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The Proof is Trivial physics edition?

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Reply 20
Original post by langlitz
I have no idea what M1 is but yeah sorry your solution is correct


M1 is the first mechanics module in A level maths. This could have been copied out of my textbook :wink:

Explain why a glass prism can separate white light.
Original post by lerjj
M1 is the first mechanics module in A level maths. This could have been copied out of my textbook :wink:

Explain why a glass prism can separate white light.


I wrote this my self in first year for an online thing we had to do where you make questions and everyone answers and rates it. You wouldn't believe how many people got it wrong haha
A sphere of mass 3 kg is rolling along a surface (without slipping). It moves with a constant velocity of 2 ms-1 until it reaches a ledge and lands in a fluid where all of its energy is converted into rotational energy and it stays spinning in the fluid. Given the radius of the sphere is 7 cm, calculate the angular velocity of the sphere after it lands in the fluid and which direction the angular velocity vector points (into screen/out of screen)
Reply 23
Original post by langlitz
I wrote this my self in first year for an online thing we had to do where you make questions and everyone answers and rates it. You wouldn't believe how many people got it wrong haha


Looks simple so you don't do it properly...

Here's a relatively simple one: I connect a buffer resistor and a small piece of silicon in series with a DC battery. Describe and justify how the electrons behave differently in each component.
Reply 24
Original post by langlitz
(..)


Is the height of the drop negligible?
Reply 25
Original post by langlitz
A sphere of mass 3 kg is rolling along a surface (without slipping). It moves with a constant velocity of 2 ms-1 until it reaches a ledge and lands in a fluid where all of its energy is converted into rotational energy and it stays spinning in the fluid. Given the radius of the sphere is 7 cm, calculate the angular velocity of the sphere after it lands in the fluid and which direction the angular velocity vector points (into screen/out of screen)


Is it cheating to look up the kinetic energy of a rotating sphere? Because I do not know it/a sphere's moment of inertia off the top of my head and my calculus isn't too good.

Not sure about direction. (Don't post a hint though!)
Reply 26
When a proton, p, collides at sufficiently high energy with another proton, a proton anti-proton pair can be created in addition to the original protons.

a). Give the equation for this interaction

b). How much addition energy is required to create these particles?
Reply 27
Original post by langlitz
A sphere of mass 3 kg is rolling along a surface (without slipping). It moves with a constant velocity of 2 ms-1 until it reaches a ledge and lands in a fluid where all of its energy is converted into rotational energy and it stays spinning in the fluid. Given the radius of the sphere is 7 cm, calculate the angular velocity of the sphere after it lands in the fluid and which direction the angular velocity vector points (into screen/out of screen)


1st attempt

Original post by lerjj
Is it cheating to look up the kinetic energy of a rotating sphere? Because I do not know it/a sphere's moment of inertia off the top of my head and my calculus isn't too good.

Not sure about direction. (Don't post a hint though!)


Moment of inertia of a sphere rotating around its centre is 2/5mr^2
Original post by Phichi
Is the height of the drop negligible?


Yeah you can disregard the drop
Reply 30
Original post by lerjj

1st attempt



I think the idea of the force acting as it hits the surface is a bit ambiguous, there is a lot more considerations to be made. But you could just ignore it for this question, as all the energy is transferred into rotational energy, and we can't make the assumption it'll rotate in the other direction. The direction though is correct.

Ill let you finish it off lerjj :smile:
(edited 9 years ago)
Reply 31
Original post by Phichi
I think the idea of the force acting as it hits the surface is a bit ambiguous, there is a lot more considerations to be made. But you could just ignore it for this question, as all the energy is transferred into rotational energy, and we can't make the assumption it'll rotate in the other direction. The direction though is correct.


Assume the force is always to the left and is to some function so that it always brings the translational kinetic energy to zero- for all the time that the ball is up to half-submerged, the force causes a clockwise rotation because it acts only on the bottom half. Once the ball is fully submerged, the fluid exerts a force to the left over the entire surface of the sphere and so doesn't cause any rotation. Although this makes some assumptions about the nature of the force, I think this is a reasonable analysis.
Original post by Phichi
When a proton, p, collides at sufficiently high energy with another proton, a proton anti-proton pair can be created in addition to the original protons.

a). Give the equation for this interaction

b). How much addition energy is required to create these particles?


a) p + p --> p + p + p + p(bar)
b) 1876 MeV
Reply 33
Original post by lerjj
Assume the force is always to the left and is to some function so that it always brings the translational kinetic energy to zero- for all the time that the ball is up to half-submerged, the force causes a clockwise rotation because it acts only on the bottom half. Once the ball is fully submerged, the fluid exerts a force to the left over the entire surface of the sphere and so doesn't cause any rotation. Although this makes some assumptions about the nature of the force, I think this is a reasonable analysis.


Ignore the force, it's just a particular problem that the translational K.E becomes zero. In reality the sphere is initially rotating clockwise, so the instantaneous direction of motion of the very bottom of the sphere before it hits the fluid is to the left, the fluid will cause a viscous torque in opposition direction. Can't mention the force really here.
Reply 34
Original post by langlitz
Moment of inertia of a sphere rotating around its centre is 2/5mr^2


then:
1/2Iω2=1/2Iω02+1/2mv2[br]2/5mr2ω2=2/5mr2ω02+mv2 1/2 I \omega ^2 = 1/2 I \omega_0^2 +1/2mv^2[br]2/5 mr^2 \omega^2=2/5mr^2\omega_0^2+mv^2

Since the ball travels a distane 2*pi*r for every 2*pi rotation origionally, we can write:
v=ω0r[br]2/5r2ω2=2/5r2ω02+ω02r2[br]ω2=7/2ω02=7v22r2[br] v=\omega_0 r[br]2/5 r^2 \omega^2 = 2/5 r^2 \omega_0^2 + \omega_0^2 r^2[br]\omega ^2=7/2 \omega_0^2 = \dfrac{7v^2}{2r^2}[br]

After actually substituting values I get: 53 rad/sec? that seems too fast...
(edited 9 years ago)
Reply 35
Original post by langlitz
a) p + p --> p + p + p + p(bar)
b) 1876 MeV


Second part is wrong.

Hint: four momentum
Original post by lerjj
then:
1/2Iω2=1/2Iω02+1/2mv2[br]2/5mr2ω2=2/5mr2ω02+mv2 1/2 I \omega ^2 = 1/2 I \omega_0^2 +1/2mv^2[br]2/5 mr^2 \omega^2=2/5mr^2\omega_0^2+mv^2

Since the ball travels a distane 2*pi*r for every 2*pi rotation origionally, we can write:
v=ω0r[br]2/5r2ω2=2/5r2ω02+ω02r2[br]ω2=7/2ω02=7v22r2[br] v=\omega_0 r[br]2/5 r^2 \omega^2 = 2/5 r^2 \omega_0^2 + \omega_0^2 r^2[br]\omega ^2=7/2 \omega_0^2 = \dfrac{7v^2}{2r^2}[br]

After actually substituting values I get: 53 rad/sec? that seems too fast...


Yep that's what I got :smile: If you consider that the final answer doesn't depend the mass, only on the inital velocity and the radius of the ball. Since the ball is travelling at 2 m/s when the radius is only 0.07 m then it spins at 4.55 revs/sec.
In the fluid it is 53 rad/sec which is 8.43 revs/sec so it isn't that much of an increase really
(edited 9 years ago)
Reply 37
Original post by lerjj

Explain why a glass prism can separate white light.


The real question is why does light slow down in an optically dense material :wink:?

Should I make a new thread shortly for this? A formal one? And hope it gets stickied.
(edited 9 years ago)
Reply 38
Original post by langlitz
Yep that's what I got :smile: If you consider that the final answer doesn't depend the mass, only on the inital velocity and the radius of the ball. Since the ball is travelling at 2 m/s when the radius is only 0.07 cm then it spins at 4.55 revs/sec.
In the fluid it is 53 rad/sec which is 8.43 revs/sec so it isn't that much of an increase really


Okay, I couldn't see what could have gone wrong since the thing making it that high was simply the radius being so small. Good question!
Reply 39
Original post by lerjj
Okay, I couldn't see what could have gone wrong since the thing making it that high was simply the radius being so small. Good question!


Do you plan on doing M4 and M5 at A-Level? If you're interested in moments of interia and such, it's good fun. It's not as hard as made out to be, I personally found relative motion the hardest thing. Was beneficial when I started my degree.

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