# Tag Info

Accepted

### What is the terminal velocity of coin

There are a couple issues Your coin might not have reached terminal velocity yet. Assuming your value of the drag coefficient, how long would it take to reach terminal velocity? Experimentally it ...
Accepted

### How does the clock postulate apply in non-inertial frames?

The clock hypothesis of relativity theory equates the proper time experienced by a point particle along a timelike curve with the length of that curve as determined by the metric. This is indeed the ...

### Length contraction and acceleration in opposite directions

When you accelerate right from rest to $0.9c$ you are quite correct that distances are Lorentz contracted so the distant objects get nearer to you. However if you now start accelerating left your ...
Accepted

### Maximum height reached by an object

Here, in the last line, from the formula s is negative, but how can s be negative? It's a typo. Also, should it not be s=u^2/2g instead of s= - u^2/2g? Yes.

### Why does velocity change or accelerate in the presence of force?

Well okay, this is a tricky one. First recall when is the equation $F=ma$ valid. You may know that it is valid in a special class of frames of reference, known as inertial frames. So now the question ...
1 vote

### How to find time taken for a faster object to cross a slower object of same length, both moving parallel to each other in the same direction?

When solving physical problems it is essential to draw diagrams. This helps to develop a concept in your mind (especially when having no idea yet about the solution). Do this before writing down any ...
1 vote

### The resultant force on a simple pendulum bob

This animation from this article demonstrates that the force/acceleration is not zero at the equilibrium position.
1 vote

### On what ground do you multiply $m$ with $v$ in the momentum equation $p=mv$?

Strictly speaking, a definition doesn’t need justification. However, some definitions are useful and important, and it is relatively easy to show why they are useful. In the case of momentum, ...
1 vote

### Free-body-diagram of a falling ball

I don't understand how you concluded that the ball is slowing down. It isn't. Its acceleration is decreasing. If it were slowing down, the velocity-time graph would curve downwards. Here it is ...
1 vote
Accepted

### Velocities - Equation 1.46 of Goldstein 3rd edition

He is using the standard chain rule for partial differentiation from calculus. The partial derivative and the total derivative are not the same. See any calculus text such as one by Kaplan, Thomas, or ...
1 vote

### How does the clock postulate apply in non-inertial frames?

But this seems problematic to me because it only ever seems to be true in inertial reference frames. For example, in the (non-inertial) frame of the traveling twin in the twin paradox, the earthbound ...
1 vote

### How does the clock postulate apply in non-inertial frames?

I think one way to resolve your conceptual issue is to consider that motion causes the effect of time dilation, and acceleration causes motion, so that it is true to say that acceleration causes time ...
1 vote

### When the mass flux is constant, why does $v d\rho = - \rho dv$?

Since $j$ is constant, $dj=\rho dv+ vd\rho=0$. Any change in the density must be compensated by a change in velocity to keep the mass flow constant.

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