Skip to main content

New answers tagged

0 votes

Buoyancy basics

The intuitive way to think about these types of buoyancy problems is in terms of the pressure due the heads of the fluids. Starting with the stated set up, the pressure under the ice cube due to the ...
KDP's user avatar
  • 6,044
3 votes
Accepted

Why doesn't Car-Parrinello molecular dynamics require an SCF calculation?

It is not entirely true that Car-Parrinello (CP) Molecular Dynamics (MD) doesn't require a self-consistent field (SCF) calculation at all. The method must start close to the SCF solution (actually a ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
1 vote

Direction of friction of a block moving down a plane

Kinetic friction will always oppose motion, not other forces. A good tip is to always remember friction (kinetic or static) as a force that tries to prevent motion as its only purpose. So, the ...
Steeven's user avatar
  • 51.4k
1 vote

Direction of friction of a block moving down a plane

friction will oppose the component of gravity acting along the ramp That's not a given. That's just a consequence of some situations where that force is "trying" to move the block, but ...
BowlOfRed's user avatar
  • 41.1k
1 vote
Accepted

Direction of friction of a block moving down a plane

If it’s kinetic (sliding) friction the friction force will act parallel to the surface and opposite to the motion of the block already underway. If it’s static friction the friction force will again ...
Bob D's user avatar
  • 73.7k
1 vote

What are the forces that do work when a spring between two masses pulls inwards?

There is no kinetic energy change in the second system (1,2, spring). So there is no net work done, so no need to ask which external forces that do the work. The constituent particles of the system do ...
Steeven's user avatar
  • 51.4k
0 votes

Would a homogeneous rod on a fulcrum oscillate or remain at rest after displacement?

If the rod were perfectly homogenous, infinitely thin, placed on the fulcrum precisely at the midpoint, and the end released without imparting any motion, the rod would theoretically remain at rest in ...
Bob D's user avatar
  • 73.7k
0 votes

Would a homogeneous rod on a fulcrum oscillate or remain at rest after displacement?

If the rod has insignificant thickness, is uniform, and is perfectly balanced about the fulcrum when level, then the weights of the parts on either side of the fulcrum will be equal no matter what the ...
gandalf61's user avatar
  • 56.4k
0 votes

A Force Perpendicular to the Velocity changes only the direction of motion?

Why does a force that remains perpendicular to velocity alaways [sic] only change the direction of the motion(circular) [sic] and not induce a velocity in the perpendicular direction??? Suppose the ...
hft's user avatar
  • 21.9k
1 vote

A Force Perpendicular to the Velocity changes only the direction of motion?

The word 'perpendicular' indicates a force direction that neither adds to nor subtracts from the forward momentum of the object in question. That is where momentum change is only in direction, not ...
Whit3rd's user avatar
  • 9,943
0 votes

Vertical Circular Motion ($u$ minimum)

If we have mass $m$ on a string of length $R$, on Earth ($g$), then the potential energy vs standard pendulum angle is: $$U(\theta) = mgR(1-\cos\theta) + C$$ where $C$ is a constant. I want to pick $C$...
JEB's user avatar
  • 35.4k
0 votes

Conservation of linear vs. angular momentum in two similar cases

*Case 2: The bullet hits at some point along the rod. Here conservation of linear momentum does not apply (it doesn't work) .... It does apply if you expand the system to include the mass of the ...
KDP's user avatar
  • 6,044
0 votes

Granular Material Piles on an Inclined Plane

The 'angle of repose' is the limit of stability for a granular pile. A pile that takes on a conical shape according to the angle of repose, has slumped into that shape. Tilting the base under such a ...
Whit3rd's user avatar
  • 9,943
0 votes
Accepted

Vertical Circular Motion ($u$ minimum)

For the vertical pendulum, the string will not be taut for speeds at the apex $v < v_c $ where $v_c$ is the minimum speed calculated by letting the tension approach zero. Therefore the bob will ...
Marius Ladegård Meyer's user avatar
1 vote

Conservation of linear vs. angular momentum in two similar cases

It is a valid question, that in case 1 it seems that momentum shouldn't be conserved due to the horizontal force of the pivot point. The simple answer to this question is that, the pivot point does ...
EagerToLearn's user avatar
0 votes

Granular Material Piles on an Inclined Plane

Consider an imaginary inclined plane that passes through a pile. The part of the pile above it is just a normal pile. The question is what happens when such a pile sits on another surface. Piles are ...
mmesser314's user avatar
  • 41.1k
1 vote

A Force Perpendicular to the Velocity changes only the direction of motion?

change the direction of the motion(circular) and not induce a velocity in the perpendicular direction One thing to be careful of in this context is the phrase “the perpendicular direction”. The ...
Dale's user avatar
  • 103k
1 vote

How to understand gyroscopes by using vectors?

About the angular momentum vector: While efficient, the concept of angular momentum vector is itself already a mathematical abstraction. In order to grasp gyroscopic precession we need to see how it ...
Cleonis's user avatar
  • 21.4k
-1 votes

How to understand gyroscopes by using vectors?

.....................................................
Pato Galmarini's user avatar
0 votes
Accepted

Mapping between generalized forces and external torque of a rigid body whose rotation is described by quaternion is not unique(?)

$\def \b {\mathbf}$ Starting with the Euler equations $$ \b I\,\b{\dot{\omega}}+\omega\times\,\b I\,\omega=\b\tau_{\rm{E}}\tag 1$$ with $$\omega=2\,\b Q\,\b{\dot{z}}\tag 2$$ where $$\b Q= \left[ \...
Eli's user avatar
  • 12.4k
0 votes

Is the kinetic energy of a rotating system just the sum of the instantaneous kinetic energies of the individual particles?

What I am trying to establish is if kinetic energy calculated treating the system as a rotating system is just an alternative point of view from treating the kinetic energy as a sum of linear motions, ...
KDP's user avatar
  • 6,044
-1 votes

What would happen to the moon's orbit if we reduce (instantaneously) its mass?

I will assume moon's orbit around the earth is modelled as a Kepler problem. The moon's mass $m_m$ is about 80 times smaller than earth's mass $M_e$. But if you don't neglect it completely, reducing ...
Mateo's user avatar
  • 426
2 votes

What would happen to the moon's orbit if we reduce (instantaneously) its mass?

Mass is a conserved quantity. Having it simply disappear violates the known laws of physics. Once you have violated the laws of physics you can no longer ask what the laws of physics predict, since ...
Dale's user avatar
  • 103k
0 votes

Is the kinetic energy of a rotating system just the sum of the instantaneous kinetic energies of the individual particles?

There is no rotation of masses, you just have movement with v=sin(wt) of several masses in different direction. The video shows, that the projection of circular motion is a sin function.
trula's user avatar
  • 6,362
4 votes

Is the kinetic energy of a rotating system just the sum of the instantaneous kinetic energies of the individual particles?

There's no "linear KE" nor "angular KE", there's only KE. KE of a particle. The KE of a particle is defined as $$K = \frac{1}{2} m |\mathbf{v}|^2 \ .$$ You just need to take the ...
basics's user avatar
  • 10.7k
1 vote
Accepted

Rotating a system

As for what most people mean when they say rotate the axis - yes, it usually implies keeping all objects in the same points in space, and just using new coordinates to express their position, velocity ...
Ido's user avatar
  • 26
0 votes

Some questions regarding a pulley

I won't attempt a point by point response to each question, since they are all somewhat interrelated and may be due, at least in part, to a misunderstanding of some basic concepts. So I will present ...
Bob D's user avatar
  • 73.7k
0 votes

With two balls connected to a string find minimum upward velocity that can be given to one of the balls such that the other leaves the ground

I have done upto this, but I don't know how to further solve this second order differential equation.
OP BOSS's user avatar
0 votes

Some questions regarding a pulley

$-T$1 refers to the reaction force (Newton's third law). Clearly, since the tension in the rope is not causing any acceleration, there must be an opposite force. Take the example of a tug of war. The ...
Ritesh Nandi's user avatar
0 votes
Accepted

Tendon excursion method application/alternative for joint represented by quaternion in biomechanics

The answer is actually in the same paper I cited. The mapping between generalized forces and spatial forces is derived using virtual work: $F_{Q} = 2G^T T'$ where $F_Q$ is a $4 \times 1$ vector of ...
blenderman's user avatar
3 votes

Why can we treat a ball as a point mass to calculate torque?

The torques can be calculated in respect to any point, and it generally requires taking integrals. However, there are some basic tricks to simplify life. One of them is choosing the rotation axis at ...
Roger V.'s user avatar
  • 60.3k
11 votes
Accepted

Why can we treat a ball as a point mass to calculate torque?

Calculating the torque on a rigid body w.r.t to the point $\vec 0$ (WLOG) with gravity pointing in a constant direction $\hat n$ is accomplished by integrating over the rigid body, with each ...
jwimberley's user avatar
  • 3,888
0 votes

Why does atmospheric drag speed up satellites?

Here's a rough mathematical description: Imagine a small body of mass $m$ orbiting at a speed $v$ around a large body of mass $M$. The radius of the circular (frictionless) orbit is given by equating ...
CompassBearer's user avatar
0 votes

Why does atmospheric drag speed up satellites?

My theory is that the work done by the drag force is insignificant in comparison to the whole energy of the orbiter...it essentially pushes it to a lower orbit while mostly conserving the total energy....
Ján Lalinský's user avatar
3 votes

A paradox while explaining the equilibrium of books

Not all friction forces labeled $f$ in the sketch are identical. I fact, they become smaller as you move towards the centre on the stack. Realizing this removes all of the mentioned issues. The ...
Steeven's user avatar
  • 51.4k
0 votes

Forces in tangentially-accelerating circular motion

Consider your experiment of a driver travelling in a circle. Now when you make the free body diagram of the driver, you can notice that the only force acting on the horizontal direction is due to ...
Science Tard's user avatar
1 vote

Relative acceleration, mechanics

Perhaps it would help to think of relative displacement and relative velocity. Suppose m and M are on the table. Suppose M is at $x_M = -1 \space m$ and m is at $x_m = 2 \space m$. You can say what ...
mmesser314's user avatar
  • 41.1k
7 votes
Accepted

Spring potential energy, conversion

The problem in your book assumes an ideal (lossless) and massless spring. That is typical of entry level physics problems. Without the ball you would need to consider, at a minimum, that the spring ...
Bob D's user avatar
  • 73.7k
5 votes

Spring potential energy, conversion

In the case where there is no ball (and assuming there is no friction/other dissipative forces), the spring will expand beyond its natural length due to the momentum it gains while unstretching. At ...
CompassBearer's user avatar
1 vote
Accepted

Confusion about sign conventions used in the Doppler formula

The Talk-page of that Wikipedia article has a section (link) about the sign conventions of several physics books. Until 2019 the article used the convention from Sears&Zemansky, University Physics,...
jkien's user avatar
  • 2,629
18 votes
Accepted

A paradox while explaining the equilibrium of books

You are correct, in a scenario with an even number of books there will be no friction acting between the pair in the middle. To see this, take the simplest case - two books. The force $F$ applied by ...
CompassBearer's user avatar
1 vote

Tension of rope over frictionless edge obstructing block

I would consider each bent part a point of interest. Starting at the last bend before the hanging mass, let's fix the rope on the edge and calculate the tension for this segment. Call it $|T_2|=mg$. ...
ABetheGammow's user avatar
2 votes
Accepted

Work Done by kinetic friction in Circular Motion

In circular motion, your total displacement $$\oint \mathrm{d}\vec{s}$$ is zero, correct. But for work, you are not integrating just the displacement $\mathrm{d}\vec{s}$, but the quantity $\vec{F}\...
CompassBearer's user avatar
-1 votes

Does a rocket moving in a circle expel exhaust at a greater velocity?

Accordingly, must not the exhaust now obtain the entirety of the spent energy? Yes. From the rocket's frame of reference, would the exhaust gases be perceived as exiting at a greater velocity than ...
BowlOfRed's user avatar
  • 41.1k
1 vote

Does a rocket moving in a circle expel exhaust at a greater velocity?

Energy is frame dependent. In a inertial frame that is momentarily comoving with the rocket, its kinetic energy is zero, and the kinetic energy of the gas is the same for both cases, if we suppose the ...
Claudio Saspinski's user avatar
0 votes

Does work differ between inertial frames?

This will help: "The kinematical roots of dynamics", S. R. Madhu Rao, Am. J. Phys. Abstract: "Whenever the total kinetic energy of an isolated collection of interacting bodies changes, ...
SennGorSan's user avatar
8 votes

Does a rocket moving in a circle expel exhaust at a greater velocity?

This is an interesting question, because when it is moving in a circle, the magnitude of its tangential velocity is constant and its angular velocity is also constant. Therefore the total kinetic ...
KDP's user avatar
  • 6,044
5 votes

Does a rocket moving in a circle expel exhaust at a greater velocity?

The energy of the fuel does become the kinetic energy of the exhaust. Let us ignore the fact that the rocket becomes lighter as it burns fuel, and that it will eventually run out of fuel. The rocket ...
mmesser314's user avatar
  • 41.1k
0 votes

When an object is thrown towards the sky it starts to gain potential energy, why?

The push force makes the object move towards the sky and the pull force of gravity makes the object come down towards the Earth's surface. You are mistaken about this part. If you throw an object ...
KDP's user avatar
  • 6,044
1 vote

Strain energy stored in a bungee cord pulled at its midpoint

When a spring is halved, its spring constant $k$ is doubled. Because I'm treating this as a two spring system I need to double the value of $k$ that I'm using. Using a value of $k=280$ N/m gives $x=....
Imperator's user avatar

Top 50 recent answers are included