horizontal direction is constant, as there is no acceleration?
 If not, could someone please clear by my misunderstanding...
Thanks

1) You are absolutely correct about your first point (due to the acceleration of gravity, the air resistance would increase vertically). The gravitational force is "stronger" than air resistance, which causes an increase in air resistance, as the object tends to accelerate faster vertically (until reaching its terminal velocity).

2) Air resistance would decrease horizontally, as there would be acceleration (as the projectile tends to slow down). If you think about it... when you throw a ball, it would start with a maximum force, and then gradually start decreasing as it goes further away from you. As it slows down, there would be a decrease in air resistance.

[NerdyPi]

1) You are absolutely correct about your first point (due to the acceleration of gravity, the air resistance would increase vertically). The gravitational force is "stronger" than air resistance, which causes an increase in air resistance, as the object tends to accelerate faster vertically (until reaching its terminal velocity).

2) Air resistance would decrease horizontally, as there would be acceleration (as the projectile tends to slow down). If you think about it... when you throw a ball, it would start with a maximum force, and then gradually start decreasing as it goes further away from you. As it slows down, there would be a decrease in air resistance.

Ah, makes sense. Thanks you again

[lzxnl]

Two features of air resistance:

1. Its direction is against the direction of motion of the object in consideration.
2. Its magnitude increases with object speed (exact dependence varies; can be linear, quadratic in speed)

So, as you throw something, initially there'll be a horizontal air resistance. However, over time the horizontal air resistance will reduce the horizontal velocity to zero and the velocity will be purely vertical. Also, as the air resistance depends on velocity, as the speed increases, so does the air resistance.

[Maz]

hey,
I'm trying to calculate the gravitational and electrostatic force between two electrons neutrons  that
are 1 x 10^-15m apart...however my answer isn't matching up with the book...can someone please help me?
the answer in th book is 0 for electrostatic and 2x10_-34 N for gravitational...

thankyou sooo much in advance 

[Syndicate]

hey,
I'm trying to calculate the gravitational and electrostatic force between two electrons neutrons  that
are 1 x 10^-15m apart...however my answer isn't matching up with the book...can someone please help me?
the answer in th book is 0 for electrostatic and 2x10_-34 N for gravitational...

thankyou sooo much in advance 

well, the electrostatic force between the neutrons is 0, as they don't have any charge, therefore, no attraction/repulsion between them. To work out the gravitational force between the two neutrons, you can easily use the formula: F = Gm^2/r^2

where:
G = 6.67*10^-11
m = 1.6749 X 10^-24 (mass of a neutron)
r = 1x 10^-15

I believe you can do the rest

[Maz]

well, the electrostatic force between the neutrons is 0, as they don't have any charge, therefore, no attraction/repulsion between them. To work out the gravitational force between the two neutrons, you can easily use the formula: F = Gm^2/r^2

where:
G = 6.67*10^-11
m = 1.6749 X 10^-24 (mass of a neutron)
r = 1x 10^-15

I believe you can do the rest

thanks...thats what i thought you did...turns out the answers in my book are wrong
thank you again 

[Maz]

Hi humans, can someone please help me on this?
It's on the standard model
An electron and a positron undergo pair annihilation. if they initially had no kinetic energy, what is the energy of each gamma produced by the annihilation? why must there be 2 gamma rays produced?
thankyou so much in advance

[lzxnl]

Hi humans, can someone please help me on this?
It's on the standard model
An electron and a positron undergo pair annihilation. if they initially had no kinetic energy, what is the energy of each gamma produced by the annihilation? why must there be 2 gamma rays produced?
thankyou so much in advance

Two gamma rays to conserve momentum. If you only have one gamma ray, you have momentum coming from a system of two stationary particles.
Energy of each gamma ray must be the same (so that their momentums cancel). You can work this out from total energy = rest energy of essentially two electrons

[Maz]

Thankyou so much 

[Maz]

hi,
we have a test on 'the standard model' coming up, and i was wandering if you could please help me with this question?
when a muon and an anti-muon collide they can annihilate each other and release their mass-energy as 2 photons. assuming that these two photons are identical,
a) what will each of their energies be
b) what wavelength will they have
c) why does there need to be 2 photons produced and not just one?
d) in what directions would they have to travel relative to each other and why?
e) in what part of the electromagnetic spectrum are they located?
i don't have any answers to these so i even the ones that i have attempted, i don't know if the answer is right or wrong

thankyou soooo much in advance

[lzxnl]

hi,
we have a test on 'the standard model' coming up, and i was wandering if you could please help me with this question?
when a muon and an anti-muon collide they can annihilate each other and release their mass-energy as 2 photons. assuming that these two photons are identical,
a) what will each of their energies be
b) what wavelength will they have
c) why does there need to be 2 photons produced and not just one?
d) in what directions would they have to travel relative to each other and why?
e) in what part of the electromagnetic spectrum are they located?
i don't have any answers to these so i even the ones that i have attempted, i don't know if the answer is right or wrong

thankyou soooo much in advance

a. Find mass of a muon. Convert total mass to an energy. Divide this energy amongst two photons evenly.
b. Find wavelength from E = hc/wavelength
c. Answered above
d. Opposite each other, conserve momentum
e. Depends on wavelength. Look up anything online that has a table of these

[shlblk]

Hi,
Can someone please explain to me (preferably with formulas) why a ball thrown at a 45 degree angle will travel further than one thrown horizontally (0 degrees) if launch speed is the same? I want to know this for my physics investigation.
I was trying to see how it works with the range formula, but since sin0 is 0, I get no solution.

Thanks!

[lzxnl]

Hi,
Can someone please explain to me (preferably with formulas) why a ball thrown at a 45 degree angle will travel further than one thrown horizontally (0 degrees) if launch speed is the same? I want to know this for my physics investigation.
I was trying to see how it works with the range formula, but since sin0 is 0, I get no solution.

Thanks!

Actually that is a solution. As sin 0 is 0, that means it won't travel anywhere.

Remember how the range formula works. You're starting and finishing at the same height. Which means you're tossing something parallel to the ground...while you're on the ground. So as you never leave the ground, your motion never starts.

[shlblk]

Actually that is a solution. As sin 0 is 0, that means it won't travel anywhere.

Remember how the range formula works. You're starting and finishing at the same height. Which means you're tossing something parallel to the ground...while you're on the ground. So as you never leave the ground, your motion never starts.

Ok, that makes sense. Could you then please explain to me why from a particular height, a ball thrown at a 45 angle will go further than one thrown horizontally, and why a horizontally thrown ball travels further than one thrown at say 30 or 60? Thanks.