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11 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

A simple example to specifically point out your error here. In general $v_p$ can change even if there's no force at all! So it seems to me that $v_p$ should keep constant as there is no force acting ...
JiK's user avatar
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9 votes

Special relativity and accelerating twins

For any observer there is a property called the proper time that is equal to the time shown by the clock they are holding. So to find out how the elapsed times of the twins compare we just have to ...
John Rennie's user avatar
6 votes

A person is pushed forward in a bus when brakes are out because of inertia or pseudoforce?

Yes, that is just two ways of saying the same thing. The different descriptions stem from using different reference frames. The pseudo force (I prefer the term "inertial force") is simply ...
Dale's user avatar
  • 102k
5 votes

Special relativity and accelerating twins

UPDATE BELOW. Among twins, the inertial one is the "preferred" one. Between two fixed events, the inertial twin visiting both events ages the most. However, in general, one has to compute ...
robphy's user avatar
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5 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

You can prove that $\mathbf{L}$ is constant performing time derivative and remembering that the force is central, $$m \ddot{\mathbf{r}} = m \dot{\mathbf{v}}= - G\frac{M \, m}{r^3}\mathbf{r}$$ i.e. ...
basics's user avatar
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5 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

System: Planet $B$ $\dots$ kinetic energy conserves for a planet in an elliptical orbit is not a correct statement. The force $F$ on body $B$ due to body $A$ is central and has a component in the ...
Farcher's user avatar
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4 votes

A person is pushed forward in a bus when brakes are out because of inertia or pseudoforce?

Be careful because there is an illusion taking place here. When the bus brakes, then the person is not being pushed forwards. Rather, the bus is being pulled backwards by a friction force - the person ...
Steeven's user avatar
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3 votes
Accepted

A theorem on page 72 in The Large Scale Structure of Space-Time

You didn’t mention the most important part of the definition of staticity, which is that $K$ must be “hypersurface orthogonal”, or in other words, the subbundle $\mathcal{K}:=\text{span}(K)\subset TM$ ...
peek-a-boo's user avatar
  • 6,605
2 votes

Motion formula gives correct answer but without formula gives wrong answer

You stepped into the magical wonderland of numerical integration. Basically, you want to estimate the area under the line $v=v_0+at$, or the integral of $v$ from 0 to 5. You chose a timestep of $\...
AccidentalTaylorExpansion's user avatar
2 votes

Terminology for time derivative of speed (not velocity)

I want the time derivative of speed. This is a useful concept, because it matches the colloquial sense of the word ‘acceleration’; if this quantity is positive then an object is speeding up (...
ryang's user avatar
  • 804
2 votes
Accepted

Why doesn't a part separated from an accelerated body accelerate anymore?

This depends on how the force is applied to the body. If it is a force field acting on all parts of the body, then what you write is not true. Then there will still be force on the separated part. If, ...
Jos Bergervoet's user avatar
2 votes

Radiation from accelerated charged in a co-accelerated reference frame

It is quite easy, despite all the difficulties the comments are warning us about, to express this question in a mathematically tractable way: (and to actually answer it!) Solve the Maxwell equations ...
Jos Bergervoet's user avatar
2 votes
Accepted

Does coordinate acceleration match proper acceleration when $v=0, s=0$?

Yes, that is the definition of proper acceleration which is the coordinate acceleration with respect to the instantaneously comoving inertial frame. In this frame, the proper acceleration has temporal ...
Vincent Thacker's user avatar
2 votes

What does the $F$ in Newton's second law equation mean?

The acceleration of an object only depends on the forces acting upon that object. Imagine a ball falling down and hitting the floor. There are two forces due to the impact: the force on the ball from ...
Mark H's user avatar
  • 24.1k
2 votes

How many $G$ of accelaration do we experience when we come to a flat surface from a slope?

As you go from the slop onto the flat you (and the bike) are accelerating. Even though your speed may be constant, the direction of your velocity is changing - it is rotating through an angle $\theta$....
gandalf61's user avatar
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2 votes
Accepted

Why does a ball bounce back even when it is released and not thrown?

When the ball reaches the ground with some momentum, it becomes compressed due to inertia (think of a spring for example). Its kinetic energy is then converted to potential energy (spring-like energy) ...
Jesse's user avatar
  • 61
2 votes

A person is pushed forward in a bus when brakes are out because of inertia or pseudoforce?

A pseudoforce alone can explain the motion of the man, but only from one very particular reference frame - the non-inertial, accelerating reference frame of the bus itself. The pseudoforce only exists ...
Nuclear Hoagie's user avatar
2 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

Angular momentum is a conserved vector, by your calculation, it is composed of two potential variable: $\vec{L} = \vec{r} \times m\vec{v}_p$, $m$ is a constant. In an elliptical orbit, $\vec{r}$ is ...
Polaris5744's user avatar
2 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

$v$, the velocity vector of the body, is not constant in magnitude because in a generic elliptic orbit it is not perpendicular to the radius. It is, however, in circular orbit which indeed has ...
ebenezer's user avatar
2 votes

What changes the velocity perpendicular to radius in an elliptical orbit?

So this question is about why $v_p$ changes when there is apparently no force in that direction. The answer is, this is polar coordinates being annoying and tricky. The answer from JiK gives a good ...
CR Drost's user avatar
  • 38k
1 vote

Why does a ball bounce back even when it is released and not thrown?

when we thrown a ball towards ground we apply an additional amount of force...When this ball reaches the ground,...the ground experts a similar amount of force on the ball. That's not quite right. ...
Solomon Slow's user avatar
  • 14.8k
1 vote
Accepted

How many $G$ of accelaration do we experience when we come to a flat surface from a slope?

The limiting case is when you have a sharp transition from a ramp to flat. Before you encounter the transition you feel the ground reaction $W \cos \theta$ which is slightly less than the weight $W$ ...
John Alexiou's user avatar
  • 38.7k
1 vote

What does the $F$ in Newton's second law equation mean?

The $F$ in Newton’s Second Law is always the force acting on the object undergoing acceleration. What you are describing in the latter half of the post sounds more like a collision to me. https://...
Nf23kdr's user avatar
  • 61
1 vote

What does the $F$ in Newton's second law equation mean?

The force in the equation $\vec F = m \vec a$ is the force acting on the body of mass $m$ and whose acceleration is $\vec a$. It is important to avoid two possible sources of confusion. Even though ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
1 vote

Why do period and damping ratio affect acceleration?

Am I misunderstanding the nature of the earthquake force? - Yes. The earthquake force is a force external to the structure and so does not change. You have forced oscillation which means that damping ...
Farcher's user avatar
  • 97.1k
1 vote
Accepted

Gravitational attraction between two bodies and gravitational accelaration

The force on the heavier object is larger but it also has more inertia (resistance to motion). The same quantity, m, is used for mass in calculating the force of gravity, F, and in determining the ...
Not_Einstein's user avatar
  • 2,801
1 vote

Does coordinate acceleration match proper acceleration when $v=0, s=0$?

Yes. We can think for example that Alice and Bob are both in a airplane. In the moment that Bob jumps, Alice has a coordinate acceleration $g$ upward for him. Bob has a coordinate acceleration $g$ ...
Claudio Saspinski's user avatar
1 vote

Is retardation reported with opposite sign as that of acceleration?

Noting that acceleration has a plus/minus sign only in straight-line motion, observe that for a particle with initial velocity $-20$m/s and constant acceleration $15$m/s², its speed at around the ...
ryang's user avatar
  • 804
1 vote
Accepted

Is retardation reported with opposite sign as that of acceleration?

This is a convention question in English and has very little to do with physics. If an object is moving faster and faster, i.e. magnitude of velocity (i.e. speed) is increasing, then under no ...
naturallyInconsistent's user avatar
1 vote

Is retardation reported with opposite sign as that of acceleration?

Acceleration is a vector, so it can be either positive or negative, depending on your coordinate system. If my car's speed is increasing by $1(m/s)$ every second and my positive $x$ axis is located, ...
Lagrangiano's user avatar
  • 1,541

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