Physics Marathon 2012 (1 Viewer)

SpiralFlex · Mar 7, 2012

HOWDY PARTNERS!

I have come to start a Physics Marathon! The purpose of this thread is to have fun and learn. This is done by asking questions, posting questions, posting solutions or socialising.

May the penguins be with you and without further to do, I shall begin with a few question and leave this thread!

[HR][/HR]Space[HR][/HR]

$$Identify TWO features from the diagram of the Space Shuttle above that aid in the safe re-entry and landing of delicious humans.$\;\;\;\;\;\fbox{2}$

$$Discuss how the features identified in the previous part, in conjunction with others built into the Space Shuttle will make re-entry safer.$\;\;\;\;\;\fbox{4}$

[HR][/HR]

$$A weight W is suspended from a ceiling by inextensible strings as shown. String OA makes an angle of theta with the vertical and OB makes the same angle theta with the ceiling. Find an expression for tension OB.$\;\;\;\;\;\fbox{3}$

[HR][/HR]

$The diagram below shows a conical pendulum with a mass of m swinging with a constant speed in the centre of the circle C. The semi-vertical angle is theta as shown.$

$$On the diagram mark the forces acting on m. Hence derive an expression for the force P, which is keeping the body moving in a circular plane.$ \;\;\;\;\;\fbox{3}$

[HR][/HR]Motors and Generators[HR][/HR]

$Consider a square, single coil of wire (ABCD) of side length 0.3 m which is free to rotate about its central axis and placed in a uniform magnetic field of 81mT. The diagram shows this looking downwards onto the plane of the coil.$

$When the coil is connected to an external circuit and a current is passed through it, the coil is held stationary when a mass of 5 grams is attached to the point P so that the weight for is directed into the page.$

$$What is the magnitude and direction of current that must flow in the coil to just balance the mass at the instant shown in the diagram? Show all working.$\;\;\;\;\;\fbox{4}$

[HR][/HR]

$Two coils of wire are situated near each other as shown. Coil A is connected in series with a source of AC current, while coil B has a small lamp in series connection.$

$$Explain why the lamp connected to coil B glowed when circuit A was switched on.$\;\;\;\;\;\fbox{2}$

$A soft piece of iron in the shape of a rod is passed through coil A and coil B. The following observations were made during the experiment:$

$- The rod became warmer.$

$- The lamp glow became brighter.$

$$Describe and account for these two phenomena.$\;\;\;\;\;\fbox{2}$

[HR][/HR]From Ideas to Implementation
[HR][/HR]
$In a vacuum a beam of electrons with a horizontal velocity of$ \;3 *10^7 \textup{ms}^{-1} \;$enters midway into a region of the electric field between two horizontal square metal plates. The length of the sides of the plates is$ 10 \;\textup{cm}. $A potential difference of 320V is applied across the plates and the separation between them is$ \; 1.6 \; \textup{cm}.$

$$Find the electric field strength between the plates.$\;\;\;\;\;\fbox{2}$

$$A electron beam reaches on of the plates at a distance d from the plates left edge. Find the distance d, by first proving the path of the electron beam is parabolic and sketch the path of the electron beam between the plates. (Assume only an electric force is acting on the electron.)$\; \; \;\;\;\;\;\;\;\;\;\;\;\;\;\fbox{5}$

$$Deduce the corresponding distance, d, if the applied potential difference is doubled.$\;\;\;\;\fbox{2}$

[HR][/HR]Note: I will steal, I mean make* a proper logo later.

After HYs.

I will also leave you guys to it, I am burnt out after making this. *dies*[HR][/HR]

SpiralFlex · Mar 7, 2012

Aww no one wants to play with Spiral again. //Dies of loneliness.

nightweaver066 · Mar 7, 2012

I'll join the fun soon, need to get some work done.

SpiralFlex · Mar 7, 2012

Okay. Spiral is still lonely.

IamBread · Mar 7, 2012

I'd play with you but I has my own homework to do

zeebobDD · Mar 7, 2012

I SHALL DO A QUESTIONNNN

zeebobDD · Mar 7, 2012

SPACE:

1) Two features on the diagram that help aid the in safe re-entry are (a) and (b),

(a) is the nose of the spaceshuttle
(b) is the black tiling made of seramic i belive

2) The nose of the space ship is considered useful in the space shuttle due to the shape it creates for the shuttle. Since the nose of the spaceshuttle will make it a conical shape, the heat that the Space shuttle absorbs as it accelerates back into the earth will be spread out evenly which creates the nose of the plane not to over heat, This in conjuction with the black carbon- carbon tiles at the bottom, this is done because when the Space shuttle is re entering the earth it gets heated to degree's exceed 1260 degree's. This Black tiling acts as an insulation device to prevent the space shuttle from overheating, which can cause the space shuttle to blow up.

NOT THE BEST ANSWER BUT its decent

SpiralFlex · Mar 8, 2012

Lonely...Spiral's lonely.

Carrotsticks · Mar 9, 2012

Question in my Fundamentals of Physics lecture:

A very thin (weightless) bag of water is held from a string.

It is then dipped into a tub of water.

What is the tension on the string?

nightweaver066 · Mar 11, 2012

Space:
2. I'm unsure if i'm supposed to take AOB as a right angled triangle or not..

costheta = W/T_{OA}

T_{OA} = W/costheta

tantheta = T_{OA}/T_{OB}

T_{OB} = W/(costheta x tantheta)

= Wcosectheta N

Motors and Generators:

1. Weight force at point P = 0.005 x 9.8 = 0.049N in to the page

Reaction force required to balance it = 0.049N out of the page

F = BIL

0.049 = 81 x 10^{-3} x I x 0.3

I = 2.02A anti-clockwise

2. a) As circuit A has AC current, there is a constantly changing magnetic flux which by Faraday's law, induces an EMF in coil B and hence a current lighting the lamp.

b) The constant change in magnetic flux, by Faraday's law, induces an EMF in the soft iron rod and hence eddy currents as it is a bulk body which oppose the change in magnetic flux (Lenz's law). The resistance of the iron rod causes the eddy currents to warm up the rod.

The addition of the iron rod amplifies the magnetic field strength which subsequently increases the change in magnetic flux. As the magnitude of an induced EMF is dependent upon the rate of change of magnetic flux (Faraday's law), there will be an even greater EMF in coil B forming greater current so the lamp will glow brighter.

Relativity!

Parvee · Mar 31, 2012

I think this needs a bump
~bump~

nightweaver066 · Apr 12, 2012

$Ideas to Implementation$

$1. E = \frac{V}{d} = \frac{320}{0.016}$

$=20000V/m$

$2. d = u_x t$

$t = \frac{d}{u_x}$

$$Using $ y = u_yt + \frac{1}{2}a_y t^2$

$= \frac{u_y}{u_x}d + \frac{a_y}{2u_x^2}d^2$

$\equiv ax^2 + bx + c$

$\therefore $ Parabolic trajectory$$
$0.008 = \frac{a_y}{2(3.7 \times 10^7)^2}d^2$

$d = \sqrt{\frac{1.6(3.7 \times 10^7)^2}{a_y}}$

$F = qE$

$= (1.602 \times 10^{-19} \times 20000)N$

$F = ma$

$(1.602 \times 10^{-19} \times 20000) = (9.109 \times 10^{-31})a$

$a = \frac{1.602 \times 10^{-19} \times 20000}{9.109 \times 10^{-31}}$

$\therefore d = 0.0789m$

$3. d = 0.0558m$

lol. I answered pretty much all of the questions..

barbernator · Apr 12, 2012

Motors and Generators question

W=mg
T=fd
W=0.005x9.8
= 0.0049N
T=0.15x0.0049=0.00735Nm clockwise.

There must be torque equilibrium for the mass to be balanced

T=BIA
0.00735=0.081xIx0.3x0.3
I=0.00735/0.3x0.3x0.081
=1.01 A from D to C

bleakarcher · Apr 12, 2012

Dude, for the weight question I got Wsin(theta)?

IamBread · Apr 17, 2012

Here's an interesting question that I thought of, I don't know if it's part of the syllabus (probably not) but it would help with some understanding.

As an objects velocity increases , it's mass dilates. $m_v = \frac{m_o}{\sqrt{1-\frac{v^2}{c^2}}}$ .
Meaning, as $v -> c, m ->\infty$ . Why is it that this increase in mass does not form a black hole?

bleakarcher · Apr 17, 2012

I'm not 100% sure with this answer as I've done close to nothing in this part of the year 12 space topic. But as v->infinity not only does m->infinity but from the length contraction equation l->infinity also. The key to creating a black hole is to trap an infinitely large mass within an extremely small volume. In this case, this is not what is done since as v->c, m->infinity and l->infinity (meaning volume is increasing to infinity). Hence, we do not get a black hole.

This is just a guess.

nightweaver066 · Apr 17, 2012

bleakarcher said:
I'm not 100% sure with this answer as I've done close to nothing in this part of the year 12 space topic. But as v->infinity not only does m->infinity but from the length contraction equation l->infinity also. The key to creating a black hole is to trap is to take an infinitely large mass within an extremely small volume. In this case, this is not what is done since as v->c, m->infinity and l->infinity (meaning volume is increasing to infinity). Hence, we do not get a black hole.

This is just a guess.

Length contraction:
v -> c, length of object in direction of motion -> 0

bleakarcher · Apr 17, 2012

^ so whats the equation for length contraction, I thought it was exactly the same?

deswa1 · Apr 17, 2012

Note, a black hole does not have an infinite mass or infinite density. It just has to have a high enough mass to make the escape velocity greater than c.

nightweaver066 · Apr 17, 2012

bleakarcher said:
^ so whats the equation for length contraction, I thought it was exactly the same?

$L_v = L_o\sqrt{1 - \frac{v^2}{c^2}}$

where Lv is relativistic length and Lo is proper length.

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