33 votes
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Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

You are right. Perfectly rigid bodies are an idealization, like point particles or massless frictionless pulleys. They do not exist. But they are useful. Plenty of objects exist that are so rigid that ...
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15 votes
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Do electrons have inertia?

Inertia is the resistance of any physical object to a change in its velocity. This includes changes to the object's speed, or direction of motion. An aspect of this property is the tendency of objects ...
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  • 221k
15 votes

Trying to understand Newton's Second Law

It is normal that an object has a constant velocity while we apply a constant force. But it is an indication that there is another force acting in the opposite direction, usually some friction force. ...
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7 votes

Trying to understand Newton's Second Law

My thinking here is that change in position means change in velocity This is wrong. A change in position does not necessarily mean a change in velocity. If an object is moving at a constant velocity ...
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  • 7,265
7 votes

Trying to understand Newton's Second Law

$a$ is the rate of change of velocity. So if $a$ is zero, $v$ is constant and doesn't change over time. So while you're applying a force, the velocity is changing and $F=ma$. After the force stops, ...
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  • 407
7 votes

Does this prove that the force and momentum formulas are wrong?

Assuming that OP means Newton's second law by force formula, Note that it says $$\mathbf{F}_\text{net}=m\mathbf{a}_\text{net}$$ In you situation, you are putting two forces from opposite sizes so that ...
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3 votes

Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

Not quite. You are correct that rigid bodies do not exist, but this is not the reason. Your assumptions imply that for a velocity to go from a positive value to a negative value, it must go through ...
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3 votes

Do electrons have inertia?

In physics, the phenomenon of inertia is in a category where an exhaustive explanation for it is not available (and may never be). The point is: in order to have a theory of physics at all the ...
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  • 15.2k
3 votes

What is the work done by friction on a body from the perspective of different observers using the formula $-μmgl$?

Actually, $l$ is not a constant. The different observers measure different values for $l$. The discrepancy in the change in KE is entirely explained by this disagreement on $l$. So, for a block ...
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  • 64.4k
3 votes

Does this prove that the force and momentum formulas are wrong?

Force does not require speed (or momentum). That is a common misconception. In your scenario, you have a stationary object, so Newton's 1st law applies: $$\sum F=0\quad \Leftrightarrow \quad F_\text{...
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2 votes
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Angular Momentum about a Point

If the angular momentum is calculated with respect to Q, both the position vector and the velocity vector must related to Q. So, the second expression is correct. If the momentum is calculated with ...
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2 votes

Can an object move with a constant velocity without balanced forces?

"How does an object move with a constant velocity on a frictionless plane (when there is no fricitional force to balance the applied force on the object)?" (assuming a rigid body) The answer ...
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2 votes
Accepted

Moving body is collided with a spring system.Why there is a difference in maximum compressed distance for different observers?

Your derivation of the difference in compression between the two frames is incorrect; after all, in Galilean relativity, spatial distances between points at a given time are the same between all ...
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2 votes

Extension in Massive Springs under gravity

The trick is to evaluate the load situation of a slice of lenght $\Delta h$. As it is at rest, the downward force at the bottom of the slice $(F_h)$, plus the weight of the slice must be equal to the ...
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1 vote

Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

No, I think you are wrong. You have forgotten the impulse conservation law $$m v_{1}+m v_{2}=0~\implies~v_{2}=-v_{1} \tag{1}.$$ The energy conservation law reads then$$\frac{1}{2}m v_{1}^2+\frac{1}{2}...
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1 vote

How does a sliding object stop moving by the effect of kinetic friction, if kinetic friction is constant and Fk can't be greater than Fapp on its own

Imagine you throw a ball vertically upward against the force of gravity , the ball would move some distance vertically and then stop due to force of gravity. When you throw the ball , you exert force ...
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1 vote

How to find the force exerted by continuous jet of water on a wall?

If its solid or liquid the law doesnt change, so lets just apply Newtons second law again. $$F=\frac{dP}{dt}=\dot{m}(v_f-v_i)$$ Where $\dot{m}$ is the amount of mass hitting the wall per unit time ...
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1 vote

Trying to understand Newton's Second Law

If I apply a force on an object at rest and it’s velocity is constant (after force is applied) wouldn’t that mean that the acceleration is 0? After the force is applied, i.e. when the force is no ...
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  • 125
1 vote

Trying to understand Newton's Second Law

There's two things to note here. The first one is that both quantities $\mathbf{F}$ and $\mathbf{a}$ are actually functions of time, not just numbers. If one wants to be explicit, one can write this ...
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1 vote

Trying to understand Newton's Second Law

If I apply a force on an object at rest and it’s velocity is constant This is not possible. Newton's first law already says that if you apply force, velocity will change. So the situation you ...
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  • 11
1 vote
Accepted

Konig's Theorems

The law relating velocities in both frames is always valid, it's only the expression of $\vec{v}_c$ that changes depending on the case. If $S'$ is in pure translation with relation to $S$, $\vec{v}_c=\...
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  • 759
1 vote

Maintaining the balance of a rod of uniform density

Approach 2 yields the correct answer. There’s a slight problem with your approach 1 that is resulting in that factor-of-2 difference. While treating the rod as a single body with the centre of gravity ...
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  • 1,811
1 vote
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Drawing spacetime diagrams assuming absolute time (page-6,Section 1.5, Schutz)

If you're considering a Newtonian/Galilean change of coordinates, the formulas are related as \begin{align} \bar{t}=t,\quad\bar{x}=x-vt. \end{align} This means take any point $p$ in the spacetime. It ...
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  • 2,492
1 vote

Drawing spacetime diagrams assuming absolute time (page-6,Section 1.5, Schutz)

On a Newtonian or Galilean Spacetime diagram (the PHY 101 position vs time graph), the circle is a horizontal line in your diagram. (In a Minkowski diagram, the circle is a hyperbola.) Lines of ...
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  • 8,048
1 vote

Are time and length independent or can one be derived from the other?

"Time" is not a quantity at all, it is a coordinate similar to "north", "east", or "up" (or "x", "y", "z"). What Rindler is saying ...
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  • 5,893
1 vote

The image contains the question. The image descriptions contains my DOUBT

No, the net force acting on both the charges at the extreme points will not be zero. In fact, it will be maximum at the extreme points. This is because, initially, when the system is released, there ...
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1 vote

A problem Understanding how a two-body system of planets starts rotating around barycentre

Your question seems to indicate that you think of the rotational motion around the barycentre as some stationary state the two-body system reaches after some short time after the initial conditions ...
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  • 23.4k
1 vote

A problem Understanding how a two-body system of planets starts rotating around barycentre

The velocity $\mathbf v_1$ of $M_1$ must be such that there is not radial component when $M_2$ is released. If this condition is fulfilled, there is a specific $|\mathbf v_1|$ for a circular orbit.
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1 vote

A problem Understanding how a two-body system of planets starts rotating around barycentre

In your model, there is no expectation of circular motion. The usual theoretical understanding is that planets form from gas and dust in an accretion disk. The gas and dust in a disk moves in nearly ...
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  • 3,261
1 vote

Does this prove that the force and momentum formulas are wrong?

When you say that force and momentum formulas are based on motion, that is not completely true. For example, Hooke's law says that the degree to which a spring is stretched or compressed is ...
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