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
28 views

Why does the bottommost point of a rolling body have a radial acceleration in the ground frame?

Let's say a solid cylinder of radius r is rolling on a stationary horizontal surface with linear velocity v, angular velocity $\omega$, linear acceleration a and angular acceleration $\alpha$ Bottom ...
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1answer
51 views

Simple physics question pls help [closed]

If two balls, A and B are projected with an angle from the same level but both reach same maximum height. The angle that Ball B makes with the horizontal line is greater than the angle that Ball A ...
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Is there a way to solve this question using instantaneous center method? [closed]

This question is about instantaneous center method of finding velocities. I saw this question on a exam paper, but i think the question may be wrong because i couldn't find a way to get IC(1,3) Or ...
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1answer
23 views

Are superconductors primarily for DC transmission?

I understand Superconductors can be beneficial due to their resistance to magnetic fields as in levitating bullet trains or MRI machines, but is our main interest for DC power distribution? I see ...
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1answer
51 views

Why is direction of train like this for car?

This is an image of a train and a car. T is the train moving towards - $x$ axis and C is the car moving towards $+ y$axis. So, my teacher made a resultant velocity with $-5\hat i$ and $-20\hat j$. ...
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2answers
53 views

What will happen to the ball in a vacuum container when suddenly the gravity after 2 seconds disappears? [closed]

Suppose that there is a ball in a vacuum container where mass of the ball is $2$ kg. The height of the container is $100$ m. Assume that the acceleration due to gravity is $10$ m per second square. I ...
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2answers
29 views

Is this correct regarding velocity of train?

So many yellow T drawn are just different position of train at different instants.The person is standing still at all those instants at one place. If a train is moving at 5m/s. Then for me , can we ...
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1answer
28 views

Direction of $V_RM$ confusion

So ,please read both the questions . They have similar originate questions that I am facing difficult in. Let us say rain is falling downwards with v = -5j and a man is moving with a velocity of 2i in ...
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3answers
51 views

Tension of string suspending a ball in Circular motion, given the mass of ball, radius and length of string [closed]

Ok so I have this question where a ball is suspended by an inextensible string and is caused to rotate in a horizontal circle which has its centre vertically below the point of suspension. What would ...
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0answers
25 views

Confusion in relative velocity with rain man problems

My major conceptual difficulty in understand this concept is mostly the direction of velocity of man with respect to rain. Let us say rain is falling downwards with v = -5j and a man is moving with a ...
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2answers
273 views

Does the SUVAT equations of motion (Kinematics) come from some differential equation?

Wikipedia says about the equations of motion that; "If the dynamics of a system is known, the equations are the solutions for the differential equations describing the motion of the dynamics.&...
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1answer
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Why the direction of the omega (angular velocity vector) is along the axis of rotation? Also for angular acceleration

I know that the direction of omega is taken along the axis of rotation but I don't understand it why it is taken? I also know that $\mathbf v = \mathbf ω \times \mathbf r$ so, all three vectors ...
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3answers
236 views

Calculating displacement from acceleration (intuitively) [closed]

If I say acceleration of car is constant at $4\; \rm m/s^2$. Then isn’t it that it covers $4\; \rm m$ in $1\; \rm s$ with velocity $4\; \rm m/s$. Then in $2\; \rm s$, the velocity is $8\; \rm m/s$. ...
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1answer
117 views

Simple pendulum with moving support in a parabola

The problem says that I have to find the equations of motion in the Hamiltonian formulation for a simple pendulum with mass $m$ and lenght $l$, which its support moves with no friction along the ...
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1answer
55 views

What happens to the momentum of this rainfall? Are these expressions describing the rainfall correct/significant?

An element of mass $\Delta m_r$ (mass of rainfall) falls vertically onto a mass $m_c$ (a cart). The empty cart starts from rest and there is no friction. A constant horizontal force $F = <F, 0>$ ...
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2answers
61 views

How to use equation of motion like this here? [closed]

So I will explain my problem exactly what it is.Please do answer in such a format by telling me places where I am wrong from the text I write.I posted this question earlier also but I noticed many of ...
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3answers
116 views

Confusion in equation of integration and differentiation for motion?

It says displacement time equation here but x is position of particle here I think. It is not displacement of particle here. But when it is given velocity of particle to us and then we integrate it. ...
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2answers
66 views

Why quantities in physics are always talking about rates? [closed]

I get the idea that physics wishes to study changes to discover new rules. But why is everything related to rates? Acceleration,Velocity? Could we use something else apart from these? What can you ...
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4answers
84 views

Why does a body falling down follow this pattern of velocity?

I was wondering about the pattern of velocity for a body falling down with initial velocity $u= 0$ and $g = 10m/s^2$. Without using the formula , I thought : Velocity would change in this manner (...
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1answer
25 views

Question of ball falling down.Difficulty in understanding the formula

A ball is thrown upward from the top of a tower 40m high. u = 10m/s.Find time for it reach AD. g = $10m/s^2$. Taking upwards direction as +ve and downward as -ve. u = +10m/s.$g=-10m/s^2$.s=-40. $-40 = ...
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2answers
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Difficult in understanding question of minimum stopping distance [duplicate]

A car moving with a speed of 40 km / h can be stopped by applying brakes after at least 2 m. If the same car is moving with a speed of 80 km / h, what is the minimum stopping distance. As I have ...
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1answer
20 views

One-dimensional harmonic move - input parameter conversion

I have one-dimensional harmonic move, but velocity is defined as function of distance from the beginning V(x) = (Vmax + Vmin)/2 + (Vmax - Vmin)/2 * cos((2 * PI / K ) * x ) Where Vmax > Vmin, Vmin &...
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2answers
38 views

Finding the velocity of a projectile thrown from a raised platform at an angle [closed]

The following question has been giving me trouble for a while now, it states A particle is thrown with speed $10 \space \text{ms}^{-1}$ from a point $2\space \text{m}$ from the ground, at an angle ...
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3answers
111 views

What axes are typically used to graph projectile motion?

I am currently trying to model a function for the 2d projectile motion of a football from the angle of elevation and the initial speed, however I am a bit confused on what the graph of projectile ...
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2answers
44 views

Why is constant velocity of a body one-dimensional motion?

If velocity of a particle is constant. It is in uniform motion. Why is it that it is a example of one-dimensional motion? It can still change its direction and have constant velocity. By saying one-...
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3answers
84 views

Why don’t we consider angle 90 and 180 as angle made by acceleration

It states that angle made by acceleration when is between 0 and 89 degrees , acceleration increases whereas in case when it is between 90 - 179 degrees, acceleration decreases. I didn’t understand why ...
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2answers
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Would the angle theta made during instantaneous speed is always same or not?

I hope you can understand that I am talking about instantaneous speed here.You can notice I have done summation of all small ds/dt in my photo. The angle which I have got it theta.I want to know is ...
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8answers
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Why do we need instantaneous speed?

I am new to this topic and was just wondering about the use of instantaneous speed. I mean, we use to calculate the speed of car let us say at 5 sec. So we take the distance travelled in 4.9 to 5.0 ...
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Discretizing kinematics of moving object

I think this is a simple question, but I'm unable to find the relationship: The relative distance between two objects at any time $t$ can be written as: $x_r(t)=x_{ro}+\int_{t_0}^{t_f}v_r(t)dt$ where: ...
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6answers
412 views

Where did the kinetic energy formula come from? [duplicate]

I was looking for a proof for it and all rely on the work-energy theorem. But the work energy theorem relies on the kinetic energy equation. Ergo circular logic. So where did it come from?
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2answers
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Calculating for total distance travelled vs position given $v(t)$ graph

It is currently to my understanding that the area under a $v(t)$ graph is the displacement of an object because $$\int v(t)dt = s(t).$$ However, some of the problems I have attempted recently give you ...
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Skipping stones on the surface of a non-static fluid — how to assess in vector space?

While skipping shells at the beach, I noticed the following: Waves approaching the shore impede the average velocity of my skip; retreating waves appear to do the opposite. This, of course, makes ...
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1answer
66 views

Why is $(p’-p)^2 = -|\mathbf {p’} - \mathbf p|^2 + \mathcal O(\mathbf p^4)$?

On page 121 of Peskin and Schroeder, they claim that $$(p’-p)^2 = -|\mathbf {p’} - \mathbf p|^2 + \mathcal O(\mathbf p^4),$$ where $p = (m,\mathbf p)$ is the incoming 4-momentum of a particle in a ...
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2answers
60 views

How to find position vector in Kepler orbit?

As the title says, I am trying to determine the position vector $r$, knowing true anomaly, semi-major axis, angular momentum and eccentricity vector. There is an equation describe the distance to the ...
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3answers
61 views

Why doesn't an initial velocity affect the final velocity?

I've been stuck on a basic kinematics problem for some time, which usually goes something like this. Three balls are dropped from some height, $h$, with initial velocity $|v|$. Ball #$1$ is given $...
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2answers
132 views

$\log\left(\frac{p\cdot p’}{(E^2-p^2)/2}\right)=\log\left(\frac{-(p’-p)^2}{m^2}\right)$?

On page 182 of Peskin & Schroeder, equation (6.17) ends with the equality: $$2\log\left(\frac{p\cdot p’}{(E^2-p^2)/2}\right)=2\log\left(\frac{-(p’-p)^2}{m^2}\right),\tag{6.17}$$ where $p=E(1,\...
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2answers
122 views

Rotational kinematics of a non-rigid body. Equation of motion? [closed]

Two masses (of mass $m$) $1$ and $2$ are mounted on a massless rod. Mass $1$ is firmly bolted to the rod, mass $2$ can slide without friction along the rod. At $t=0$ the system is brought into ...
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67 views

Dimensionality of fluid flow

In some textbooks flow is classified as one, two, or three-dimensional depending on the number of space coordinates (i.e. x,y,z) required to specify the velocity field while according to some other ...
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Momentum Vectors in Bondi coordinates

In the Bondi-Sachs formalism, we can define the notion of 'retarded' time via a coordinate transformation of the usual Minkowski metric $$ d s^{2}=\eta_{\mu \nu} d x^{\mu} d x^{\nu}=-\left(d x^{0}\...
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2answers
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Is energy equivalent to motion? [closed]

Energy can be described in many contexts, such as the energy required to move something - be it translational or rotational - being kinetic energy, temperature/heat, electricity, light, etc. However, ...
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4answers
140 views

How to find motion equations using energy and angular momentum? [closed]

I'd really appreciate some help with an exercise. The exercise presents a system of two bodies, $m$ and $M$. Both are connected with a weightless rope; the former is rotating (with a given initial ...
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0answers
47 views

Average velocity of rocket [closed]

A rocket is accelerating straight up from the earth's surface. At $1.15$ s after liftoff, the rocket clears the top of its launch platform, $63$ m above the ground. After an additional $4.75$ s, it is ...
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1answer
41 views

Distance of slowing object [closed]

I have object with a given mass $m$, which is moving with the velocity $v$ and there is force $\bf F$ pushing against movement of the object. How far will object travel until it stops and how long ...
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5answers
65 views

Vertical motion under gravity

If we project an object vertically under gravity it travels vertically upwards until vertical velocity zero, why is that? Why does the velocity become zero?
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2answers
35 views

Equation for Work required to achieve a certain velocity [closed]

If we are trying to find the work required to get an object moving at velocity $v$, and we start with $w = f\cdot d$, we can then make the following substitutions: substitute $f$ with ma: $w = m\cdot ...
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3answers
74 views

Computing the $x$-$y$ position of a vehicle for constant acceleration

I want to very simplistically plot the $x$-$y$ position of a vehicle in 2D that can accelerate/brake and drive a constant speed while turning etc. The basic equations for determining the position for ...
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3answers
76 views

Why are the seat belts designed the way they did? [closed]

I understand the importance of seat belts in vehicles. Since our body is moving at the same speed as the car, there should be something holding us tight in our seat when we step on the brake. Most 3-...
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1answer
64 views

Equation for four-acceleration

Wikipedia states that $$ \begin{align} \mathbf{A} =\frac{d\mathbf{U}}{d\tau} &= \left(\gamma_u\dot\gamma_u c,\gamma_u^2\mathbf a+\gamma_u\dot\gamma_u\mathbf u\right) \\ &= \left(\gamma_u^4\...
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2answers
72 views

How to maintain the same initial velocity in trials with projectile motion experiment?

If I would like to experiment with projectile motion to determine the relationship of launch angle and horizontal range (for instance, I would throw a ball multiple times from a height with varying ...
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3answers
488 views

Problems regarding the equations for work done and kinetic energy

For example consider that I am driving a car with a mass of $200$ kg moving at $10$ m/s relative to an outside observer, with an assumed constant opposing friction of $50$ N, for simplicity (it won't ...

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