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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Energy threshold for photon

I just read https://en.wikipedia.org/wiki/Annihilation#Examples and there is the popular $ e^+e^- \to \gamma \gamma $ reaction described. Is it always possible to produce a $ \gamma $? A photon does ...
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What is a point object? [duplicate]

I am currently studying Kinematics and there's a little bit confusion about the definition of point object given in my course book NCERT(it is a standard textbook in India) which is as follows : ...
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Is it possible that this electromagnetic field propulsion system would not violate any conservation laws? [on hold]

I have recently come up with a design of a conceptual electromagntic field propulsion system which should not violate any conservation laws, particularly the Law of Conservation of Momentum and the ...
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If acceleration $a = v\cdot\frac{\mathrm dv}{\mathrm dx}$ , then why isn't acceleration always zero when velocity is zero? [duplicate]

We know that, $$\begin{align} a&=\frac{\mathrm dv}{\mathrm dt}\\[3pt] &=\frac{\mathrm dv}{\mathrm dx}\cdot\frac{\mathrm dx}{\mathrm dt}\\[3pt] &=v\cdot~\frac{\mathrm dv}{\mathrm dx} \end{...
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mass m on a inclined plane [closed]

If the figure shows a smooth inclined plane what should be the minimum magnitude of acceleration which the mass m dropped freely vertically to the ground.
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How does an object gain energy when it enters a potential field?

Conservation of energy says total change in energy is $0$. Let's assume a small object is in outer space (maybe like a football). It moves towards a planet (assume planet has no atmosphere for ...
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5answers
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Can the average speed of a moving body be 0? [closed]

My book states that- "Average speed of a moving body can never be 0 except when time approaches infinity" I understand that Average speed = (Total dist./ Total time) and that if time is infinity, ...
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Is it possible to have a situation in which velocity of the particle is never 0 but its average velocity in an interval is 0?

My book states that-" It is not possible to have a situation in which speed of the particle is never 0 but the average speed in an interval is 0" I would like to know if the same holds true for ...
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Four-velocity of an accelerating particle

In the rest frame of a particle, the acceleration four-vector is $$ \mathbf{a} = (0, g) $$ (see https://users.physics.ox.ac.uk/~smithb/website/coursenotes/rel_A.pdf , Eq.3.51). By definition $$ \...
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1answer
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An Evil Vector Problem! [closed]

A person is walking on straight road starting with a velocity $v$, the direction of of air ,observed by him makes an angle $\alpha$ with his is direction of motion. Now during the race, he finds ...
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What is the “energy associated with torque?”

I'm told that when a magnetic dipole is placed in a magnetic field $\vec{B}$, it experiences a torque $\vec{\tau} = \vec\mu \times \vec{B}$ and an associated energy $ H = -\vec{\mu} \cdot \vec{B} $, ...
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4answers
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How to represent the direction of the velocity of a high speed moving object like a plane in real life application?

Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, being a coherent derived unit whose ...
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Accelerometer measures 1g along z-axis

I have an accelerometer which measures 1g along the z axis when resting on a table. I don't understand why this is because the net force on the accelerometer is zero along the z-axis as the force of ...
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5answers
127 views

Calculating mean velocity: using time or velocity? [on hold]

Background I have been running tests which involve timing an object moving a certain (fixed) distance, $s$. Each test has been repeated 3 times, and the 3 times ($t{_{1}}, t{_{2}}, t{_{3}}$) for the ...
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1answer
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Meaning of time derivative of the Lorentz factor $\gamma$?

This question about the Lorentz factor $\gamma$ in special relativity. I know what $\gamma$ means and how to drive. I'm wondering if I have time derivative of $\gamma$, what dose it mean conceptually?
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Height of projectile and horizontal distance

So i think I have finally derived some expression relating the maximum horizontal splash displacement and initial drop height of a ball from using some information on the threads in this forum namely ...
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2answers
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Calculate acceleration and lateral G force from GPS coordinates [closed]

I have a data logger that records time (in tenths of seconds), latitude and longitude. I would like to calculate my acceleration and lateral G force based on the collected data. Is someone able to ...
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Is it possible to guess this solution just by looking at the system? [closed]

Show that, if a projectile is shot from a height $h$ with speed $v_0$, the maximum range is obtained for launch angle $\theta=\arctan\left(\dfrac{v_0}{\sqrt{2gh+v_0^2}}\right)$ For the problem shown ...
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1answer
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Is reversal of velocity always equivalent to reversal of time?

Let us imagine there is a container full of small particles which are allowed to collide with each other and the container walls (in 2D). If I initialise the system with given velocities and positions ...
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Extended Essay related to cars?

this is somewhat a follow-up from my previous post but I was wondering if anyone could recommend any university level research questions I could do with limited resources (mercedes ml 350, the general ...
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5answers
100 views

Units of variable acceleration

If we have a function, which describes, how a displacement in space along a line varies as a function of time: e.g.: $s(t)=vt$, its units are meters because $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
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4answers
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A man with a rifle shoots bullets at a speed [closed]

Hello all So I was doing this question. I had a doubt that, initially as both the (man and rifle ) system and the bullet were at rest we put 0 instead of m1u1 + m2u2 in equation of conservation of ...
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1answer
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Projectile motion problem with only angle and distance given [closed]

I have a cannon that shoots a projectile at an angle of 30°, the projectile hits a wall at 2.5m and it does so when the velocity only has a an horizontal component. I need to find the initial speed, ...
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1answer
34 views

Lorentz boost in light-cone coordinates

Consider a particle with momentum $p^{\mu}=(p^+,p^-,p_{\perp})$, where the momentum is written in light cone coordinates defined as, \begin{align} n^{\mu}&=(1,0,0,1)& \bar{n}^{\mu}=(1,0,0,-1) ...
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2answers
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What does it mean to velocity be a derivative of position? Isn't position a point, not a function? [closed]

So in mass-spring simulation I encountered that one simulates particles by using positions and velocities of particles etc. By mechanics: Velocity is the derivative of position. But, isn't "...
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Maximum acceleration on a turn

The question $\\$ Suppose a car enters a turn with an initial speed $\vec{v_1}$, and comes out with a final speed $\vec{v_2}$ perpendicular to the direction of $\vec{v_1}$. The driver does everything ...
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1answer
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Basic mechanics, motion in one dimension [closed]

I've just started to read Physics by Serway et al. In the problems section in chapter 2 there's the following question: The driver of a car slams on the brakes when he sees a tree blocking the road....
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1answer
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What does this description about kinematics mean?

In my $Theoretical \ Mechanics$ book I recently read, it gives the following description about "the path" in a certain time interval: $$\vec{r}(t)=\vec{r}(t_0)+\vec{v}(t_0)(t-t_0)+\int_{t_0}^tdt^{\...
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3answers
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Is the acceleration of car greater when hitting the accelerator, or the brakes? [closed]

Is acceleration of car greater than when pedal the pushed to the floor or when break pedal is pushed hard? I do understand that the signs would change in either but I am more considered about the ...
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1answer
53 views

General question about momemtum / motion

I do not have a Physics or science background. I have a question which is related to physics and I am sure this is the right forum for the answer. Imagine a big circle with a radius of 10cm and in ...
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1answer
62 views

Terminology for time derivative of speed (not velocity)

Is there any standard terminology for the derivative of the magnitude of velocity with respect to time (suitable for use in first-year Calculus)? The word ‘acceleration’, in its technical sense, is ...
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3answers
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Doubt in a question in Newton's Laws of Motion [closed]

The following question is from JEE (Joint Entrance Examination) 2007. As it is clear from the solution of the problem, they have included only the horizontal component of acceleration and not the ...
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3answers
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Is a rotating object moving or stationary?

Because it rotates in situ, its center of mass does not move, so it is static, but it is rotating, so it is not static, then is it static or moving?
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1answer
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Total energy of a two particle system [closed]

Question: Velocity of center of mass of two particles is $~v~$ and the sum of the masses of two particles is $~m~$. Kinetic energy of the system is: A$)~~=\frac{1}{2}mv^2~$ B$)~~\ge \frac{1}{2}mv^2~$...
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4answers
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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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2answers
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relative acceleration of ball in a train [closed]

A boy is standing in a train with a ball in his hand. The train is having a constant acceleration equal to x m/s^2. While still in train he throws the ball with a constant velocity of 20 m/s at an ...
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1answer
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How to estimate minimal efforts required to update trajectory of a moving ball?

Let's imagine we've got a uniformly sampled curved trajectory of a moving ball in 3D space without gravity. Let's assume that some dynamic force is applied to the moving ball to update its trajectory ...
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0answers
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Maximum range of a projectile (launched from the cave) [closed]

A cannon located under a cave forms an alpha angle with the horizontal launching a series of projectiles. The cannon is at point A, at a distance L from the base of the cave (point B). If the initial ...
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2answers
104 views

How do you correctly apply $a=v\cdot\text dv/\text ds$? [closed]

I know that $a=v\cdot\text dv/\text ds$ can be deduced by simple algebra and calculus and is correct. But once I was analyzing the motion of a ball projected straight up in free gravity. If I apply $...
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What is the difference between position, displacement, and distance traveled?

Suppose the question is somewhat like this: If $v=8-4t$ and the position at time $t= 0\ \rm s$ is $2\ \rm m$, find the distance traveled, displacement, and final position at $t=3\...
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1answer
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Relative motion between two particles

I already know the relation of relative velocity as : Vab=Va-Vb So is there any derivation for this relation or is just how it is defined? Relating to this i am finding some hard time to imagine why ...
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1answer
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If we say “an object is at rest,” can we also say “the object is moving at a constant velocity?” [closed]

If we say "an object is at rest," can we also say "the object is moving at a constant velocity?" Of course, the constant velocity would be zero, so it's mathematically sound. However, the wording ...
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1answer
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What is the advantage of using a polar coordinate system with rotating unit vectors?

What is the advantage of using a polar coordinate system with rotating unit vectors? Kleppner's and Kolenkow's An Introduction to Mechanics states that base vectors $\mathbf{ \hat{r}}$ and $\mathbf{\...
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Trajectories of a thrown object (with bumps considered)

Assume that I stood at the boundary line of a football / baseball / cricket ground(without any turf/pitch/grass in it) - just a plain ground - like play ground. I have a ball that can bounce when ...
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1answer
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How much lateral acceleration during cornering is needed to flip a car?

I'm doing my IB HL Math Exploration, where I use a bit of physics. Say I have a car with mass m, center of gravity at height h, and width of 2b, and I am turning a corner, how much centrifugal ...
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How to find force needed to lift two wheels off ground during cornering?

I'm attempting to calculate the maximum velocity at which I can safely drive around a corner. One scenario I'm considering is where all four wheels maintain contact with the ground, but slips. The ...
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1answer
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How to Translate Forces Between Points?

Consider a force $\vec{F}$ acting at distance $\vec{r}$ from a center of mass $CoM$ . Is it possible to somehow express the forces acting on $CoM$ from the known variables? The torque will obviously ...
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1answer
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Deriving vector equation for average acceleration

How can I derive the vector equation $$\vec{a}_{avg} = \frac{\vec{v}_2-\vec{v}_1}{t_2-t_1} = \frac{\Delta \vec{v}}{\Delta t}$$ for average acceleration? I know how to derive the average $x$ ...
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2answers
101 views

If vector $a$ rotate about vector $b$, does vector $b$ also rotate about vector $a$?

For $2$ vectors $\, \vec a,\vec b$, both originate at $[0,0,0]$: If vector $\vec a$ rotates about vector $\vec b$ when observed from a coordinate system fixed to vector $\vec b$, does vector $\vec b$ ...
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1answer
67 views

Free fall time after being accelerated [closed]

An elevator car whose floor-to-ceiling distance is equal to $2.7m$ starts ascending with constant acceleration $1.2 m/s^2$; $2.0 s$ after the start a bolt begins falling from the ceiling of the car. ...