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January 29, 2020, 10:00:16 pm

### AuthorTopic: Harder Physics Questions  (Read 9459 times) Tweet Share

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#### Edmund

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##### Re: Harder Physics Questions
« Reply #90 on: March 07, 2010, 11:04:29 pm »
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Aah thanks, I calculated the density with the timber floating horizontally =/
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#### Edmund

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##### Re: Harder Physics Questions
« Reply #91 on: March 20, 2010, 09:07:51 pm »
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GAMSAT over! Came across some interesting questions I might like to share

For 1 mark (1.5 minutes),

The amount of energy needed to raise 1 mL of water by 1°C is 4.2 J. One meal served in a restaurant is equivalent to 8000 kJ. If all the energy is converted to heat, how much ice water with a temperature of 0°C must a diner (body temperature =37°C) drink to maintain his body temperature (or counteract this increase in energy input)?

Feel free to clarify since I might not remember the question too well.

The options were 50L, ~70L, cant remember the other two options though...
2007-2008 VCE ATAR 90.15
2009-2011 BSc (Unimelb)
2012-2015 DDS (Unimelb)

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#### appianway

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##### Re: Harder Physics Questions
« Reply #92 on: March 20, 2010, 09:16:04 pm »
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Energy that the water gains = energy converted to heat.

Energy that water gains = volume of water * change in temperature * 4.2J
Change in temperature = 37 degrees

37 * 4.2 * V = 8 * 10^6
V = 8* 10^6 / (37*4.2)

And yeah, just plug it in.

#### Edmund

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##### Re: Harder Physics Questions
« Reply #93 on: March 20, 2010, 09:20:20 pm »
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Well that looks right, I probably forgot the formula and guessed it wrong
2007-2008 VCE ATAR 90.15
2009-2011 BSc (Unimelb)
2012-2015 DDS (Unimelb)

Booksale: Drugs That Shape Society, Forests in a Global Context

#### iamdan08

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##### Re: Harder Physics Questions
« Reply #94 on: March 20, 2010, 10:12:10 pm »
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From memory, I'm pretty sure i got 50L. Can't remember the exact question though. :S
2007-08 VCE - Accounting, Texts & Traditions, Methods, Chem, Physics, Lit

2011 Bachelor of Biomedicine (Completed) @ The University of Melbourne
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#### /0

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##### Re: Harder Physics Questions
« Reply #95 on: May 07, 2010, 01:33:17 pm »
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A nice problem back from my first quantum problem set:

Show that, if a photon collides with a stationary electron, they cannot both emerge
from the collision along the same trajectory.

Useful Formulae:
Relativistic momentum: $\gamma mv$
Rest energy: $mc^2$
Relativistic total energy: $\gamma mc^2$
Photon Energy: $E = pc$

Where $\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$

(I'll post up a really good question after I hand in my current thermo assignment xD)
« Last Edit: May 07, 2010, 01:49:31 pm by /0 »

#### schnappy

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##### Re: Harder Physics Questions
« Reply #96 on: May 10, 2010, 08:22:36 pm »
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GAMSAT over! Came across some interesting questions I might like to share

For 1 mark (1.5 minutes),

The amount of energy needed to raise 1 mL of water by 1°C is 4.2 J. One meal served in a restaurant is equivalent to 8000 kJ. If all the energy is converted to heat, how much ice water with a temperature of 0°C must a diner (body temperature =37°C) drink to maintain his body temperature (or counteract this increase in energy input)?

Feel free to clarify since I might not remember the question too well.

The options were 50L, ~70L, cant remember the other two options though...

Trick question the water is in a solid state so it can't be readily consumed

#### /0

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##### Re: Harder Physics Questions
« Reply #97 on: May 11, 2010, 07:13:00 pm »
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How long do black holes last?

1) The Schwarzchild radius of a Black Hole is the radius of the 'event horizon', past which even light cannot escape.
Find an expression for the Schwarzchild radius of a black hole using the notion of escape velocity (non-rigorous derivation :-o)

2) Over time, black holes lose energy and hence mass via "Hawking Radiation". The power emitted by a Black hole is:
$P = \sigma e AT^4$, where $\sigma = \frac{2\pi^5k^4}{15hc^3}$, and $h$ is Planck's constant, $k$ is Boltzmann's constant, and $c$ is the speed of light, $A$ is the surface area, $e$ is emissivity, and $T$ is temperature.
Since we may assume a black hole to be a perfect blackbody (perfectly absorbs and emits radiation), $e = 1$.
Given that the temperature of a black hole is: $T = \frac{hc^3}{16\pi^2kGM}$, where $G$ is the Gravitational constant, and the energy of a black hole is $Mc^2$, form a differential equation to find the lifetime of a black hole in terms of its initial mass.

3) How long would a black hole the mass of the sun last ($2 \times 10^{30}kg$)? How about the mass of a proton ($1.67 \times 10^{-27}kg$)?
« Last Edit: May 11, 2010, 07:50:37 pm by /0 »