General questions about the way objects move and interact. This tag should be used when the tags for certain kinds of mechanics (newtonian-mechanics, classical-mechanics, quantum-mechanics, etc.) are too specific.
4
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3answers
319 views
How fast do I need to go in order to avoid being seen by the Police?
I was driving down the road at roughly the speed of traffic. I saw a police officer parked on the side of the road, and also noticed that a Semi was traveling in the lane right next to him. This got ...
4
votes
2answers
359 views
Is instantaneous velocity an abstraction?
In introductory analysis, the discussion the derivative emphasizes that while average rates of change are measurable, instantaneous rates of change are a "limiting abstraction". While this makes ...
4
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3answers
156 views
Microgravity: how powerful is an astronaut
In 2008, NASA astronaut Clayton Anderson tossed a 635-kg tank of ammonia coolant overboard from the International Space Station (ISS). Subsequently the tank burned up in earth atmosphere as planned.
...
4
votes
3answers
281 views
Calculating stress without strain
I am working on an algorithm for a real-time simulation.
I would like to calculate to extremely permissive tolerances approximate values for the stress within a 2D geometry. It will not be difficult ...
4
votes
3answers
387 views
An example of non-Hamiltonian systems
I am preparing for the exam. And I need to know the answer to one question which I can't understand.
"Give an example of non-Hamiltonian systems: in case of infinite number of particles; for a finite ...
4
votes
4answers
692 views
How do levers amplify forces?
This is really bothering me for a long time,
because the math is easy to do, but it's still unintuitive for me.
I understand the "law of the lever" and I can do the math and
use the torques, or ...
4
votes
3answers
467 views
List of Scattering Phenomena
While at lunch with my lab group we got into discussion of the different types of scattering phenomenon that we encounter in everyday life and physical experiments. We ended up listing about a dozen ...
4
votes
1answer
84 views
(Re-)use of a space elevator (basic mechanics and potential energy source)
It's said that if a space elevator were made then it would be much more efficient to put objects in orbit. I've always wondered about the durability of a space elevator though. I don't mean the ...
4
votes
1answer
79 views
Finding the acceleration of a cart rolling on a table
The cart is rolls frictionless on the table. It has a mass of $1 kg$. Attached to it are 2 strings, that go through two frictionless sheaves. The weights have masses as in the picture.
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2answers
1k views
Shape of Fan Blades
How is the shape of the blades of an air fan determined? Trial and error, or is there a theory behind it? What are they trying to maximize, volume of air dislocated per rotation?
4
votes
2answers
177 views
Why is the Lagrangian quadratic in $\dot{q}$? [duplicate]
My teacher said we only consider Lagrangians which are quadratic in $\dot{q}$, and we don't take other Lagrangians. I couldn't understand why. Can anyone please explain this?
4
votes
1answer
672 views
Rubber Band Forces
I have a question regarding the force a band places on an object. Say I have a rubber band wrapped around 2 pegs at a certain distance, and at that distance I know the pounds of force per inch it is ...
4
votes
3answers
150 views
Does more rain strike a vehicle while moving or while stopped (or neither)? [duplicate]
Assume there is a rainstorm, and the rain falling over the entire subject area is perfectly, uniformly distributed. Now assume there are two identical cars in this area. One is standing still, and ...
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0answers
51 views
Scaling arguments for the Contact mechanics between two elastic spheres
I am studying a bit granular dynamics and I have seen that two spheres of radius $R$ in contact with a contact area of radius $a$ would need an applied force $F$ on this two spheres that is nonlinear ...
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0answers
787 views
Why does Guillotine have of 45 degreed blade rather than the one parallel to the ground? [closed]
What is the point of having 45 degreed blade in Guillotine? After all, the pressure-Force over Area-blade imposes to the neck is the same either way.
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votes
3answers
352 views
What makes the earth keep spinning?
I am thinking of what makes the earth keep spinning. Is there anybody here know the answer?
3
votes
4answers
546 views
“Regular” 20-sided die, vs “life counter” 20-sided die. Same probabilities?
Regular dice are made such that opposite sides of the die add to 1+the number of sides.
For example, a 20-sided die has 14 and 7 opposite of each other, adding to 21.
For certain types of games, ...
3
votes
4answers
555 views
Oscillate a swing without touching the ground
Is is possible to set a swing into oscillations without touching the ground?
This occurred to me while watching the second pirates movie. There is a scene where the ship's crew is suspended in a cage ...
3
votes
1answer
124 views
Why is the $\langle v_{x}^{2} \rangle=\frac{1}{3} \langle v^2 \rangle$?
For a randomly moving particle. Or, I suppose that 1/3 could generalise to 1/n, where n is the non rotational degrees of freedom for that particle.
Related reference Kinetic Theory of Gasses.
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1answer
300 views
Soliton mechanics
How much of classical mechanics can be modelled with solitons?
What I am aware of is that single solitons behave in a way like free particles: they move along as stable entities with constant ...
3
votes
2answers
109 views
Extended Rigid Bodies in Special Relativity
I was reading Landau & Lifhsitz's Classical Field Theory and I noticed that they mention that an extended rigid body isn't "relativistically correct".
For example, if you consider a rigid rod ...
3
votes
2answers
558 views
Question about moment of inertia and velocity
First off, I swear this is not homework. I'm doing some practice problems because I got an exam coming up. I'm stuck on this one:
I figured I would use energy conservation for this problem. So since ...
3
votes
1answer
397 views
Does the friction force change directions with a change in reference frame?
In a basic friction problem with Block A sliding on top of Block B, the direction of the friction force is usually explained as being simply the opposite of the direction of motion. So if Block A is ...
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votes
2answers
144 views
Universe Expansion and two tennis balls
Clear the universe of all matter except for two tennis balls. Place the two tennis balls in the same inertial frame 1 Mpc apart.
Are the tennis balls getting further apart?
Will the tennis balls ...
3
votes
2answers
184 views
System of Particles and Moment of Mass
I recently came across the definition of the Center of Mass of a system as the point about which the first moment of mass is zero.
Further, it defined Moment of Inertia as the second moment of mass.
...
3
votes
1answer
520 views
Integrals of Motion
Landau writes in chapter 2 of his Mechanics book "The number of independent integrals of motion for a closed mechanical system with s degrees of freedom is 2s-1". Then he goes on to show how -- "Since ...
3
votes
1answer
497 views
How do anti-lock brakes know when to brake?
When you come to a stop normally, the brakes don't pulse when you stop. Since the car can only know its speed by the rotation of the wheels, how can it distinguish between the car is stopped normally ...
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4answers
374 views
Rolling stone on a frictional surface
Consider a spherical rigid stone rotating with angular velocity $\omega$ being dropped vertically onto a horizontal rigid surface with the coefficient of friction $\mu$. Can the stone roll on the ...
3
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2answers
362 views
What is the mathematical formulation for buckling?
Argument: Buckling is an engineering concept that can only be applied to thin columns with compressive loading.
(Is it possible to) Prove the above sentence right or wrong with mathematical ...
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votes
3answers
323 views
Why trimming the mainsail gives a larger torque to head up
I am learning sailing on a 5m catamaran (Nacra 5).
I am familiar with basic aerodynamics and the physics of the sail and keel.
We learned that when sailing closed hauled, too tight a mainsail tends ...
3
votes
2answers
671 views
Formula for implementing a simulated weighing scale
Background:
I'm on the pledge drive committee for a non-profit group, and there is an issue that is always contentious within this group; thankfully the group is amicable, although competitive, about ...
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votes
3answers
63 views
Friction on roads
I have a question with which I am having trouble.
A 71m radius curve is banked for a design speed of 91km/h. Given a coefficient of static friction of 0.32, what is the range of speeds in which a car ...
3
votes
4answers
326 views
Difference between torque and moment
What is the difference between torque and moment? I would like to see mathematical definitions for both quantities.
I also do not prefer definitions like "It is the tendancy..../It is a measure of ...
3
votes
2answers
349 views
Bernoulli's equation and reference frames
So I was thinking about this while driving home the other day.
I've never been quite clear on why when you drive with the windows down air rushes into your car. I thought this might be explained by ...
3
votes
2answers
278 views
How did L.H. Thomas derive his 1927 expressions for an electron with an axis?
I'm looking at the 1927 paper of Thomas, The Kinematics of an Electron with an Axis, where he shows that the instantaneous co-moving frame of an accelerating electron rotates and moves with some ...
3
votes
1answer
179 views
Coincidence detectors in Bell tests: How close is close enough?
When is a coincidence a coincidence? We know that to identify entangled photons, the electronics is set to look for simultaneous clicks at opposite detectors. The size of the window is to some degree ...
3
votes
1answer
103 views
Intuition behind Work
I have a doubt in understanding the intuition behind the concept of work. First of all, I think this isn't duplicate, I've searched on the site, and the closest thing I've found was this post which is ...
3
votes
1answer
504 views
What phrases describe collisions with coefficients of restitution less than zero or greater than one?
The coefficient of restitution describes the elasticity of a collision:
1 = perfectly elastic, kinetic energy is conserved
0 = perfectly inelastic, the objects move at the same speed post impact
...
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0answers
90 views
Obtaining point of application of Ground Reaction Force with use of a hyperstatic load-cell array platform
I'm looking for the theory of an experiment that is giving me a hard time to perform.
I have an instrument composed of a rigid horizontal square plate rigidly supported under each corner by a ...
3
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0answers
116 views
Symmetries of separable potential
For separable potential, say $x^4+y^4$, its symmetry are degenerate.
Is that a generic case to every separable potential? I will explain my question:
The potential $x^4+y^4$ has $A_1, B_1, A_2, B_2, ...
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0answers
142 views
Does a thermally expanding torus experience internal stress?
I'm trying to learn continuum mechanics and thermo-mechanics.
As we know, heating an object increases the mean atomic distance $a_0$ of the atoms in a rigid body. Let's assume it is a linear elastic ...
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7answers
2k views
What are the Constraints on Building a Tower to Space?
When I was a boy I used to daydream about building a tower so tall that the top of it would stick out of the top of Earth's atmosphere project into near space.
There would perhaps be a zero gravity ...
2
votes
2answers
265 views
What is the purpose of iron bars in concrete?
What purpose do iron rods in concrete serve?
Do these iron rods impart any strength to the concrete apart from defining the framework for the concrete to solidify upon initially?
2
votes
4answers
330 views
pendulum on a rotating table. Will a torque be generated?
Here is the set up. Very simple. A flat (i.e. horiozntal table, there is no gravity) and rounded table that spins on its axis (through the center of the table). A spring mass system is now put on the ...
2
votes
2answers
462 views
Does the phase space (configuration and momentum space) of particles have a Euclidean norm? Does it have a useful meaning of “distance”?
Often in engineering physics, different vector spaces are used to visualize the trajectories (evolution) of systems. An example being the 6n dimensional phase space of n particles. It is not very ...
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votes
4answers
307 views
Which angle causes an object to land quickest?
Say a canon is where the circle is, and it shoots two different canonballs at different angles, but at the same speed, which angle would make the cannonball hit the ground first?
Intuitively I'd ...
2
votes
3answers
2k views
Torque vs Moment
I was wondering, why in Newtonian physics torque is called "torque" while in static mechanics they call it "moment"?
I prefer by far the term "torque", for not only it sounds strong, but also ...
2
votes
3answers
658 views
Initial vs Constant Orbital Velocity
I am working on some basic physics simulation for a game and need to simulate gravity. I have a system working that is behaving more or less correctly so far, but I want to see if I can send a ...
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4answers
880 views
Physics behind Wheel Slipping
Lets say that I'm in a car and I apply full acceleration suddenly. Now, the wheels would slip and hence the car doesn't displace much.
But If I start with some constant acceleration, slipping doesn't ...
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2answers
202 views
What sustains a rigid body's rotation at its constant angular(rotational) speed?
Continuing from the following scenario from my previous question Centripetal force of a rotating rigid body? :
Consider someone pushing a roundabout in a playground. Initially the
roundabout is ...
