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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2answers
6k views

Pair production - mathematically?

Allover the web i am only seeing a statement similar to this: Pair production is not possible in vaccum, 3rd particle is needed so that conservation of momentum holds. Well noone out of many ...
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4answers
530 views

Do time dilation and length contraction cancel out?

I've recently been getting into special relativity by myself. I noted something interesting: We know that the factor by which time dilates and lengths contract is the Lorentz factor $$\gamma = \frac{1}...
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2answers
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Why is velocity squared in kinetic energy? [duplicate]

I'm not looking for how it's derrived. I'm wondering why it works out this way that doubling the speed quadrouples the KE. Here's my thought process: A car is moving down the road (ignoring air ...
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3answers
2k views

Can there be motion without the flowing of time? [closed]

tldr - Can we reject the notion of the passage of time, but still accept that motion is real? This question is a follow up to comments between myself and the author of this answer to the question, ...
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2answers
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Difference between Speed and Velocity

What is the difference between Speed, Velocity and Acceleration? Could any one describe it pictorially?. I am more over confused even after investigating many times. I am unable to relate myself ...
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6answers
11k views

Zero velocity, zero acceleration?

In one dimension, the acceleration of a particle can be written as: $$a = \frac{dv}{dt} = \frac{dv}{dx} \frac{dx}{dt} = v \frac{dv}{dx}$$ Does this equation imply that if: $$v = 0$$ Then, $$\...
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8answers
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Why isn't average speed defined as the magnitude of average velocity?

Speed is usually defined as the magnitude of (instantaneous) velocity. So one could assume that average speed would be defined as the magnitude of average velocity. But instead it is defined as $$s_{...
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2answers
2k views

Does a force really act on a body during a collision?

Consider two bodies A(black) and B(red) having equal mass. A is moving at a constant speed towards B, which is stationary. At certain point of time, they collide elastically, $\therefore u_{A}=v_{B}$ ...
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3answers
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Projectile motion maximum distance to origin

Is there an elegant or easy way to derive the maximum distance from the origin to a point of the parabola created by projectile motion (assume initial position is the origin)? Other than ...
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2answers
8k views

Increase in velocity by loss of mass? [closed]

A trolley of mass 300kg carrying a sand bag of 25kg is moving uniformly with speed of 27km/h on a frictionless track. After a while, sand starts leaking out of a hole on the floor of the trolley at ...
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4answers
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How is acceleration measured if there is change only in the direction of velocity & not magnitude?

The formal definition of acceleration is It is the measure of how fast the velocity changes with respect to time. Now, when the magnitude of velocity changes, acceleration $$\vec{a} = \frac{d \...
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9answers
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Why isn't the acceleration at the top point of a ball’s journey zero?

When I shoot a ball vertically upward, its velocity is decreasing since there is a downward acceleration of about $9.8\,\mathrm{ms}^{-2}$. I have read that at the top most point, when $v = 0$, the ...
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3answers
906 views

What is the kinematics of a particle with complex mass?

particles with real-mass have time-like kinematics ($ds^2 > 0$). particles with zero-mass have light-like kinematics ($ds^2 = 0$). particles with imaginary-mass have space-like kinematics ($ds^2 &...
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3answers
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Intuitive meaning of factor 2 in formula of vertical throw max height $h=v^2/2g$

This is a question about a simple thing. The simplified expression for maximum height in vertical throw is $h=\frac{v^2}{2g}$ , could anyone explain intuitively (analogies are welcome) why there is a ...
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1answer
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How to calculate the kinetic energy of supernova ejecta?

I believed I could use $0.5\times M_{ej} V_{ej}^2$, with $M_{ej}$ being the ejected mass and $V_{ej}$ being the velocity of the ejected mass. But I noticed in this and this that the the mean velocity ...
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2answers
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How can an object move from point A to point B?

If I was at point $A$ and I wanted to walk directly to point $B$, I would have to walk half way to point $B$, but before that I would have to walk half way to halfway to halfway to point $B$ and half ...
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2answers
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Galileo's law of odd numbers

The Galileo’s law of odd numbers states that the distances traveled are proportional to the squares of the elapsed times. In other words, in equal successive periods of time, the distances traveled by ...
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8answers
697 views

Does momentum change when Kinetic energy changes?

$K.E=\frac{1}{2}mv^2$ , $P=mv$ thus there is a relation between them. I really can't understand why there is no decrease in momentum when kinetic energy is decreased in an inelastic collision. I am ...
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2answers
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What does $\bar{y}$ with a line over it represent?

I've been asked to complete this chart and have never come across this symbol before, nor can I find anything about it on google: What does the $\bar{y}$ with the line over it represent? And also, ...
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9answers
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Can velocity be an undefined quantity?

We have the image below displaying the uniform velocity by time-distance graph. At every point velocity is constant but what if distance and time both become zero as at origin in the graph is? The ...
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2answers
3k views

A block on a wedge

The system is as follows - Friction exists only between the 2 blocks. I am trying to find out the accelerations of $m_1$ and $m_2$. Let $a_2$ be acceleration of $m_2$, and $a_x$ and $a_y$ be the ...
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8answers
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Real world examples for projectile thrown upwards or downwards

I am preparing a physics course for high school about projectile motions. If a projectile moves with initial velocity $v_0$ in the gravitational field of the earth, the equation $$ s(t) = \frac{1}{2}...
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1answer
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What is the difference between negative acceleration and decreasing acceleration?

According to Reisnick and Halliday, retardation is the acceleration in the negative direction of time axis ie. opposite to the motion. Decreasing accleretion is the decrease in the magnitude of time ...
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3answers
399 views

What is really instantaneous?

How can a body travel at an instant and what does instantaneous speed tells us? What really is meant by speed of an object at an instant if an object does not travel at an instant? I would like a ...
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2answers
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Instantaneous Centre of Rotation

Let's say a body is undergoing both rotational and translational motion. I know that ICR of the body as a whole will be the point about which the body is doing pure rotation, so basically will be the ...
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2answers
362 views

What is function of force $F$ and work $W$?

I know that : Position $$r:\mathbb{R} \to\mathbb{R^3} \ \ \ \ \ ; \overrightarrow{r}(t)$$ Velocity $$v :\mathbb{R} \to\mathbb{R^3} \ \ \ \ \ ; \overrightarrow{v}(t)$$ Acceleration $$a:\mathbb{R} \...
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1answer
444 views

Rigid Body Mechanics In a vacuum (no gravity) [closed]

I am writing a physics engine to map the rotary and translatory movements of a uniformly dense solid cylinder within 3d space. If a vectored thrust is applied to one end of the cylinder at an ...
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1answer
1k views

How to obtain the distance traversed by a free falling body equation?

I know that the distance that a free falling body has traverse through time is given by $d=0.5*g*t^2$. I would like to know how to get to this equation to study a bit more how it was obtained. I have ...
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4answers
23k views

Deriving equations of motion using integration

Please refer to my school textbook pg48 (of the book, and not the pdf counter) here: http://ncertbooks.prashanthellina.com/class_11.Physics.PhysicsPartI/ch-3.pdf My doubt is in this context: (right ...
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2answers
7k views

Why does a plane look like it's slower than a car?

Recently, I was in a car and noticed a plane in the sky. What was interesting was that the plane seemed to go slower than my car because I passed the plane. Is there a physics reason to this?
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4answers
487 views

Question about $a = v\ \mathrm dv/\mathrm dx$

Consider $\vec{v}$ Now differentiating this w.r.t time, $$\vec{a} = d/dt( \vec{v}) = \vec{v}(d\vec{v}/dx)$$ Now this multiplication of vectors obviously makes no sense. This along with the fact that ...
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1answer
407 views

How was the torque-force relationship discovered

I understand the equations of angular motion and how they relate to one another, however it is not clear to me where the torque equation comes from. We have $F=ma$. By solving for the angular ...
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3answers
301 views

What is the behavior of the normal force during uniform circular motion?

A roller coaster car has a mass of 1360kg when fully loaded with passengers. As the car passes over a circular hill with radius 20m, its speed is not changing. A.) What are the magnitude and ...
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2answers
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Non-Constant Acceleration due to Gravity

Recently, I had the first physics lab for my university physics course. This lab was fairly simple, as we were merely using a computer and a distance sensor to graph the position, velocity, and ...
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2answers
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Why does a stream of water behave like this when aimed against a spoon or into a glass?

I noticed while doing everything except the dishes that water behaves quite strange. These two things might be two different phenomenon, but the water acts in a similar way in both cases. Spoon and ...
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7answers
15k views

In calculating work done by a constant force over a constant distance, why doesn't the subject's initial velocity matter? [duplicate]

Assume a point-mass $m$ is travelling in a straight line, and a force $F$ will act on $m$ (in the same direction as $m$'s velocity) over a constant distance $d$; why doesn't $m$'s velocity matter to ...
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2answers
14k views

Why do cars gain lift while going at a high speed?

I've been researching the physics involved with spoilers and I've just been turning up the same information about spoilers giving the back wheels of a car more traction by acting as an up side down ...
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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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2answers
121 views

Are all oscillatory motions periodic motions?

For example, if a pendulum system in the real world oscillates its amplitude and period will eventually decrease. Does that still count as a periodic motion? Since the motion doesn't have the same ...
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1answer
194 views

What is the meaning of the integral of position? [duplicate]

Jerk, $j(t)=\dddot x(t)$, acceleration, $a(t)=\ddot x(t)$ and velocity, $v(t)=\dot x(t)$ are derivatives of position, $x(t)$. What is the meaning of the integral of position? For example, one could ...
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2answers
14k views

Why can velocity and acceleration be negative?

Why is speed and acceleration negative when $V_1$ of an object is say 150m/s, $V_2$ is 0 m/s and $\Delta d=0.50\,\rm m$? I found the time it takes which is 0.0033s and the acceleration to be 90909.09 ...
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3answers
839 views

Ball flying towards me - Newton's third law is violated?

I was trying to answer the question of the flying ball here on the basis of Newton's third law and momentum conservation. Here is what I have tried. Lets take a ball mass of $m_1$ (index 1 is the ...
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1answer
1k views

Derivation of formula of normal acceleration

Consider a particle that is described by $x(t)=(2.0-0.1t) \cos(0,5t)$ and $y(t)=(2.0-0.1t) \sin(0,5t)$ t in seconds and x,y in meters. In previous subquestions we were asked to determine an ...
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3answers
39k views

Kinetic energy and momentum conservation in an explosion?

My physics book says, "A firecracker sliding on ice has the same total momentum before and after it explodes." I understand this part. This is because of Newton's 3rd law, and no external forces. This ...
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4answers
2k views

Area under and slope of the motion graphs

I wanted to ask in general what area under the graph means. Also which physical quantity is highlighted by area under distance vs time graph. I'm confused that area is a 2 dimensional concept and it ...
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1answer
2k views

Jerk and Snap in Circular Motion

I can understand the physical existence of centripetal acceleration, the orthogonality of that with the linear (tangential) velocity - but how to physically understand jerk and snap in case of uniform ...
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3answers
197 views

Why does this example seem to contradict conservation of energy?

Consider a 10 kg box on the bed of a truck accelerating at 1 m/s^2. Consider a reference frame where it starts at 0 m/s and ends at 10 m/s and then consider a reference frame where it starts at -5 m/s ...
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3answers
86k views

How to find stopping distance of a car? [closed]

I am trying to calculate the minimum stopping distance of a car once the brakes are applied. I know that $F = ma$, and the braking force is $F = \mu N = \mu m g$, so $$a = \mu g.$$ Next, by applying ...
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5answers
166 views

Why should the particles meet at a common point? [closed]

I saw a question in my physics book asking for the time when all the three particles (each at the corner of an equilateral triangle and each having constant velocity v along the sides of the triangle) ...
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0answers
53 views

Web utility for viewing $x$-$t$, $v$-$t$, and $a$-$t$ graphs

I'm wondering if there exists any utility on the web which displays $x$-$t$, $v$-$t$, and $a$-$t$ graphs for, say, a number of well considered example animations. The utility should allow pausing, ...

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