Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright.
-1
votes
1answer
98 views
I need help with this question on Heat Capacity
A calorimeter has a Heat Capacity of $70 J/K$. There is $150g$ water with a temperature of $20^oC$ in this calorimeter. In this, you put a metal cube of $60g$ with a temperature of $100^oC$. The ...
3
votes
1answer
230 views
Finding the diameter a parachute must be to land a probe on Mars at specific speed
Hi I'm trying to solve this textbook example but I don't know where to begin;
NASA has decided to send an experimental probe to Mars. Its weight on earth is $40 kg_f$. When the probe is near ...
2
votes
2answers
1k views
Minimum possible Kinetic Energy of a confined electron
The problem is this:
Consider an electron confined in a region of nuclear
dimensions (about 5 fm). Find its minimum possible
kinetic energy in MeV. Treat this problem as
one-dimensional, and ...
0
votes
1answer
211 views
How should I apply conservation of energy to this problem?
A block rests on a table. Then the block is pushed by a spring and slides across the table until it falls to the ground. The mass of the block is $1.30\text{ kg}$, the spring constant $550\text{ ...
0
votes
1answer
143 views
Spring-mass physics homework question [closed]
I've been having trouble with my physics homework. The problem is:
You may have measured the properties of a simple spring-mass system in the lab. Suppose you found ks = 0.9 N/m and m = 0.01 kg, ...
2
votes
1answer
169 views
Inhomogeneous Effective Mass in a 2D Lattice
Consider a tight-binding square lattice in 2D. This lattice has two different nearest neighbor tunneling rates along the x and y directions; call them $J_{x}$ and $J_{y}$. All longer range tunneling ...
1
vote
1answer
142 views
Clarification on a Goldstein formula steps (classical mechanics)
At page 20 of Classical Mechanics' Goldstein (Third edition), there are these two steps given between eqs. (1.51) and (1.52):
$$\sum_i m_i \ddot {\bf r}_i \cdot \frac{\partial {\bf r_i}}{ \partial ...
1
vote
0answers
168 views
Simple heat transfer question [closed]
You add an unknown volume of milk of $5.2 ^\circ C$ to a cup of coffee ($40 mL$ of water, temperature: $80.3 ^\circ C$). After a while of stirring the temperature reaches $73.2 ^\circ C$. The ...
0
votes
3answers
2k views
Finding Angular Acceleration of rod given radius and angle
A uniform rod is 2.0 m long. The rod is pivoted about a horizontal, frictionless pin through one end. The rod is released from rest at an angle of 30° above the horizontal. What is the angular ...
2
votes
1answer
158 views
Finding Max Radius of Propeller (angular velocity)
I am looking at a question from University Physics
The given answer
Whats the intuition behind using the below diagram
of finding $v_{tip}$? I was looking the the $v_{tan}$ not knowing how ...
3
votes
1answer
305 views
How do I find the initial velocity in this problem?
An X-ray tube gives electrons constant acceleration over a distance of $20\text{ cm}$. If their final speed is $2.0\times 10^7\text{ m/s}$, what are the electrons' acceleration?
I know this ...
0
votes
2answers
108 views
Solar Thermal/Solar Photo-voltaic calculations [closed]
This is my final high school assignment. I'm asked to prepare a research document on Solar Thermal and Solar Photo-voltaic, to prove that these two are feasible alternative power sources to power the ...
0
votes
1answer
430 views
Maximum Kinetic energy of a spring
The block in the figure below lies on a horizontal frictionless
surface and is attached to the free end of the spring, with a spring
constant of 35 N/m. Initially, the spring is at its relaxed ...
2
votes
0answers
86 views
Bandgap Spacing in Photonic Crystals
I am doing some self-study on photonics and have encountered the following question:
We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic ...
0
votes
2answers
263 views
Linear motion with variable acceleration
Consider the following problem
I pull a mass m resting at x = 0 on a frictionless table connected to a spring with some k by an amount A and let it go. What will be its speed at x=0?
I know how to ...
1
vote
1answer
341 views
Find magnetic scalar potential for superconducting sphere
In regions where $J = 0$, the curl of the magnetic field $B$ is necessarily zero (since $\nabla \times B = \mu_0 J$). Therefore $B$ can be written as $B = -\nabla V_m$, where $V_m$ is a scalar ...
2
votes
1answer
4k views
Abiotic oil vs the traditional theory of oil deposit formation [closed]
I am curious to see what people think of the abiotic theory of oil deposit formation versus the traditional theory.
I have long wondered how enough organic material became trapped underground to ...
0
votes
0answers
499 views
Finding the period and frequency for simple harmonic motion [closed]
A 1 lb weight is suspended from a spring. Let y give the deflection (in inches) of the weight from its static deflection position, where “up” is the positive direction for y. If the static ...
2
votes
2answers
614 views
Why might the normal force on a box not be equal to its weight?
Very simple homework question which I managed to get wrong:
"The weight of a box sitting on the floor points directly down. The normal force of the floor on the box points directly up. Need these ...
0
votes
1answer
95 views
Is Force equal to components in different dimensions of Force or distance of those components
I'm trying to understand to find my homework problem. Its a simple concept of finding force. I wonder if you have two formulas then is the total Force = force from x + force from y. Or ...
0
votes
4answers
181 views
Find total energy and momentum of an moving electron in a rest frame
I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$.
How do I find the total energy and momentum of the electron in the rest frame $S$?
I ...
0
votes
0answers
80 views
How to figure out an elastic constant? [closed]
Im doing this study and i have this question which I'm not 100% sure on, got me pretty stumped. anyone think they can help me?!
When a bowstring is pulled back in preparation in preparation for ...
3
votes
1answer
173 views
Can I find a potential function in the usual way if the central field contains $t$ in its magnitude?
I'm working on a classical mechanics problem in which the problem states that a particle of mass $m$ moves in a central field of attractive force of magnitude:
$$F(r, t) = \frac{k}{r^2}e^{-at}$$
...
0
votes
0answers
183 views
Finding transcendental equation for the energy of a particle in delta potential well near infinite potential barrier [closed]
I'm having trouble finding the transcendental equation for a particle in a delta potential settled near an infinite potential wall. The potential is given by
$$
V(x) =
\begin{cases}
\infty & x ...
0
votes
0answers
85 views
Wireless signal strength
My question is possibly somewhat misplaced, but I'll try to explain as best as I can. Suppose I have a transmitter with a frequency of 2500MHz and a power of 1W. It radiates uniformly in all ...
-2
votes
1answer
410 views
Calculate acceleration and time given initial speed, final speed, and travelling distance? [closed]
A motorcycle is known to accelerate from rest to 190km/h in 402m.
Considering the rate of acceleration is constant, how should I go about calculating the acceleration rate and the time it took the ...
2
votes
2answers
806 views
Calculating work done on an ideal gas
I am trying to calculate the work done on an ideal gas in a piston set up where temperature is kept constant. I am given the volume, pressure and temperature.
I know from Boyle's law that volume is ...
1
vote
3answers
267 views
Same momentum, different mass
The question is: if
A bowling ball and ping pong ball
are moving at same momentum
and you exert same force to stop each one
which will take a longer time? or some?
which will have a longer ...
2
votes
1answer
112 views
helium balloon tied to a car [closed]
A helium balloon is tied to the seat of a car. The doors and windows of the car is closed. If the car now starts moving, in which direction will the balloon move- front or back?
0
votes
2answers
241 views
How does one calculate the volume of a nucleus and the volume of an atom (in this case hydrogen)?
The hydrogen atom contains 1 proton and 1 electron. The radius of the proton is approximately 1.0 fm (femtometers), and the radius of the hydrogen atom is approximately 53 pm (picometers).
0
votes
1answer
336 views
Calculate the UPS Capacity in amp-hours [closed]
I am trying to find out the UPS capacity in amp-hours for my HP UPS system.
I've already done some calculations based on the UPS information from the HP Power Manager software.
Bellow are my ...
-1
votes
1answer
262 views
Internal forces in a truss and its geometry
I'm to work out the internal forces in a truss, but I can't get my head around the geometry of the truss itself. I'm starting to think there may have been information on the diagram which I missed. ...
0
votes
1answer
59 views
1D Acoustical Relations beyond nearest neighbor couplings
Consider some 1D Lattice of atoms with nth neighbor coupling of strength k_{n}. I'm looking for the dispersion relation for acoustical phonons under these conditions.
I start with the Lagrangian,
...
6
votes
0answers
224 views
Capacitance of this unusual capacitor
This capacitor is composed of two half spherical shelled conductors both with radius $r$. There is a very small space between the two parts seeing to that no charge will exchange between them.
...
1
vote
1answer
45 views
Rays in Symmetric Resonator
I'm having some trouble figuring out how to get started on this question:
If I have a symmetric resonator with two concave mirrors of radii $R$ separated by a certain distance, after how many round ...
0
votes
2answers
375 views
Deriving the Poynting Theorem
I am trying to derive the Poynting theorem. So far, I've only been able to narrow down which equations I think I'll need to do so. These are the equations:
Maxwell's Equations:
$$
\nabla\times{\bf E} ...
0
votes
0answers
74 views
How do I figure out the normal force on a person on an accelerating platform? [closed]
My question is an elaboration on this question: Force on rope with accelerating mass on pulley
The elaboration is to determine what the Force that the platform exerts on the person. Assume the ...
1
vote
1answer
209 views
What is the general relativistic calculation of travel time to Proxima Centauri?
It has already been asked here how fast a probe would have to travel to reach Alpha Centauri within 60 years. NASA has done some research into a probe that would take 100 years to make the trip. But ...
2
votes
3answers
303 views
Jumping on earth versus jumping on the moon
Given the following problem:
On the moon the acceleration due to gravity is $g_m = 1.62 m/s^2$. On earth, a person of mass $m = 80 kg$ manages to jump $1.4 m$. Find the height this person will ...
0
votes
1answer
319 views
How to get the gradient potential in polar coordinate
In polar coordinate,
$$\nabla U = \frac{\partial U}{\partial r}\hat{\mathbf{r}} + \frac{1}{r}\frac{\partial U}{\partial \theta}\hat{\mathbf{\theta}} .$$
Can anyone show me how to get this result?
1
vote
1answer
164 views
Bouncing Ball Pattern
If a ball is simply dropped, each time a ball bounces, it's height decreases in what appears to be an exponential rate.
Let's suppose that the ball is thrown horizontally instead of being simply ...
1
vote
2answers
165 views
Proof of $Dq-qD=1$ where $D=\frac{\partial }{\partial q}$ is the differential operator
Can anyone provie me the proof of $Dq-qD=1$ where $D=\frac{\partial }{\partial q}$ refers to the differential operator?
Or if it's something special to quantum mechanics, why is it?
Is this ...
0
votes
2answers
88 views
Proof of $T=\sqrt{2y/a}$ in uniformaly accelerating object [closed]
Suppose that there is a object that does a y-axis-only free fall to ground. The initial distance from the ground is defined as $H$.
How does one prove that time the object takes to reach the ground ...
0
votes
0answers
74 views
Describing the movement of the object in a particular situation in Lagrangian way
Suppose there is a object M, (sliding motion) moving by the initial speed $v$ and the initial location $x_0$. Otherwise noted, friction is assumed to be nonexistent. It then meets a circular mold ...
2
votes
3answers
209 views
How do I integrate $\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$
How do I integrate the following?
$$\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$$
where $C$ is a constant.
I'm supposed to get a Gaussian function out of the above by integrating but don't ...
1
vote
1answer
153 views
Questions regarding solving the Brachistochrone problem using Lagrangian
brachistochrone problem: Suppose that there is a rollercoaster. There is point 1 ($0,0$) and point 2 ($x_2, y_2)$. Point 1 is at the higher place when compared to the point 2, so the rollercoaster ...
0
votes
2answers
127 views
Frequency of a tuning fork in a vacuum
Consider this equation of a damped harmonic oscillator such that:
$$
\ddot{x}+2\gamma\dot{x}+\omega^2_0=0
$$
with: $\gamma=\frac{b}{2m}$ and $\omega_0=\sqrt{\frac{k}{m}}$
Finally, we know that the ...
1
vote
1answer
376 views
Adiabatic expansion [closed]
I'll start off by saying this is homework, but I ask because I don't understand how the math should work (I don't just want an answer, I'd like an explanation if possible). I understand if this is ...
0
votes
0answers
112 views
Antenna Power and gain calculation [closed]
I have a wireless security related question, the second part confused me:
Your wireless network usually has a range of 100 feet. However you
are having a (confidential) meeting in a 10’ x 10’ x ...
0
votes
1answer
1k views
Distribution of charge on a hollow metal sphere
A hollow metal sphere is electrically neutral (no excess
charge). A small amount of negative charge is suddenly
placed at one point P on this metal sphere. If we check on
this excess negative charge a ...
