# Tagged Questions

The time rate of change of the position of an object

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### confusion regarding force applied at different angles and work thus done? [closed]

considering $f=1$ and $s=1$ in the work-energy theorem , $W= \cos(\omega)$ this shows that as we increase the angle we do less work for moving an object to the same distance. It also means that ...
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### What's the nature of intraction between hydrogen atoms in the interstellar medium?

Interstellar densities are about 1 atom/cm$^3$. Do these atoms even move? If they do, why do they move? What would be their average speed? (Free expansion)
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### If mass increases as velocity increases, does gravity increase as well? [duplicate]

The way I understand it, velocity and mass are tied together with relativity. As I increase in velocity, I increase my mass until my mass is so great that it would take more energy than the entire ...
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### Velocity of propagation of an EM field in vacuum

I am asked to find the speed of propagation $\vec{v}$ of an electromagnetic field, in a region of empty space. The magnetic field is described by $\vec{B} = B_0e^{ax}\sin{(ky-\omega t)} \hat{z}$. I ...
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### How to use Special Relativity in simple numericals? [closed]

So I was recently thinking about what an electron's velocity would be after it was accelerated through a potential difference of 1 V. Doing simple calculations like just dividing its energy (which ...
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### What's the equation for the magnetic field created by a single moving electron?

Suppose we have an electron moving in a straight line with velocity $v$. My question is: What is the equation that describes the magnetic field generated by this movement. I'm unsure how to proceed ...
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### Velocity of water column

If an air filled tube 5 km long with a diameter of 20 m was submerged vertically under water one end 5 km underwater and the other end at the surface, how fast would the water column rise if the ends ...
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### Velocity selector and charge to mass ratio of an electron

I recently read that J.J. Thompson used a velocity selector to find the charge to mass ratio of an electron. Having learnt the principle behind the velocity selector, I see no relation between these ...
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### How to explain to a child why can acceleration be positive or negative? [closed]

I want to find out how can acceleration be negative? I have an answer and this answer is: $$a=\frac{v_f-v_i}{t_f-t_i} < 0$$ because $v_f < v_i$ where $v_f$ is final velocity and $v_i$ is ...
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### is photon direction affected by source velocity? [closed]

let's suppose the red ball in the below image is an object with V' velocity, the car and the surface travel in V (0.5 c). based on relative velocity laws, the red ball will hit the green pointer ...
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### Can empty space be a frame of reference to measure velocity?

I am wondering if there is only one object in the universe than does it make any sense to talk about its velocity. If empty space can be thought of as a reference to measure its velocity than it might....
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### Force impact when lifting objects traveling at a relative velocity of 0

During a break my colleagues and I came up with the following Gedankenexperiment I would appreciate if you guys helped us find the solution for: Truck (weight 5t) traveling at 80 kph with a tow-...
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### (+)/(-) Acceleration & (+)(-) Velocity

Does a negative acceleration mean an object is slowing down? Does a negative velocity basically mean you are moving at an opposite direction? The sign is just direction?
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### Why did they use two separate vectors to calculate the magnitude and angle of the initial velocity?

The question/solution are on the attached images. (It's dealing with projectile motion) I already understand how to work out part a, but I'm having trouble understanding why they used the final ...
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### Ball Bounce Acceleration at the Point of Bounce [closed]

simple question but I wanted to confirm my approach of thinking about the problem. Question: A rubber ball is dropped from the fifth story of a building and free falls to the ground, after which ...
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### Can absolute speed of object be measured? [closed]

Let's say there are two objects in space - Station and huge non-transparent sphere. Sphere starts to move from station 5000 km/h. Inside the sphere, beam of light is produced in direction the sphere ...
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### How a wheel move on the earth?

I was wondering how a wheel move on the earth ,I mean if friction causes it moves so what causes it stops after a while without any pushing force? I think that the direction of the friction force is ...
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### if mass is dependent on velocity, and velocity is relative, how can we determine rest mass [closed]

If I'm not mistaken, $m=\gamma*m_0$ ($\gamma$ being the lorentz transformation) But $\gamma$ depends on velocity. How do we determine velocity if velocity is relative? If we are currently being flung ...
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### Relative velocity

Question is from Barron's SAT subject test Physics "A flat railroad car is moving to the right at 5m/s. A person standing on the car throws a ball straight upward at 20m/s. If air resistance is ...
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### Portal 2 physics - one moving portal [closed]

A problem is posted and solved so many times on 9gag - till yet no good answer. The problem: you have two portals (one moving and one still standing). The one moving reach a still standing cube: what ...
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### Help with Gravity question

I need some help with this question from my homework. An Oort cloud comet moves on an orbit so eccentric that it can be well approximated by a radial straight line through the Sun. The comet starts ...
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### What is the physical significance of taking derivative with respect to proper time?

I would like to know if there is any physical significance associated with the derivative of a quantity with respect to proper time or is it just a mathematical trick. Since proper time is measured in ...
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### Position from velocity time graph [closed]

I have taken Calculus before but unfortunately no equation is given so I have to do it the old way. An object is at x = 0 at t = 0 and moves along the x axis according to the velocity-time graph ...
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### Trouble with Simple Average Velocity [closed]

I have a super simple question on average velocity that either I am not setting up correctly, or is itself graded incorrectly in a huge online system used by thousands of students for a very long time....
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### Time taken to cover a point object in a circular region

I'm assuming a finite width rotating beam with increasing width as it gets closer to the circular edge. I'm trying to find the time it takes the beam to cross a point object at some distance from the ...
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### What is meant by 'Phase velocity is $c^2/v$' [closed]

Is phase velocity $c^2/v$ or $v/2$? Is there only one phase velocity as indicated by the above formula? If yes then why is wavepacket said to be superposition of 'waves' whose velocity is termed as ...
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### Time and velocity

Suppose two cars are traveling at a velocity of $c/2$ ($c$=speed of light) in two straight lines parallel to each other but in opposite direction in vacuum. Then what will a person sitting in one of ...
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### Does the velocity of steam affect the transfer of heat?

I am analysing a heat exchanger system which uses steam travelling through a finned tube to warm air. I am currently looking at reducing time lags in the system so as to raise the temperature of the ...
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### Are all conserved scalars proportional to relativistic mass? [closed]

I have read in Rindler's relativity book that all scalars depend only on the magnitude of velocity of a particle and that are conserved are proportional to relativistic mass. m(\vec{v})=m_{0}/\sqrt{...
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### Empty space and velocity

If there was a universe without anything in it except you in a space rocket how you would you know what your velocity if you kept the engine on?
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### Objective velocity (Relativity theory) [closed]

Suppose that I have a velocity of (approximately) $c$. Then I decelerate with $c$ per hour for exactly one hour. Then I would know that I would be standing still in the end, because the decelleration ...
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### Calculating electricity used for weights on an elevator

Given an elevator with a motor driven by a power supply, torque ability and a pulley does it take more electricity to lift more people? I have some of the calculations but do elevators lift at a set ...
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### Finding acceleration using components method

The problem I am working on gives me an initial velocity of 2[E15N] m/s and a final velocity of 4[E35N] m/s after 30s. I need to find acceleration so I create a right angle triange for both of these ...
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### Having trouble understanding projectile motion [closed]

So, I learned about projectile motion in class a fe days ago, and am having trouble understanding. I main problem is seeing how velocity works in projectile motion. For example, why is the velocity in ...
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### How can a net force of 0 still lead to movement and how is movement possible with interaction pairs? [duplicate]

So I always thought of movement as interaction pairs. For example, I thought that a rocket moves by applying a force of hot exhaust gases, which then apply a reaction force back on the rocket ...
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### Difference between escape velocity and speed [duplicate]

What is the difference between escape velocity and escape speed , is their any error if we use them interchangeably?
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### Question about acceleration & deceleration in real world?

have searched the net and am not satisfied with any answer I have found to calculate the following:- Here is an example just for reference. Suppose I can accelerate my car from 0-100km/h in 10s. That ...
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Throughout schooling we are given problems in which we are to solve for velocity, force, mass, etc. I have a real life situation in which I would love to solve these exact things. Please consider: Car ...
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With the rapidity $\phi$ defined so that $\frac{v}{c}=\tanh{\phi}$, say you have 3 parallel moving reference frames $S$, $S'$ and $S''$ with a constant but different velocity/rapidity. If the ...
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### motion with constant acceleration question! [closed]

Consider a motorboat maneuvering on the surface of a lake, Is it possible for the boat to have accelerated motion in the x axis and unaccelerated motion in y direction?
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### Velocity of a liquid jet, coming out of a container

Think of a container which has a hole beneath it. And we have filled the container in height $h$ with a liquid of density $\rho$ and cross section of the hole $a$. (look at the pic) Then what is the ...
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### How does a bikerider turn by leaning on one side? [duplicate]

I am trying to understand the following image: Could someone please help me understnding it. What I understood so far: Your angular velocity mutyplied by the impulse gives you the impulsemoment. By ...
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### Why is the potential independent of the generalized velocity?

In Goldstein, Classical Mechanics, Chap. 1.4 we derive Lagrange's equations from D'Alembert's Principle. My question is regarding the last part of the derivation, specifically the part where he ...
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### Calculate velocity with acceleration caused by drag

Maybe someone can help me with calculating the velocity of a sphere falling down. Initial: Acceleration is $10 ~\rm \mathrm{m/s^2}$. Timestep is $1 ~\rm s$. Approximation of drag = \$-v_{...
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### Jumping from a moving train

If you were to jump from a moving train at 20 km/h, would it be better to run in the direction of movement before jumping to maintain inertia or in the opposite direction of the train ?
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### Real and relative velocity formula

What i need is a thing more complicated than the usual relative velocity formula. Imagine a radar. You are at the center, moving at some angle, but lets make it clear, you are always at the center. ...