Questions tagged [kinematics]

The description of the movement of bodies by their position, velocity, acceleration (and possibly higher time derivatives, such as, jerk) without concern for the underlying dynamics/forces/causes.

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1answer
171 views

Circular motion - vectors [closed]

We are asked to show that, if $$\frac{{\rm d} \mathbf{r}}{{\rm d}t} = \mathbf{c}\times\mathbf{r},$$ where $\mathbf c$ is a constant vector, then the body moves in uniform circular motion. I have ...
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Projectile motion formulas

Assuming: The equations for the vertical and horizontal components of my initial velocity. Are: $v_{x,i} = v_i \cos\theta$ and $v_{y,i} = v_i \sin\theta$ And the displacement components are ...
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225 views

Stellar outflows in the Milky Way

There appears to be a lot of evidence that gas flows outward in Galaxies. I've been trying to parse through the available data and am unable to get a clear answer to this question: is there any ...
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Finding resultant and direction of resultant

In this question- A motorboat is racing towards north at 25km/h and the water current in that region is 10km/hr in the direction of 60 degree east of south. Find the resultant velocity of the boat. ...
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When one kid runs into another and they bump heads, why is the slowest mover in most pain? [closed]

In my experience, if one kid runs really fast and accidentally bumps into a second kid who's standing still, and they bump heads, the kid who was 'standing still' (relative to earth) always seems to ...
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What would happen if an unstoppable force hits an immovable object? [closed]

I realize that the question a rather large paradox, but I do wonder if such a thing were true what would happen, assuming that neither of these "objects" can be destroyed by each other?
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Could a fish in a sealed ball, move the ball?

If you had a glass ball filled with water, completely sealed and containing a fish, could the fish move the ball?
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251 views

View from a helicopter rotor: why is the horizon distorted?

This video ("rotor panorama") was captured by a camera attached to the rotor head of a radio-controlled helicopter, with the frame rate set to the rotor's frequency. During a long segment of the video,...
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2answers
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Why do we equate an indefinite integral to a specific value?

Suppose we want to obtain a displacement vector defined as $\mathbf s(t) = x(t)\mathbf i + y(t)\mathbf j + z(t)\mathbf k$ from the components of a velocity vector $\mathbf v(t) = \dot x(t)\mathbf i + \...
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5answers
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If an object moved 5 meters in a second, how can its velocity be 10 m/s? [duplicate]

On physicsclassroom.com, an assignment gives data for an object experiencing acceleration: This is the corresponding chart for the object's velocity: At $0$ seconds the position of the object is $0$...
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If an electric car were to drive without having to stop, would the range be greatly affected by the speed at which the vehicle is moving?

Of course aerodynamics factors into this question, and the faster you are moving the more air you have to push out of your way, the more energy you use. But would the difference be only a small ...
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234 views

Does $y$-motion really have nothing to do with $x$-motion?

I am watching a Physics 1 for physical science majors on coursera.com, and in one of the concept tests there is a question that goes like this; "A bullet is fired horizontally from a rifle on the Moon ...
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How long does it take to optimally change position and velocity?

A spaceship moving in two dimensions is at position $(x, y)$ and has a velocity $(v_x, v_y)$. It also has a maximum acceleration $a_{max}$. Its goal is to be at position $(x', y')$ with a velocity of $...
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1answer
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What is the origin of the naming convention for position functions?

In physics, position as a function of time is generally called d(t) or s(t). Using "d" is pretty intuitive, however I haven't ...
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Is $\dfrac{dx}{dt}$ a fraction or not?

I am new to calculus and during my mathematics class my sir defined $\dfrac{dx}{dt}$ as $$dx/dt=\lim_{t\to t_1}\dfrac{f(t)-f(t_1)}{t-t_1}$$ and my sir made a clear statement that $\dfrac{dx}{dt}$ ...
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How would you move without gravity?

I'm wondering how would you move without gravity? You would still have air pressure at 1Atm. Would you "swim" in the air or would you have do something else?
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1answer
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Hypothetical special relativity with mass conservation [closed]

I thought of this question while studying inelastic collisions in special relativity, where kinetic energy is converted into mass-energy. I was wondering if it's possible to formalize a version of ...
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2answers
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Time reversal in simple *solution* to equation of motion

Consider the solution to the equation of motion for a particle with a constant acceleration: $$ x(t) = x_0 + v_0t + \frac{1}{2}at^2.$$ If I let $t \rightarrow -t$, then the equation becomes: $$ x(-t) ...
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2answers
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Doubt on series springs [closed]

Here is a description of the motion of two springs in series. The premise is that the force on the two springs is the same. This is derived from the following reasoning: when I pull the mass with a ...
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2answers
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Displacement vs. Distance

I'm new to physics, and I just learned what velocity is. From what I learned, velocity is, displacement/time. But, I'm confused about what displacement and distance is, and how is affects velocity. So,...
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4answers
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How to find tangential/radial/angular velocity for motion in any curve? [closed]

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 ...
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1answer
624 views

Why do three particles placed at the vertices of an equilateral triangle, moving along the sides, meet at the centroid?

I just read an example of vectors in my book which is confusing me. Three particles A,B and C are at the vertices of an equilateral trinagle ABC. Each of the particle moves with constant speed v. A ...
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5answers
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Instantaneous velocity

So here’s a question I’ve been thinking of for a while. Suppose we say, “an object is having an instantaneous velocity along a particular direction ( say 10 m/s along the $x$-direction)” . Is it fair ...
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2answers
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Why does the sound of my tea stirring go up in tone the faster I stir?

I've noticed that when stirring my tea, as I stir faster the tone generated by the stirring goes up. Why is that? Is it something to do with the Doppler Effect? EDIT -------------------------- ...
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0answers
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When this CD is spun so fast that it shatters, why does the warped shape move slower than the surface?

In this video where a CD is spun abnormally fast to the point of shattering, one notices from the writing on the CD that the surface is spinning faster than the warp "shape" is. In other words, the ...
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3answers
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For circular motion in a vertical plane, why does Net Force = Centripetal Force?

I'm struggling with some of the concepts pertaining to the forces and acceleration associated with circular motion in a vertical plane (only concerned with what happens at the 'top' and 'bottom' of ...
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4answers
154 views

Can we do a thought experiment to prove that the horizontal and vertical motion of a projectile are independent of each other?

It seems really hard for me to grasp the idea that horizontal and vertical motion of a projectile are independent of each other. Intuitively I feel that they should affect each other. How can it be ...
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1answer
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Determine acceleration from experiment (Newton 2nd Law)

I have done a physics experiment (setup below). And was asked to determine the experimental and theoretical acceleration. The data I've got Ok, am I right to say Experimental acceleration = $2(...
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2answers
87 views

Angular velocity as $r \to 0$

I know that angular velocity is defined as the rate of change of angular displacement and is related to tangential velocity via $$\omega = \frac{r\times V}{|r^2|}$$ My question is what will be the ...
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3answers
516 views

Does a constant 4-acceleration implies constant acceleration?

Before asking the question I explain here my line of thoughts: I'm almost sure that a constant acceleration $\bf{a}$ implies constant 4-acceleration $w^\mu$. So I assume constant acceleration, then, ...
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1answer
811 views

Relativistic kinematics of particle decay

Suppose a particle decays to three other particles. The masses of all particles are assumed to be known and we work in the rest frame of the parent particle. So there are 12 parameters for this ...
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2answers
2k views

Why can't some light ever reach earth? [duplicate]

Note: I'm a layman in physics. Excuse me if this question is a duplicate, I lack the knowledge in technical terms to find the answer to this by research (I've tried). Let's suppose there's a star ...
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2answers
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Help with a solid sphere sliding and then rolling

Initially at time $t=0$, a solid sphere slides with velocity $v$ along a horizontal surface. The coefficient of friction is u. Find the required time for the sphere to stop sliding (the sphere will ...
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0answers
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Why does the Vlasov equation not consider higher order time derivatives?

The Vlasov equation is given by: $$ \frac {df}{dt} = \frac {\partial f}{\partial t} + \vec{v} \cdot \frac {\partial f} {\partial \vec{x}} + \vec{a} \cdot \frac {\partial f} {\partial \vec{v}} $$ But ...
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2answers
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Why is the velocity vector perpendicular to the radius vector for projectile motion when the distance from origin is maximal?

After reading this question I have a question of my own: A solution for the furthest distance from the origin for a projectile motion is to solve $(\frac{y}{x}) \cdot (\frac{dy}{dx})=0 $. Why is it ...
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1answer
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Newton 2nd Law: Does vertical force (mass) affect the horizontal acceleration?

I learnt before that if 2 forces are perpendicular to each other, they should not affect each other. However in a recent experiment setup (asked in another question): I believe the theoratical ...
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3answers
243 views

Looking for the curve traced by a moving bicycle when its steering bar is fully rotated

I am looking for a curve traced by a moving bicycle when its steering bar is fully rotated either clockwise or anti-clockwise. How to model it mathematically? Is the curve a circle? My attempt is ...
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2answers
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If change in position over time is average velocity, why doesn't change in position over time squared equal average acceleration?

For example, let's say a car is experiencing an acceleration of $1$m/s$^2$, for $6$ seconds so it goes $18$m. Now the average velocity is found through dividing $18$m by $6$s which is in line with the ...
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2answers
652 views

The force causing galaxies to accelerate away from each other?

The galaxies in the universe appear to be accelerating away from each other according to the Doppler shift that is present in the light we receive from other galaxies. According to Newtonian ...
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4answers
968 views

In perfect vacuum (no external force), if I apply a small amount of force to a body, will it continue to accelerate forever?

According to the equation $F = ma$, the greater the force applied to a body, the greater its acceleration. But in a place where there is no gravity or any other kind of external force, if I apply a ...
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6answers
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Why do we care about the difference between speed and velocity?

I teach 7th grade students about the difference between speed and velocity. One of them ask me why do physicists create the concept of velocity. I cannot answer. I don't know precisely why do we care ...
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Slope of $x$-$t²$ curve (displacement vs time squared)

I know that slope of $x$ vs $t^2$ curve gives $\frac{a}{2}$. How do I prove it? This is what I did : $x = ut + \frac{at^2}{2}$ $x = \frac{at^2}{2}$ (for $u=0$) $\frac{dx}{dt^2} = \frac{a}{2}...
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2answers
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Uniform Circular Motion w/ Tension and Friction

So I had a problem today which I couldn't make any sense of. I don't have access to it at the moment but this is a pretty accurate approximation. Basically, a mass (m) is swinging horizontally on ...
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4answers
63 views

Motion between two particles in a relative manner

Suppose a particle A is travelling in east direction with velocity of x m/s and another particle B is travelling with velocity y m/s in the west direction. Why does the the particle B appears to move ...
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1answer
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Trajectory of projectile thrown downhill

I'm teaching myself mechanics, and set out to solve a problem determining the optimum angle to throw a projectile when standing on a hill, for maximum range. My answer seems almost plausible, except ...
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1answer
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Intercept a moving object

Object A can move at 50km/h, wants to intercept object B (currently $15^{\circ}$, east of north from A) moving at 26km/h, $40^{\circ}$ east of north. What angle should A take to intercept B? AB is ...
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2answers
119 views

Obtaining a general equation for velocity (in 2D projectile motion)

I'm trying to obtain a general equation for the instantaneous velocity of a projectile moving on a Cartesian plane. I began with the equation for a projectile's trajectory (air resistance neglected): ...
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1answer
1k views

Using Kinematic Equations of Motion to find Velocity in Spring-Block System [closed]

Suppose a block of mass, m is connected to a spring of force constant, k. The block is moved on a frictionless surface through distance, b with the help of a uniform external force, F so that the ...
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1answer
327 views

why does perpendicular motion to the direction of someone' s approach does not affect the distance between them

I was reading about the the four turtles/bugs math puzzle Four bugs are at the four corners of a square of side length D. They start walking at constant speed in an anticlockwise direction at all ...
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2answers
702 views

Why does the Fluid inside a Water Balloon Continue to Move even when the Balloon Being Pulled has Stopped?

If there is a rubber balloon filled with water attached to a string, and the string is pulled in 1 direction, causing the balloon to also be pulled, why does the water inside continue to move even ...