# Questions tagged [velocity]

The time rate of change of the position of an object

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7answers
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### Relative velocity greater than speed of light

Is it possible a relative velocity between 2 objects to be greater than the speed of light? I know that it is impossible to reach the speed of light considering us to be immobile but is this ...
1answer
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### Is the velocity gradient of a homogeneous viscous fluid constant?

The formula for the viscosity of a fluid (or even a gas) is equal to \begin{align*} \frac{F}{A} =\mu\frac{du}{dz} \end{align*} where $F$ is the force applied to the top layer of the fluid, $A$ is the ...
1answer
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### What does speed of an object relative to water mean?

A swimmer swims with the velocity 1.25 m/s "relative to water". He has to swim across the river which is 150 meters wide. If his direction of velocity is perpendicular to the stream, he is taken away ...
1answer
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### If two surfaces slip on each other then their velocities along the common normal are equal?

If two surfaces slip on each other then their velocities along the common normal are equal?
0answers
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### Which ball reaches the end first? [closed]

This question came up in a physics paper: there are two tracks A and B, with the direction of gravity specified. If two balls begin to move at the same uniform velocity at the same time, which of the ...
1answer
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### If 2 spaceships approaches each other at 0.99c, how much blue shift would they see? [closed]

Imagine these 2 spaceships moving at 0.99c toward each other and speed isn't additive how much blue shift would they see? I know how to calculate when the scenario is spaceship vs Earth but how about ...
2answers
132 views

### Four-velocity and the metric tensor

The metric tensor $g_{{\mu}{\nu}}$ has this property $$g_{{\mu}{\nu}}g^{{\mu}{\nu}}=4$$ and the four-velocity, $U^{\mu}=\frac{dx^{\mu}}{d\tau}$ which has this property $$U_{\mu}U^{\mu}=c^2.$$ So ...
1answer
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### How do we write free body diagram for an object that is rebounding?

How do we write free body diagram for an object that is rebounding? Here's an illustrated example Let us assume that the object has an initial force. So, I want to draw free body diagram for each ...
1answer
56 views

### What is the interpretation of speed of a travelling wave, based on $\frac {\partial x}{\partial t}$ formula

Motion satisfies the wave equation if the following holds true $$\nu^2 * \frac {\partial ^2 \psi} {\partial x^2} = \frac {\partial ^2\psi} {\partial t^2}$$ where $\psi$ is the vertical displacement ...
1answer
279 views

### What if a spaceship going half the speed of light turned on its headlights in both directions?

If the spaceship, travelling at c/2 were to turn on only its forward facing headlights, then after one second (relative to us, the stationary external observers) we would see that the ship has moved ...
14answers
20k views

### Why doesn't a braking car move backwards?

The net force on an object is equal to the mass times the acceleration, $F = ma$ When I brake on a (moving) car, the net force is negative, therefore causing the resulting acceleration to also be ...
2answers
104 views

### Question about derivation of four-velocity vector

In order to describe a notion of rate of change of positon, in four-dimensional spacetime, we have to introduce the concept of four-velocity. So, consider the following: For a massive particle ...
3answers
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### What would be the minimum velocity of a particle performing S.H.M.?

We were asked a simple question on a test: What is the maximum and minimum velocity of a particle performing an SHM? Note here that we're talking about a generic standard SHM here. If the maximum ...
1answer
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### What's the velocity for a person falling from a building?

I saw that in the neighborhood a person fell our of 25th floor down the street (ground floor). I was intrigued by some questions about such situation, but all of them are out of one question: What's ...
1answer
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### Are my observations about the differences between average velocity and speed, in 1 dimension, correct?

I've been struggling to understand the differences between average velocity and average speed in 1 dimension; by doing some exercises I think I have figured it out, but i want to check if I'm correct. ...
2answers
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### If the speed of light is a limit, is there a way to tell which direction we are moving through space and how fast?

Say two photons start at point A, and go in opposite directions. They will be traveling apart at 2x the speed of light correct? So if this limit is how fast you are able to travel through space (and ...
2answers
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### Can two electron moving in opposite direction move faster than light? [duplicate]

Suppose we are moving two electrons in opposite direction both having a speed of 1.6e8 meter per sec.When they will cross each other then their relative velocity will be 3.2e8 ms_1.Isn't it faster ...
2answers
69 views

### Speed of a falling object

When an object is dropped, does it accelerate to terminal velocity? or is it at terminal velocity immediately?
1answer
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### Work done on box [closed]

I am having a hard time grasping this. W = Fd = mad, and given v vs t graph, the area under the curve will give us d. When the velocity graph draws a horizontal line, a=0. Thus I thought that A = C ...
1answer
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### Types of Collisions and velocity

I know that there are three different collisions elastic, inelastic and perfectly inelastic. I wanted to know how the three velocities would rank respective to the type of collision. For example, if a ...
1answer
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2answers
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### Can acceleration at a point be non-zero if angular velocity is zero?

I have just started studying rotational kinematics.
1answer
270 views

### How is the power of water flowing through a pipe directly proportional to $v³$ and not $v²$?

How is the power of water flowing through a pipe directly proportional to $v^3$ and not $v^2$? (Where $v$ is the velocity of water) I have been trying to write power in terms of $v^3$ but I'm ...
1answer
156 views

### Is this Question correct? Newtonian Motion - Relative Motion of Rain

There is a question in a textbook which states: "A cyclist is riding north at 12km/h when it starts to rain. The rain appears to be falling towards her at an angle of 10 degrees relative to the ...
3answers
216 views

### What is the fastest possible acceleration?

Context & Question In physics, the fastest possible speed is accepted as the speed of light. No other mass has been recorded to travel as fast as the photon. What is the acceleration of the ...
0answers
25 views

### When is the group velocity of a wave packet the velocity of energy flow?

In particular, I'm asking this for anisotropic media
3answers
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### Why speed = distance/time? [duplicate]

Is it merely an arbitrarily chosen definition that we created in order to quantitatively measure the speed of an object or is it some other way around. I want to know what's the reason behind such a ...
1answer
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### A Question about Index Notation of a Scalar Product [closed]

Could anyone please explain why an index $k$ is added to the scalar product of the velocities (squaring the velocity) in the figure below? Can't we use the same index $j$ for the second factor? \...
1answer
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### What is the fastest tangential velocity?

What is the fastest tangential velocity, that was reached by a man-made object? I have only found examples of angular velocity. More specifically, I am looking for the speed of the rim of spinning ...
1answer
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### Speed of light, can it be not constant? [duplicate]

What if we switch on a flashlight while travelling at the speed of light (we are travelling in vacuum, in a straight path)? What will happen and what will be the speed of light from the flashlight? ...