All Questions
178
questions
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2
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53
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Newton's laws on constant velocity [duplicate]
We know that Newton's second law tells us $F=ma+v\frac{dm}{dt}$ so, if a body has a constant velocity but a changing mass, then the equation tells us that the net force acting on the body is $F=v\frac{...
14
votes
7
answers
4k
views
Why does a charge need to have a velocity to experience force in a magnetic field?
While studying magnetism, I learned that force on a charged particle inside a magnetic field is
$$\mathbf{F}=q(\mathbf{v}\times\mathbf{B})$$
where all symbols have their usual meaning, which implies ...
- 161
1
vote
2
answers
114
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This model of friction implies the speed decays exponentially, but shouldn't it actually vanish within a finite amount of time?
A snooker player strikes the cue ball, so that it starts moving with constant speed $v_{0}$. We assume that spin plays no role in its motion. Due to friction between the ball and the cloth of the ...
- 743
0
votes
3
answers
71
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How do mass and velocity relate to momentum? [duplicate]
So this is more of a clarifying question. A lot of online definitions state that momentum is a measure of how hard it is to stop or swerve an object, which makes sense. However, the formula for ...
- 539
0
votes
1
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43
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What is the physical significance of this generalised potential?
Consider a generalised potential of the form $U=-f\vec{v}\cdot\vec{r}$ where $f$ is a constant. This potential should not contribute any internal forces between particles as
\begin{equation}
\vec{F}=-\...
- 838
0
votes
1
answer
92
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If a force depends on velocity, then why is the force not conservative? I need a formal proof [duplicate]
I am currently an undergraduate taking a course on Newtonian mechanics. The lecturer defines a force to be conservative if there exists a scalar function (we call it potential function), say $V(x,y,z)$...
- 175
0
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1
answer
83
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Why does my 3rd method not show me that power = force dotted with velocity?
I have derived 3 expressions for the power that a force, $\vec{F}$, transfers to a body. (A) and (B) are consistent, but (C) isn't. Where have I gone wrong with (C)? EDIT: THE MISTAKE HAS NOW BEEN ...
7
votes
9
answers
2k
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Why does force perpendicular to the velocity change only its direction; not the speed?
While analyzing the case of a force and consequently an acceleration acting perpendicular to the velocity of a given body, I do understand that force's component along the velocity will be 0 causing ...
- 177
2
votes
5
answers
1k
views
Why do we have momentum?
Momentum is the defined as the product of mass and velocity and can be thought as measuring how much motion something has. However, it is not clear to me why we need momentum and why force is not ...
0
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3
answers
85
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When a person falling reachs terminal velocity, the body moves in a constant velocity. But shouldn't the body not move bcs there are equal forces?
Why is the person not staying stationary in the air when there is equal forces from both sides?
0
votes
0
answers
61
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How can the equation for generalized force be derived assuming the system is in equilibrium?
I have been going over sections 1.4-1.5 of Goldstein's Classical Mechanics where the equation for generalized force
\begin{equation}
Q_j=-\frac{\partial U}{\partial q_j} +\frac{d}{dt}\frac{\partial U}{...
0
votes
1
answer
29
views
Finding the velocity when the force is in terms of position [closed]
(The unit of position, mass, time is m, kg, s.)
On a horizontal line, a force $F=-6x$ (where $x$ is in meters) is applied to a particle of mass $M=3$. When $x=3$, the velocity $v=6$. What is the ...
- 115
0
votes
1
answer
99
views
Does $Power=F\cdot V$ imply $K.E=mv^2$?
The work done on a body with mass is equal to the constant force applied on the mass, multiplied by the distance over which that force is applied ($W = Fd$). Dividing both sides by the time over which ...
0
votes
1
answer
89
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Is a velocity-dependent force $\vec{F}$ that doesn't do any work on an object a conservative force? [duplicate]
Let's consider a point like object with mass $m$ upon which acts a force $\vec{F} = \vec{c} \times \vec{v}$ ($\vec{c}$ is supposed to be a constant vector). Given that $\vec{F}$ is perpendicular to $\...
- 3
0
votes
2
answers
157
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Why do we write force = mass × acceleration? [duplicate]
Force is directly proportional to mass and rate of change in velocity i.e., acceleration. So why do we write force as mass times acceleration? I mean why do we multiply mass by acceleration?
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