# 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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### Why do kinematic equations only work with constant acceleration?

People say that the equations are derived assuming a constant acceleration. I just don't see how this is the case. (I am new to calculus.)
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### Why do equation of motion fail to apply in non-inertial frame even after applying pseudo force?

I considered two cars of equal mass moving towards each other with speed 30m/s and 18 m/s respectively and when they are 100 meters apart they both start de accelerating at 6m/s² each. So if we ...
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### Minimum seperation of moving objects doubt

Let there be $2$ objects $P_1$(initial velocity $u$ $ms^{-1}$ & acceleration $a$ $ms^{-2}$) & $P_2$ (initial velocity $U$ $ms^{-1}$ & acceleration $A$ $ms^{-2}$) initially separated by ...
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### Can the speed of the wind influence the speed of sound propagated in it?

The question came up with a exercise (Page 5, exercise (vi) of Pratice Exercise 1.1) from the book Physics Galaxy Volume 1 Ed. 1, Ashish Arora. The question proposes the following: Two junglemen are ...
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### Why power of engine must be equal to power of friction?

Suppose a car with electrical engine moves uniformly with friction between wheels and the ground (neglect air resistance, energy loss, etc.). So why the car's engine power must be equal to friction's ...
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### Strange kinematics result

I'm currently doing some algebra-based physics problem sets and have come across this strange result. Problem A ball is tossed upwards with an initial velocity $\vec{v}_0 = 48.9\text{ m/s}$. Assuming ...
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### Lagrangian Dynamics of an inverted Spherical Cart Pendulum

Introduction I have to come up with a PD-controller for an inverted Spherical Cart Pendulum, therefore I tried to compute the Dynamics of such a Pendulum. The Spherical Cart Pendulum is a hybrid ...
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### A question regarding equations of motion

Suppose I drop a body from a height $h$. The initial velocity is zero. Now it reaches the ground and stops and finally becomes zero. So now u=0 and v=0. If I use the equation of motion $v=u+at$ to ...
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### How can I find the amount of time required for this to stop? [closed]

A particle starts from rest and moves with a constant acceleration of 5 m/s2. It goes on for 10 s. Then, it slows down with constant acceleration for 500 m until it stops. How much time does it take ...
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### How to find time taken for a faster object to cross a slower object of same length, both moving parallel to each other in the same direction? [closed]

Can you explain me how to determine the time taken for a faster object to cross a slower object when they're both of same length and are moving parallel to each other in the same direction?
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### Is the deceleration a result of friction, a constant value?

Frictional Force = km, therefore k = a Would this mean that no matter the mass of an object (if they are the same material) They will decelerate at the same speed with friction, therefore if u is the ...
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### Angle-free two-dimensional inelastic collision formula?

I'm trying to calculate velocities (by components - x, y) of two objects (balls) after inelastic, two-dimensional collision. I've successfully implemented the angle-free formula for elastic two-...
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### Are bicycles the fastest possible human-powered vehicles? [closed]

It seems to me that bicycles have proven themselves to be the fastest human-powered vehicle in common usage. By “human-powered” I mean it has no fuel or power source other than the inputs given by a ...
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### Deriving Formulas for a running person and a moving bus [closed]

A bus is going along a straight line with velocity v1. You are located at a distance D from the bus and distance S away from the road. You can run with a maximum speed v2 in any direction. a) In ...
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### They accelerated covariantly [closed]

Show that if the 3 acceleration vector of a relativistic point mass is zero for a relativistic inertial observer then it is also zero for all relativistic inertial observers. This implies that ...
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### Find the equation for the angle $\theta$ in which the particle leaves the semicircle. No Friction [closed]

I think I missed something in this mechanics problem. We're given a polished (no friction) and homogeneous hemicircle which has mass $M$ and a particle of mass $m$ laying on the top of it. There is ...
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### Running of the fine structure constant

In Measurement of the Running of the Fine-Structure Constant, the L3 collaboration reports their fine structure constant data for various values of momentum transfer which they call $Q^2$. This is ...
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### Why does solving the differential equation for circular motion lead to an illogical result?

In uniform circular motion, acceleration is expressed by the equation $$a = \frac{v^2}{r}.$$ But this is a differential equation and solving it gets the result $$v = -\frac{r}{c+t}.$$ This doesn’t ...
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### Is it possible for a point-like system to behave like $x(t) = \frac{t}{2}\log(t^2)$ near $t=0$? (infinite speed) [closed]

Is it possible for a point-like system to behave like $x(t) = \frac{t}{2}\log(t^2)$ near $t=0$? (infinite speed) I know beforehand that relativity theory forbids anything with mass from travel faster ...
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### When should smartphone be declared as static using accelerometer reading? [closed]

I wish to know when should I declare an smart phone static by reading its 3 axis accelerometer reading. I have notice that modulo of all 3 axes gives 9.8 when ever smart phone is static. What does it ...
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### Why is there a difference between single and double compound pendulum's kinetic energy equation?

When calculating equation of motion of single compound pendulum, kinetic energy is taken as $K= I\dot{\theta}^2/2$ (See). But when it is double compound pendulum, kinetic energy of the first pendulum ...
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### How can one detect a device (phone) falling using its 3-axis accelerometer?

I am working on an Android app to use its 3-axis inbuilt accelerometer to find if the phone falls. I googled and found that if the following value is less than 2: $$a = \sqrt{x^2 + y^2 + z^2}$$ (where ...
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### How do you find the final velocity when acceleration is changing between two values over some distance? [duplicate]

How do you calculate a final velocity of an object when given its initial velocity and the object is accelerating between an initial and final acceleration over some given distance?
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### The rate of change of speed when turning back along a straight line

I was trying to find how the rate of change of spped at $t$ of an object moving in a straight line relates to its maginitude of it acceleration. The case when it is moving in the direction before and ...
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### Constant velocity and linear acceleration

Given an object moving in a straight line with velocity that can be modeled by a parabola, would the answer to the question "when is the velocity constant" be the vertex of the velocity ...
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### How is there kinetic energy at the bottom of the pendulum swing if velocity = 0 at the lowest point?

Say a mass $m$ is attached to a massless string that is attached to the ceiling. The mass is pulled 30 degrees to the left of the vertical and then let go. At the lowest point of its swing it has ...
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