The time rate of change of the position of an object

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34
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9answers
13k views

Why does a free-falling body experience no force despite accelerating?

Note: For the purposes of my question, when I refer to free fall assume it takes place in a vacuum. From my (admittedly weak) understanding of the equivalence principle, falling in a gravitational ...
1
vote
1answer
58 views

How find/estimate Parasitic Area (CDA) in order to calculate V(L/D)max?

I am trying to calculate the speed which maximizes lift/drag for an aircraft. Using Carson's paper, it looks to me like the only unknown variable is "f", the parasitic area, which Carson describes as ...
0
votes
0answers
34 views

How to calculate position of object that have linear and angular acceleration at the same time?

Let's imagine that we have a boat with two engines: main and rotary engine. Main engine gives boat acceleration $a$ m/s² up to constant linear speed $V$ m/s. When engine turns off, boat slows down ...
2
votes
2answers
277 views

How to find Tangential/Radial/Angular Velocity for motion in any curve?

Is the radial velocity responsible only for changing distance between objects and the component perpendicular to it only for change in direction? If so why? Please try to give a different explanation ...
0
votes
1answer
5k views

What factors affect the velocity of an object moving down an inclined plane?

If you have a small toy car rolling down an wooden plank, what factors would increase or decrease the velocity of said car. In the experiment my team originally stated that adding mass to the car ...
-2
votes
1answer
40 views

special relativity question [closed]

How can I find the velocity of an object given the time elapsed of trip from the frame of reference of the object and the distance from the frame of reference of Earth.
-1
votes
1answer
28 views

The relationship between velocity and radius [closed]

I have learned that for an object moving in a circle, there must be an inward force acting upon it in order to cause its inward acceleration. This is sometimes referred to as the centripetal force. ...
0
votes
0answers
11 views

Determining velocity and angular velocity after a collision between two 2D rectangles [closed]

Say I have two rectangles, which each have four vectors (which represent their vertices), a velocity, a mass, an angular velocity and a rotation (which is only used to simplify calculations as I don't ...
-1
votes
2answers
67 views

When Does Mass Matter? [closed]

I have been trying to wrap my head around this idea for a month or so, and I am not really sure how mass plays into the velocity of a moving body. I have not taken physics in a while, but one thing I ...
10
votes
4answers
182 views

Problems in the relation $a=v\frac{\mathrm{d}v}{\mathrm{d}x}$

We all know $a = \frac{\mathrm{d}v}{\mathrm{d}t}$. A little application of the chain rule leads to the relation $$a=v\frac{\mathrm{d}v}{\mathrm{d}x}$$ But the above equation shows that $a=0$ whenever $...
0
votes
3answers
112 views

Why centripetal force does not increase the value of tangential velocity? [on hold]

I found in a text book that the value does not change because the centripetal force is perpendicular to the tangential velocity. But I am confused, because a vector can have a component, which is ...
0
votes
3answers
48 views

Why is the sign of the tangential component of acceleration negative in this problem? [closed]

Since the train is slowing down at a constant velocity, shouldn't the acceleration equal zero? Why is it negative in this case?
4
votes
1answer
555 views

Angular velocity relative to different frames

In Goldstein it is said "It is intuitively obvious that the rotation angle of a rigid body displacement, as also the instantaneous angular velocity vector, is independent of the choice of origin of ...
-1
votes
1answer
48 views

Is it possible to introduce 4-velocity very early on?

I was wondering if it would be possible to teach new physics students about 4-velocity straight away at the beginning of their studies. Keeping the regular notion of the three dimensional velocity $\...
2
votes
2answers
116 views

Equation for a falling body including terminal velocity

I'm making an app that times how long it takes a pebble to fall, then calculates the distance it fell. I noticed that the simple $f(t) = \frac{1}{2}gt^2$ was becoming increasingly inaccurate as the ...
0
votes
2answers
109 views

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_{...
0
votes
1answer
44 views

Equation confusion regarding finding acceleration [closed]

Physics newbie here, sorry if this is super basic... I am learning about kinematic equations and am having difficulty reconciling two of the equations. Let's say I begin driving and after 1 second ...
1
vote
0answers
21 views

How to determine the three-velocity measured by a single observer?

In this question the issue of extracting the three-velocity of a test particle in some frame arose. The answer provided by @PaulT and in this paper (arXiv) seems to me to give rise to ambiguity when ...
0
votes
1answer
140 views

Does speed in space really matter?

This is about validating the science from a science fiction novel. My understanding is that speed is all relative. So while I am in a car on the freeway my speed relative to another passenger is ...
2
votes
2answers
45 views

Juno probe's speed is in relation to what frame of reference?

When describing the velocity of space probes in general (the Juno probe in particular) what frame of reference is the velocity quoted in? This is in relation to the news of the Juno Satellite ...
1
vote
0answers
64 views

Two impulses on body

Suppose I have a physics simulation with a single body with three degrees of freedom (two axis and rotation). I have two impulses of equal magnitude in opposite directions. When applying these ...
0
votes
2answers
205 views

Kinematics equations of motion find x position in 2D from height

I have a question and I thought of 2 possible ways to answer it however the resulting answers are different which means I must be doing something wrong in my calculations. All the below happens 2 ...
0
votes
1answer
84 views

Why is mass times position not used in physics? [closed]

The momentum $\vec{p}=m \vec{v}$ plays a very prominent role in physics. Why is the same not true for the quantity $\vec{q}=m \vec{r}$? E.g. one can write angular momentum as $\vec{L} = \vec{r} \...
1
vote
2answers
37 views

Is the magnitude and direction of gravity on an object the same as the objects vertical velocity?

I need to make a diagram for an entry level college physics class. I need to show the magnitude and direction of gravity on a flying object and also show the magnitude and direction of the objects ...
0
votes
1answer
37 views

Can rotational velocity be transformed into directional movement?

If I attach a weight to a string, and spin it, I notice that the faster I spin it, the more it pulls away from me. So, I imagine what would happen if I arranged a very large machine, designed so that ...
-1
votes
1answer
72 views

What do things move relative to?

When someone says that a spacecraft in otherwise empty space is traveling at a constant velocity of 10 km/h (for the sake of convenience) then what is the reference point for which this measurement ...
0
votes
3answers
64 views

Why is speed defined as coordinate derivative over proper time rather than observer's time in STR?

In special theory of relativity, why is 4-velocity defined as: $$ u^\mu = \frac{dx^\mu}{d\tau} $$ and not as $$ u^\mu = \frac{dx^\mu}{dt} $$ where ${\tau}$ is proper-time and t is time in some ...
0
votes
1answer
34 views

Group Velocity of guassian packet equals time derivative of mean position?

If I look at a gaussian wave package, and then interpret (in the usual Quantum-Mechanics Way) its square value as the propability density, then I can calculate a mean value for the position: $$ x_{...
0
votes
1answer
45 views

If the applied force is constant, what is the graph of the velocity?

This is a sample interview question from Oxford: A ball, initially at rest, is pushed upwards by a constant force for a certain amount of time. Sketch the velocity of the ball as a function of ...
1
vote
3answers
126 views

Velocity of a body in a pulley problem involving a spring fixed to the ground

So the picture shows the whole situation. My question is, how do you find the velocity of the object after it had descended 10 cm (0.1 m) ? Pulley's radius 0.2 m, moment of inertia 0.2 kg/m I have ...
0
votes
2answers
119 views

Speed of light and relativity [duplicate]

Suppose there are two observers $A$ & $B$ both are in motion, now $A$ sees $B$ is moving with speed $'u'$. A says that another object $'d'$ is moving with speed $c+u$ w.r.t. A in the same ...
3
votes
2answers
28 views

Why is the damping force acting on an oscillating system opposite in direction to velocity and not acceleration?

So far I know that the damping force is a frictional force that opposes motion and so it acts in the opposite direction to velocity . Bit why can't the same be said for acceleration doesn't the ...
0
votes
1answer
46 views

Even when the drift velocity of the electrons is very small, how is current in the circuit established immediately?

So I have this question in my textbook and Ive also got it in my test. Ive written a wrong answer. My friend wrote the drift velocities of each particle add up and he got full marks. Is it right? Can ...
8
votes
3answers
17k views
1
vote
2answers
126 views

Special Relativity - oscillator paradox

I am reading about the Special relativity and the original Einstein papers from 1905 and 1920 where he derives the Lorentz transformation and the effects of the time dilation and space contraction ...
3
votes
2answers
79 views

What is the velocity in the Navier-Stokes equation?

I have been looking at the Navier-Stokes equation, and can't seem to find anywhere a clear description of what velocity it represents. From what I have read it could be any of the following: The '...
4
votes
1answer
64 views

Vary action with respect to velocity

Variation of the action $S$ corresponding to a Lagrangian e.g. $L(x(t),\dot{x}(t))$ gives the Euler-Lagrange equations: $$ \frac{\delta S}{\delta x(t)} = 0 \\ \int du \left ( \frac{\delta L}{\delta x(...
9
votes
2answers
1k views

Functional derivative in Lagrangian field theory

The following functional derivative holds: \begin{align} \frac{\delta q(t)}{\delta q(t')} ~=~ \delta(t-t') \end{align} and \begin{align} \frac{\delta \dot{q}(t)}{\delta q(t')} ~=~ \delta'(t-t') \end{...
0
votes
1answer
37 views

instantaneous velocity center

The instant center of rotation, also called the instantaneous velocity center is the point fixed to a body undergoing planar movement that has zero velocity at a particular instant of time. For ...
5
votes
2answers
9k views

Calculate speed from accelerometer

I am trying to use my accelerometer on my mobile device (smart watch to be specific) to calculate a persons arm swing speed. The data returned from the accelerometer is in $m/s^2$. Since the ...
0
votes
2answers
96 views

Why $F_\mu V^\mu=0$ in special relativity? [closed]

I'm trying to prove why $F_\mu V^\mu=0$. Where $F^\mu$ is the four force and $V^\mu$ is the four velocity. Can anyone help me?
0
votes
1answer
26 views

What are the Radial, Tangential and Linear Velocities in this Context?

From the pictures below, it has been said that the tangential component (represented by the tangent at $B$) the linear velocity of a particle moving from $A$ to $B$ is $r \omega$, which is equal to ...
-2
votes
1answer
29 views

Velocity ; Acceleration [closed]

A ball was thrown vertically upwards with a speed of 3m/s. The ball reached a highest point before falling down to the ground. What is the acceleration of the ball at the highest point? A 0m/s2 B ...
0
votes
4answers
90 views

Explain $\Delta x = v_0t + \tfrac{1}{2}gt^2$ please? [duplicate]

$g = \Delta v/t$, so $\Delta v = gt$. $v = v_0 + \Delta v$, so $v = v_0 + gt$. So if $\Delta x = vt$, then $\Delta x$ should be $v_0t + gt$. Why the $\tfrac{1}{2}gt^2$? I'm really confused, so this ...
0
votes
1answer
34 views

Acceleration at rest when ball is thown [duplicate]

If a ball is thrown vertically upward and it comes to rest for a moment, then its velocity decreases and becomes zero. Does its acceleration also becomes zero? If yes then how ball falls down?
0
votes
2answers
1k views

Small Animal Terminal Velocity

I recently saw a cat fall probably 100 feet like in this video Cat Falls. It seemed as if the cat reached terminal velocity by the time it hit. Does this mean that cats (and other small animals) could ...
0
votes
2answers
86 views

Components of Velocity in polar co-ordinates

Consider a point moving along a curve in a plane. The position of a point P on a coordinate system can be specified by a single vector $\vec{r}$=$r\hat{r}$. A rough sketch describing the situation is ...
0
votes
1answer
37 views

Maximum velocity of a ball down a slope

I have the following situation: The dimensions of the ball are significantly smaller than the dimensions of the slope, and there is no friction. I'd like to know the maximum velocity of the ball ...
1
vote
2answers
57 views

Is distance considered only for objects which have already moved?

I have this confusion concerning distance and speed. Typically, we express displacement and distance relative to a reference point, which we regard to be an unmoving location. As displacement has a ...
1
vote
0answers
66 views

Meaning of a Dirac delta speed in 1d shallow-water equations

A 1d thin layer of a fluid of constant density in hydrostatic balance can be modeled by shallow-water equations: $$\begin{align*}\partial_tu+\partial_x\left(v+\frac{u^2}{2}\right)&=0,\\ \...