0
votes
1answer
65 views

Landau Mechanics equation 16.9

I am having trouble deriving the equation 16.9 from Landau's Classical Mechanics book. This equation is the maximum kinetic energy of a particle if a massive particle with mass $M$ disintegrates into ...
3
votes
1answer
52 views

Non-deterministic particle system

This question is in the spirit of Norton's dome, an example of an apparently non-deterministic system in Newtonian mechanics. Under certain restrictions, the Picard–Lindelöf theorem guarantees the ...
1
vote
2answers
55 views

Pulling on a weakened rope - where will it tear?

Let's say I have a rope of 10m length and it is weakened in 3 spots: at 2.5m, at 5m and at 7.5m. Weakened means that if enough tension is applied it will tear at these points (all points are equally ...
1
vote
1answer
63 views

Two masses on frictionless table with one string connecting both masses on ground and the other string [closed]

From Morin's Introduction to Classical Mechanics page 342: A solid cylinder of mass $m$ and radius $r$ lies flat on frictionless horizontal table, with a massless string running halfway around ...
0
votes
1answer
18 views

belts and balls, correct size of holes

I'm working on a robot that has to transport a set of balls up at a 60 degrees angle. In order to do this I want to use a belt system with holes in it. Now my question is how big do these holes have ...
0
votes
2answers
49 views

How to take in to account torques applied at different points

Suppose we have a rigid body with known moment of inertia through some axis ($J$) and that there are multiple torques being applied in different points in that body. I know that for a rigid body, the ...
5
votes
5answers
914 views

How does an object's motion behave if dropped from an aeroplane travelling diagonally upwards?

Imagine an aeroplane travelling with velocity $v$ at some angle $\alpha$ from East to North. A box is dropped from the aeroplane. What would the projectile of the box be? Would it be a parabola with ...
0
votes
1answer
22 views

Torque in a 2-segment robot leg

I have a (hypothetical) robot with two legs, each with 2 segments, with a joint at the body of the robot and in the 'knee' of each leg, as in this (poorly-drawn) diagram: (Sorry about the quality and ...
2
votes
3answers
86 views

Unstable equilibrium in a pendulum

Consider a pendulum with a bob and a massless, rigid, hinged rod attached to the bob. The bob is at rest at the bottom most position. Neglecting friction, is it possible to impart such a velocity ...
4
votes
2answers
118 views

Newton's Third Law Exceptions?

Lately I've been brushing up on some of my old Physics texts from college. Most recently, I've been rereading parts of "Classical Dynamics of Particles and Systems (5th ed.)" by Thornton and Marion. ...
1
vote
0answers
70 views

Lagrangian mechanics is different form Newtonian? [duplicate]

I am a post graduate student and had completed my classical Mechanics with Newtonian mechanics to Hamiltonian mechanics. I have better understanding of Newtonian, Lagrangian and Hamiltonian. But I ...
2
votes
1answer
63 views

Equation of motion for system of springs

I need to find the equations of motion for the following system. If $x_1$ is $m_1$'s extension and $x_2$ is $m_2$'s, then, I feel like for $m_1$ we just need to consider $x_1$ giving $$m_1 a_1 = ...
0
votes
3answers
80 views

If a body is floating in a static fluid, then the volume of the displaced fluid equal to the volume of the inmerse part of the object (proof)

Suppose an arbitrary body is floating in a static fluid, either totally or partially immersed in it, then the volume of the displaced fluid equal to the volume of the immersed fraction of the object. ...
6
votes
1answer
81 views

Movement of a cylinder filled with water

Not long ago I was pretty bored at a dinner and I started playing with a water bottle that was not empty: I've been quite interested in its behavior when putted on its side and pushed: the bottle of ...
0
votes
0answers
31 views

Effect of an asymmetric weight distribution on a hack squat machine

In a hack squat machine (see figure above), does it matter if I put more washers on one side? May this asymmetry cause an asymmetry of my effort in performing this exercise? For example, if I put ...
0
votes
0answers
25 views

Restricted three body problem

I have the restricted three body problem, which corresponds, to the equation $ \ddot a-2\dot b -a= \displaystyle\frac{\partial U}{\partial a} $ $ \ddot b +2 \dot a-b = \displaystyle\frac{\partial ...
0
votes
0answers
32 views

Confusion regarding a method of writing constraint equations

I came across a method for writing the constraint equations known as "The Virtual Work Method".I am quoting the exact language of the text(well,not exactly the exact)- Consider the atwood machine ...
32
votes
5answers
4k views

What are washers for? [closed]

When you attach a bolt to something using a nut, it is clear what the roles of the nut and bold are. The more you tighten the bolt the more secure your fastening. However, you are often also told ...
-3
votes
1answer
79 views

If it takes less than a year to accelerate to the speed of light at 1g why will it take the Voyager 10,000 years to reach Alpha Centauri?

Today I was doing my physics homework and there was a problem involving a space ship falling at 9.8 m/(s^2) to simulate gravity, and it asked how long would it take for the ship to reach to speed of ...
1
vote
2answers
197 views

Force Diagram for K&K 2.13 [closed]

I have been (independently) working on Problem 2.13 in Kleppner and Kolenkow's An Introduction to Mechanics and come to an answer which conflicts with the hint the authors provided in the book. The ...
3
votes
2answers
86 views

What is the “associated scalar equation” of equations of motion?

In an essay I am reading on celestial mechanics the equations of motion for a 2 body problem is given as: $$\mathbf{r}''=\nabla(\frac{\mu}{r})=-\frac{\mu \mathbf{r}}{r^3}$$ Fine. Then it says the ...
0
votes
1answer
37 views

What does an $n$-body system with constant $T$ and $U$ look like?

Can someone give an example of a system where the kinetic $T$ and potential $U$ energy are constant (but not zero)? Here's what I have in mind: Say you have $n-1$ satellites of negligible mass ...
17
votes
3answers
3k views

How do crocodiles jump?

In a video (Here), I saw crocodiles jump vertically about three meters without using any solid surface. The wonderful thing is that when they start to jump, their vertical velocity is approximately ...
1
vote
1answer
35 views

Indicate if objects after collision will stick

Is it possible to indicate if objects after collision will stick together knowing their properties(materials,hardness,etc)?
1
vote
0answers
55 views

point-particle vs rigid-body [closed]

As pointed out here point-particle-based modeling can lead to very inaccurate predictions. Could you give an example where point-particle-based model describes reality accurately enough and one where ...
0
votes
1answer
66 views

What indicates if object will be reflected - certain example

If I throw a small rock(1kg) at a big rock(100kg) the small rock is reflected; Let's say my weight is 80kg - if I would jump into a big rock instead of being reflected I would move in the same ...
4
votes
1answer
46 views

Physics of Snow Globe

What is the physics behind snow flakes inside a 3D snow globe? If I were to implement a snow globe in computer graphics what kind of model do I need to the flakes motion like a real snow globe? What ...
0
votes
2answers
104 views

Electrical force between two objects

I tried to solve the following problem: There are 2 objects . The object m1 with charge q and the object m2 with charge q.(same charge).The object m2 is connected with a rope to the ceiling. at the ...
1
vote
4answers
99 views

Basic buoyancy question

If I have a cup of water filled with air at the bottom of a pool, then when the cup is "upside down" the air doesn't leave because the water pressure is pushing it up against the top of the container. ...
0
votes
1answer
42 views

(air pressure and displacement) Isn't this image wrong?

Isn't this figure wrong? P(x,t) = -B(dy/dx) . If the derivative of air displacement has a maximum, then this is where the pressure is minimum, not maximum as this figure suggests. Could someone ...
4
votes
3answers
333 views

Quantum Mechanics or Classical Mechanics? [closed]

I'm just a student of grade 11 but, I was interested in knowing about Physics much deeper. In order to start my interest in Physics, I watched this video of Quantum Physics NOVA : Quantum ...
2
votes
1answer
78 views

Walking & Swinging

How can I show that the most convenient way to move the arms while walking is swinging them back and forth, alternatively? To pose the question in another way: can I prove, starting from the ...
3
votes
2answers
50 views

Constraint and Applied forces

In D'Alembert principle forces are classified into constraint and applied forces ? Is this classification different from internal-external forces ?
1
vote
1answer
93 views

In a 2-body problem, when is the moving path closed?

In a 2-body problem is it true that, in all situations, the moving path is closed? In which cases are the paths closed? Fixing the coordinate system or fixing one of the bodies gives us different ...
1
vote
0answers
56 views

Tension on an object [closed]

According to a website, $T_3 = T_2 + T_1$. Also, the rope and pulley device are mass less. But I don't really understand this. Let me try and explain how I see the problem. $M_2$ receives tension ...
0
votes
1answer
46 views

Lifting an Atwoods machine from the ground? [closed]

If we have an Atwood machine where masses $m_A$ and $m_B$ rest on the ground, then we apply an upwards force $F$ to the Atwoods machine. What is the acceleration of the blocks when $F=124N$, $m_A = ...
1
vote
1answer
59 views

Calculating forces efficiently in Lagrangian formalism

I will Illustrate the question using an example problem: We have a mass $m$ connected to a mass $m$ by a rod of length $l$, and also to a mass $4m$ by another rod of length $l$. The rods are ...
1
vote
1answer
38 views

Contra-rotating propellers torques

Please look at the following mechanism for contra-rotating propellers: YouTube video When a CCW torque acts on the upper gear and a same torque acts on the lower gear (both seen from above), the ...
0
votes
2answers
55 views

Rocket towing an object

I'm not sure if my question makes sense, but I don't know where to start. A crate of mass $m_2$ is connected by a massless rope of constant length $l$ to a rocket of mass $m_1$. We take into account ...
2
votes
2answers
105 views

Why fundamentally does classical mechanics lead to second order dynamics? [duplicate]

What's so special about second order equations in classical mechanics? I have a basic understanding of the Lagrangian and Hamiltonian formulations of classical mechanics, so I'm not looking for ...
-5
votes
1answer
87 views

Why $\vec F=m\vec a$ instead of $\vec F=m\vec v$? [duplicate]

$\vec F=m\vec a$ ,for moving object with 10 $km/s$ in a constant manner ,where acceleration is zero. ie No force on object ? http://www.physicsforums.com/showthread.php?t=622711
1
vote
2answers
89 views

Deriving Bernoulli's equation via conservation of E

So I'm not OK with how some people derive this equation. These people consider a pipe whose endings have cross-sectional areas and heights which are different. They then use the conservation of ...
3
votes
4answers
200 views

Solving for motion of rotating rod using only Newton's laws?

I have a question that's been bothering me for years. Given a rod of uniform mass distribution with total mass $M$ and length $L$ that lies on a horizontal table (with one end fixed to the table ...
0
votes
0answers
52 views

Question about an example of non-integrable constraints

The example is a thin disk rotating of an inclined plane. The disk can roll not only down the plane, but also "sideways". Let $(x,y)$ be the position of the CM, where the $y$ axis is down the slope ...
4
votes
2answers
193 views

Ten-ping bowling: Can a ping pong ball knock over a bowling pin?

In this video we see a rather unorthodox bowling technique on display. The gentleman appears to knock over all of the pins using only a ping pong ball. It's a fake, of course: you could never knock ...
0
votes
1answer
27 views

Modulus of action-reaction forces

I read somewhere that action-reaction forces satisfy the form $$ \mathbf{F}_{12}=-\mathbf{F}_{21}=f(|P_1-P_2|)(P_1-P_2) $$ meaning that the modulus can only depend on the distance between the two ...
7
votes
6answers
462 views

How do traveling waves pass through a standing wave node, if the node doesn't move?

I'm having trouble with the explanation that a standing wave in a string is the superposition of traveling waves. The nodes in the diagram above are points where the particles of the string's ...
7
votes
2answers
271 views

How do we know if a formulation of classical mechanics is correct?

For example, the Lagrangian formulation. I may be missing something, i.e. not having done it in enough detail, but here is my issue: from the definition of the lagrangian ($\mathcal{L}$) and from ...
0
votes
2answers
186 views

An intuitive proof of Bertrand's theorem

Is there a way to see that Bertrand's theorem is true intuitively. I mean without getting into too much mathematics ?
1
vote
2answers
106 views

Universal gravity at small distance

Could it be that there is simply a maximum gravitational force that two bodies of finite mass can exert on one another? This would occur at $r=0$, so maybe there is some really really really small $a$ ...