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.
5
votes
2answers
65 views
Conceptually, what is negative work?
I'm having some trouble understanding the concept of negative work. For example, my book says that if I lower a box to the ground, the box does positive work on my hands and my hands do negative work ...
1
vote
0answers
24 views
Finding the force of an object? [closed]
Sorry if my initial question was seen as a "homework question" but I don't really know how to go about asking this type of question. I have a feeling I have to use Newtons law but there is no ...
2
votes
3answers
587 views
Example in the book: A simple accelerometer
A simple accelerometer
You tape one end of a piece of string to the ceiling light of your car and hang a key with mass m to the other end (Figure 5.7). A protractor taped to the light allows you ...
0
votes
0answers
42 views
How fast could a football drop? [closed]
Suppose we have a perfect spherical football with d=22.5cm and m=420g. The football has a initial vertical speed of v(around 20m/s, not sure if it will cause a turbulence) and zero angular speed. If ...
0
votes
1answer
44 views
if a simple pendulum is dropped in a elevator with a acceleration greater than acceleration due to gravity then what will be its frequency
if a simple pendulum is dropped in a elevator with a acceleration greater than acceleration due to gravity then what will be its frequency ? We know time period depends on frequency.
3
votes
2answers
112 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
4answers
336 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 ...
1
vote
1answer
50 views
How large of a solar sail would be needed to travel to mars in under a year?
I'm attempting to approach this using the identity
$$F/A = I/c$$
I can solve for Area easily enough
$$A = F(c/I)$$
and I know the distance $d$ is
$$d=1/2(at^2)$$
But I'm having difficulty trying to ...
5
votes
1answer
64 views
Calculating how a polygon bounces off a plane
I'd like to calculate how polygons bounce off a plane. In this picture, the square doesn't bounce straight up, but instead it bounces somewhat to the right and starts spinning. But I have no idea ...
-1
votes
0answers
19 views
Ignoring rotational inertia of a ballistic pendulum
When calculating the initial velocity of a steel ball by using a ballistic pendulum, but by ignoring the rotational inertia of the pendulum rod, a systematic error is introduced. My question is will ...
0
votes
0answers
6 views
Finite Element, NASTRAN, how to print the differential stiffness matrix in .f06 output file [migrated]
We are working on an optimization problem in which we can approximate the eigenvalue calculation by assuming a constant eigenvector, using the formula:
$$
...
0
votes
0answers
15 views
Row of pivoted magnets and energy scale
This question is about a system involving a horizontal row of length L of equally spaced pivotable magnets, each with a pole at either end. These magnets will often be referred to as units.
So each ...
2
votes
1answer
138 views
Mechanics question
The following is a question from a past exam paper that I'm working on, as I have an exam coming soon. I would appreciate any help.
A fairground ride takes the form of a hollow, cylinder of radius ...
0
votes
3answers
3k views
Yield Strength versus Ultimate Strength
What is the qualitative difference between these two:
As seen on the table Typical yield and ultimate strengths.
I am trying to resolve the meaning of the phrase "contact yield stress" from C. ...
0
votes
1answer
49 views
Measuring vibration and converting to force (N)
The test is:
To have a rotating machine, bolted into a factory floor.
To measure the vibration on 3 axis (output of accelerometers can be acceleration or velocity in $\mathrm{m/s}$ or ...
0
votes
1answer
90 views
Shear Flow corresponding to Eccentric Shear Force of a Closed-section Beam (Structural Analysis - Mechanics)
Been stumped with this question for way too long... its a beam with a thin-walled rectangular cross-section, and a shear force is acting at a distance from the shear center. I know my decomposition of ...
0
votes
1answer
34 views
What lifting mechanism is likely to have the best energy recovery ratio?
Suppose I was designing an apparatus which needed to lift 250kg 5cm high, hold it there for a few seconds, and then lower the object back to the original height. Such a process would need to be ...
2
votes
2answers
66 views
Hamiltonian of Harmonic Oscillator with Spin Term
We have the usual Hamiltonian for the 1D Harmonic Oscillator:
$\hat{H_{0}}=\frac{\hat{P^2}}{2m} + \frac{1}{2}m \omega \hat{X^2}$
Now a new term has been added to the Hamiltonian, $\hat{H} = ...
1
vote
1answer
85 views
Why can we analyse force balance on a dislocation?
Dislocation (like screw or edge dislocation) is not a 'real' thing, while Newton's laws only apply to a real object (no matter macroscopic, like stars, or microscopic, like atoms).
In the derivation ...
-1
votes
1answer
29 views
Terminal velocity and force pull [closed]
I can't figure out this problem .
Buoyancy force and gravity remain constant and viscous force by is $-kv$. And these forces all balance, but data isn't given according to that, or I am not able to ...
1
vote
1answer
50 views
Floating Objects and Weight
The Situation:
A ball is placed in a beaker filled with water and floats. It is also attached to the bottom of the beaker via a string.
The Question:
The ball is attached to the beaker, thus ...
1
vote
1answer
33 views
Mechanical shock resistance as a function of shape
I have a system where I'm dropping glass tubes filled with some sample from a certain height, along a track. I can apply a back-pressure of air to push them down faster, and in general the faster they ...
2
votes
2answers
153 views
Cantilever Beam - Maximum Shear of the Beam
A cantilever beam $3\ \text{m}$ long is subjected to a moment of $10\ \text{kNm}$ at the free end. Find the maximum shear of the beam.
The answer is "There is no vertical load, shear is zero"
...
1
vote
1answer
37 views
Finding the coffecient of restitution
A ball moving with velocity $1 \hat i \ ms^{-1}$ and collides with a friction less wall, afetr collision the velocity of ball becomes $1/2 \hat j \ ms^{-1}$. Find the coefficient of restitution ...
0
votes
0answers
79 views
How to calculate mechanical advantage of a worm gear?
How to calculate mechanical advantage of a worm gear?
My textbook simply use the turn ratio as the mechanical advantage, but I'm not sure how that works.
My thinking: If the worm has a radius of ...
0
votes
2answers
102 views
How do you tell what forces do no work?
The total mass of the children and the toboggan is 66 kg. The force the parent exerts is 58 N (18 degrees above the horizontal). What 3 forces/ components do no work on the toboggan?
I said the ...
0
votes
1answer
61 views
A sphere rolling down a rough wedge which lying on a smooth surface
A sphere of mass $m$ and radius $r$ rolls down from rest on an inclined (making an angle $\phi$ with the horizontal ) and rough surface of a wedge of mass $M$ which stays on a smooth horizontal floor. ...
-1
votes
1answer
94 views
Confusions about rotational dynamics and centripetal force
I am a high school student. I am having confusions about the centripetal force and rotational motion . I have known that a body will be in rest or in uniform velocity if any force is not applied. But ...
3
votes
2answers
185 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.
...
1
vote
1answer
686 views
Finding angular acceleration from torque
We have to analyze this video
Givens:
An applied net torque due to the wind on the windmill is equal to 1500 N*m.
Each (of the 3) propeller props weighs approximately 45 Kg and has a Moment of ...
4
votes
0answers
54 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 ...
4
votes
3answers
151 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 ...
2
votes
4answers
79 views
Would a phone move upon vibration in a completely uniform situation?
I was sitting down yesterday and saw my phone vibrate on a side, and it moved about a centimetre per vibration.
I wondered why it moves, and thought perhaps that the side it was on had a slight ...
3
votes
3answers
65 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 ...
2
votes
2answers
117 views
Whats the anti-torque mechanism in horizontal take-off aircraft?
In most helicopters there is the anti-torque tail rotor to prevent the body from spinning in the opposite direction to the main rotor.
What's the equivalent mechanism in horizontal takeoff single ...
0
votes
1answer
56 views
Hollow stone columns provide more support?
In history class in elementary school I remember learning that the Greeks would build their stone columns hollow because they thought this provided more support. Is it true that a hollow column is ...
4
votes
1answer
85 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
2answers
178 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?
3
votes
1answer
104 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 ...
0
votes
1answer
716 views
Speed of a fly inside a car
A couple of weeks ago I was travelling in a car (120 km/h approximately) and I saw a fly flying in front of me (inside the car, near my nose, windows closed). I wonder how was that possible.
Does it ...
1
vote
2answers
155 views
Is there any case in physics where the equations of motion depend on high time derivatives of the position?
For example if the force on a particle is of the form $ \mathbf F = \mathbf F(\mathbf r, \dot{\mathbf r}, \ddot{\mathbf r}, \dddot{\mathbf r}) $, then the equation of motion would be a third order ...
3
votes
2answers
145 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 ...
0
votes
0answers
134 views
Maximum Shear on a Beam - beam with fixed support on one end and hinge on other end
A beam $\displaystyle 3m$ long with fixed support on one end and hinge on the other end is subjected to a uniform load of $10\ kN/m$. What is the maximum shear of this beam?
The solution is this one:
...
4
votes
1answer
82 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.
...
2
votes
1answer
67 views
Why is $dL = L d\epsilon$?
Let's say there's a random elastic material. It's length is $L$ and it's tensile strain $\epsilon= (L-L_0)/L_0$
Now, when one pulls on it the following is true:
$dW_{tot}=FdL =\sigma AdL=\sigma A L ...
13
votes
6answers
5k views
What is the difference between Newtonian and Lagrangian mechanics in a nutshell?
What is Lagrangian mechanics, and what's the difference compared to Newtonian mechanics? I'm a mathematician/computer scientist, not a physicist, so I'm kind of looking for something like the ...
5
votes
1answer
163 views
Drag on a spinning ball in fluid
I am a physics newbie (high school level) and I am wondering what happens when a spherical object is spinning on the spot in a bunch of gas (no gravity here, just an imaginary physics sandbox).
Am I ...
1
vote
2answers
69 views
What fraction of peak horsepower do typical 4 door passenger vehicles use?
I was surprised when I looked at the power rating of the engine used on a Humvee. It's only ~190 horsepower, which is exceeded by many sedan engines.
So an obvious question is why doesn't my Camry SE ...
2
votes
1answer
67 views
Atomic physics through classical resonance?
I have a rather general question regarding the theory of Quantum Mechanics. To preface this question, consider a violin string. When a violinist exposes the string to a bow, this is exposing the ...
2
votes
4answers
206 views
Pseudo force in rotating frames
A bug of mass $m$ crawls out along a radial scratch of a phonographic disc rotating at $\omega$ angular velocity. It travels with constant velocity $v$ with respect to the disc.
What are the forces ...
