Questions tagged [velocity]

The time rate of change of the position of an object

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7answers
269 views

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 ...
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1answer
86 views

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 ...
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1answer
155 views

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 ...
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1answer
111 views

Gamma factor in special relativity

I try to derive the Lorentz transformation of a Lorenzt transformation frame an inertial frame $O$ to the frame $O'$ of a moving particle at constant speed v. We have four vectors $\textbf{x}'=\Lambda ...
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3answers
129 views

Surviving a fall into a balloon-filled pit [closed]

My brothers and i have been debating this question for years. If an average person fell at terminal velocity into the center of a pit that is 1 mile deep x 1 mile in diameter, and the pit is filled ...
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0answers
46 views

Where does this equation come from?

I am investigating about viscosity of fluids and I found the following equation to calculate the viscosity: $$\nu = \frac{2(\rho_s-\rho_l)\cdot g \cdot r^{2}}{9\cdot\Delta v} $$ The experiment ...
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1answer
83 views

How can one experimentally measure the velocity of water flowing through a pipe?

I am planning to investigate the effect of bend curvature of a pipe on velocity of water. For this, I will need to find the velocity of water through various pipe bends. Would placing an object in the ...
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1answer
164 views

How to know what the area under curve represents?

Is there a way to find out what the area under the curve represents? For eg. If i gave you a graph of $v$ with respect to $t$ would you be able to tell me what the area under the curve represents ...
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1answer
34 views

Should there be a significant differential in wind velocity indoors vs outdoors in relation to Earth's rotating atmosphere?

What is the behavior of a rotating Earth's atmosphere indoors? Should there be any differences experienced between the atmosphere spinning in lockstep with the Earth outdoors vs. closed-off indoors? ...
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3answers
70 views

Rule of addition of velocities in water

Is the speed of light still the same for all inertial observers in water? If not, what are the rules of addition of velocities in water according to special relativity?
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4answers
197 views

Why is escape velocity independent of direction of projection?

However, for any body projected with a velocity less then the escape velocity, the final maximum height will vary with variation in angle of projection. But why is this not the case in case of ...
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0answers
42 views

PG-Metric dr/dt

at the moment, I investigate some time in learning General Relativity and there I saw the Painlevé-Gullstrand Metric which is given as $ds^2 = -dT^2 + \left(dr+\sqrt{\frac{r_s}{r}} dT\right)^2 + r^2 ...
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0answers
26 views

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?
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0answers
167 views

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 ...
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1answer
52 views

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 ...
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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 ...
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1answer
29 views

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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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3answers
291 views

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 ...
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1answer
36 views

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 ...
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1answer
48 views

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. ...
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2answers
102 views

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 ...
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2answers
51 views

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 ...
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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?
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1answer
74 views

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 ...
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1answer
76 views

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 ...
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1answer
92 views

Relation between strain and velocity

The strain tensor writes $\epsilon_{ij}=\frac{1}{2}\Big(\frac{\partial u_i}{\partial x_j}+\frac{\partial u_i}{\partial x_j}\Big)$ with $u_i$ the displacement in the $i$ direction. Then $\frac{\...
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2answers
96 views

Confused with derivative and partial derivative

suppose $x=f(t)$ with a constant acceleration. Then does $\frac{\text d x}{\text d t} = \frac{\partial (x)}{\partial(t)}$ since the position in $x$ only changes with time? Then the acceleration in ...
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3answers
103 views

Velocity of massless particles

Due to the equation $E = \frac12{mv}^2$, can a massless particle travel at an infinite speed in a vacuum (as its mass would be $0$ so its energy would also be $0$)?
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1answer
86 views

How does an object in space travelling at constant velocity have a net force of zero acting upon it?

If the definition of balanced forces is "two opposing forces that are equal" and an object with a net force of zero acting upon it means that the forces are balanced on the object, then it should ...
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1answer
108 views

Using $v_f = v_0 +at$ for objects in free fall [closed]

I have a question about the difference of using $v_f = v_0 +at$ and $s = v_0t + \frac{1}{2}at^2$ for objects in free fall. I'm trying to solve a problem where there's a ball rolling along an inclined ...
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1answer
245 views

How to get the four-velocity components from a given metric tensor?

I’m a little bit confused about how to get the four-velocity components from a given metric tensor (or line element). For instance, which are the components of the four-velocity in the Schwarzschild ...
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1answer
175 views

What happens to the fluid flow velocity and concentration in a tube that branches out

Suppose that liquid is flowing through a tube with a constant velocity of, say 10 m/s. Within the liquid, there is a constant concentration of some substance, [S], lets say of 20 uM. Then, at some ...
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1answer
70 views

Why is velocity mathematically describes as a division? [duplicate]

I want to know why, in kinematics, is velocity described as $v = \frac{\Delta x}{\Delta t}$, and why it is not described as any other expression (like a multiplication), why does a division is the one ...
26
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4answers
6k views

With what velocity are we moving along the time dimension?

Does the question make sense? Velocity along time axis means $v_t=\mathrm dt/\mathrm dt$? If it doesn't, please explain where the flaw is. Taking time as measure like length? Or do we need to ...
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5answers
804 views

Why does relativistic velocity subtraction produce a larger relative velocity than classically?

I got a question for homework saying that there were two rockets on a parallel track, heading towards earth. Rocket A was in front of Rocket B. Rocket A was travelling at a velocity of $0.75c$ from ...
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2answers
79 views

Why is force related to acceleration instead of velocity?

I know $F=ma$, but it's never really made sense to me. Here's an example: Hypothetical scenario- I push a bowling ball ($m = 4\,\rm kg$) sitting on the floor with $5\,\rm N$ of force. (Assume no ...
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1answer
166 views

Using the Minkowski metric show that $u_au^a = -c^2$

Using the Minkowski metric show that $u_au^a = -c^2$. I am trying to solve the above problem. Firstly $4$-velocity can be defined as $$u_a = \frac{dx^a}{dt'}$$ where $dt' = dt\sqrt{1-\frac{v^2}{c^2}...
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2answers
176 views

Can acceleration at a point be non-zero if angular velocity is zero?

I have just started studying rotational kinematics.
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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 ...
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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 ...
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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 ...
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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
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3answers
127 views

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 ...
3
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1answer
54 views

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? $$\...
2
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1answer
67 views

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 ...
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1answer
57 views

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? ...