# Tagged Questions

The time rate of change of the position of an object

1answer
26 views

### special relativity question [on hold]

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.
1answer
28 views

### The relationship between velocity and radius [on hold]

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. ...
0answers
10 views

### Determining velocity and angular velocity after a collision between two 2D rectangles [on hold]

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 ...
2answers
65 views

### When Does Mass Matter? [on hold]

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 ...
4answers
180 views

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 ...
2answers
108 views

2answers
36 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 ...
1answer
35 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 ...
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 ...
3answers
63 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 ...
1answer
34 views

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{...
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 ...
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 ...
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?
1answer
24 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 ...
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 ...
4answers
89 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 ...
1answer
33 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?
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 ...
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 ...
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 ...
1answer
36 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 ...
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 ...
0answers
64 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,\\ \...
0answers
23 views

### Representing variable velocity in cellular automata

I've read through some of the more non-physicist accessible material on cellular automaton, touching on digital physics etc and I am somewhat confused about methods to reason with variable velocity in ...