# 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### Motion equations for Woodpecker toy (multibody system)

I am trying to understand a rigid multibody model of a Woodpecker toy (see figure below). Now I am not going to go into details about the model or justify this approach, I am just trying to understand ...
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### 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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### 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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### In circular motion of a particle is the velocity of particle always perpendicular to the centripetal acceleration?

In circular motion of a particle is the velocity of particle always perpendicular to the centripetal acceleration? Are there any exceptions? Why or why not ?
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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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### 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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### 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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### 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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### 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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### If an object in uniform motion with zero net force, can its mass change?

According to newton laws if something is in uniform motion and equivalent force is equal to zero then its mass cannot change. Our book says an object stays in state of rest or motion unless compelled ...
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### 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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### 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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### 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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### 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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### What is the difference between these two ways to calculate average velocity?

Average velocity: $$v_{\rm avg,1}=\frac{v_{\rm final}+v_{\rm initial}}{2}$$ and average velocity: $$v_{\rm avg,2} =\frac{\rm total\;displacement}{\rm time \;taken}=\frac{\Delta x}{\Delta t}$$ ...
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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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### How to approximate acceleration from a trajectory's coordinates?

If I only know $x$- and $y$- coordinates of every point on a trajectory without knowledge of time information, is there any way to approximate Cartesian acceleration angle at each point? Time interval ...
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### 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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### Conservation of linear momentum magnitude along a trajectory

I was once criticized for "taking angular momentum as momentum going in a circle". I was loosely trying to state, in classical mechanics, that in using conservation of momentum, one can switch between ...
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### Is two cars colliding at 25 mph the same as one car colliding into a wall at 50 mph in reference to injuries?

This question has been asked using 50 & 100 mph reference, see this Phys.SE post and links therein. However, I am interested in the potential injuries to occupants of the autos. As the one going ...
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### Why is the direction of net force on an object and the direction of acceleration of that object different in this problem?

A $2.0 kg$ box of cucumber extract is being pulled across a frictionless table by a rope at an angle $\theta=60°$ (from positive direction of $x$ axis, we have taken horizontal surface of table ...
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### tangential acceleration for uniform circular motion

I understand that circular motion is defined by 2 components of acceleration, one tangential and one radial and their resultant is what causes circular motion. I am confused though as to why it is ...
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### How can a horizontally fired bullet reach the ground the same time a dropped bullet does?

I studied projectile motion and now I know that we can treat each component of motion independently. Since gravitational acceleration acts on both a horizontally launched bullet and a vertically ...
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### Time taken for collision [closed]

We have three particles at the vertices of equilateral triangle of side $d$. At $t=0$ they start moving in such away that at all instant of time each of them has a speed $v$ towards adjacent one. We ...
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### Functions of Time

1) Is position a function of time only or also velocity? Likewise, is velocity a function of time only or also the position? 2) The following are functions of time: $s(t)$ = distance a particle ...
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### Angle required to hit the target in projectile motion

Consider the problem of projectile motion where the angle to hit a target $(x,y)$ is asked, once given the initial velocity magnitude $v_0$. The projectile is fired from the point $(0,0)$. Here is ...
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### 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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### What do negative times and negative displacements represent in parametric projectile equations?

There are negative times and negative $x$ and $y$ displacement in parametric equations; what do they represent? And is it possible to tell the object's velocity, direction and acceleration from just a ...
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### Solar Sail Questions

I'm just looking for a good (better) analogy. How do massless photons put pressure on a surface especially when it's a mirror? Using the analogy of the wind (atmosphere) on a sail breaks down when I ...
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### Can the initial time be non-zero?

I am just slightly confused about how initial time is chosen in mechanics. I keep thinking I have understood it but some doubts come up later. Basically when I derived the constant acceleration ...
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### Why do most office chairs have 5 wheels?

(Inspired by Why are four legged chairs so common?) I've been wondering for a while... Why do most wheeled office chairs have 5 wheels? My guess would be that while stability vs. simplicity results ...
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### How does a mobile phone vibrate without any external force?

How does a mobile phone vibrate without any external force? By Newton's law, any body can't move without any external force
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### Extra vibrational mode in linear molecule

When calculating the number of vibrational modes for a molecule, the formulas differ for linear $(n = 3N - 5)$ and non-linear $(n = 3N - 6)$ molecules, where $n$ is number of modes and $N$ is number ...
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### Does rotation increase mass?

If an object is rotated on its axis near the speed of light would its mass increase? Normally if the object was moving (in relationship to the Earth for example) I would agree that its mass would ...
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### What stops giant cruise ships toppling over in rough seas?

A week ago, 2 of the most gigantic cruise ships in the world docked near my city. If you have seen one, or been on one, you will know how large they are. They look extremely unsafe to me, although ...
In different contexts, my book(Principles of Physics by Resnick, Halliday ,Walker) , they wrote For torque, the path need no longer be a circle and we must write the torque as a vector $\vec{\tau}$...
I understand that if the velocity is constant (acceleration=$0$) throughout the course of motion (where graph shows a rectangle) then it would simply be like playing with equation: (1). velocity=...