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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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Throw a ball AT a moving train, in what direction does it reflect?

You are standing at a platform, with a ball. a train approaches and u throw the ball at it in a straight line from a small distance. in what direction des the ball reflect off? heres what i think, ...
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1answer
27 views

Velocity of an object orbiting on a string

First, I'm sorry if this question is dumb, incoherent or vague - I never really studied any physics and this might be out of my grasp, but I tried to apply some of the math I know and figure these ...
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1answer
23 views

How would the displacement of an object be calculated using only initial speed, coefficient of kinetic friction and the slope of the hill?

A sled takes off from the top of a hill inclined at 6.0° to the horizontal. The sled’s initial speed is 12 m/s. The coefficient of kinetic friction between the sled and the snow is 0.14. Determine how ...
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2answers
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Is motion smooth? [duplicate]

It's obvious that for every particle velocity is smooth i.e it cannot undergo sudden finite change in its position in infinitisiminal time. Similarly any particle's velocity cannot undergo a change ...
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1answer
18 views

Why are the vectors canceled out in this scenario for angular momentum of a particle?

I have a study guide for our next test, and I'm trying to understand the professors answer but I don't understand why i^ * i^ = 0? Here is his work, Why do we know that the P Vector is on direction ...
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1answer
16 views

Radial component of velocity at extreme distances

Suppose I am given that a planet's position with respect to some star is of the form $\textbf{r} = r\textbf{e}_{r}$. Then of course $\textbf{v} = \dot{r}\textbf{e}_{r} + r\dot{\theta}\textbf{e}_{\...
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2answers
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An object moves with constant acceleration. What would be the derivative of the unit velocity vector of that object with respect to time? [closed]

there was a question saying that: "An object is moving with constant acceleration a. which of the following are also constant?" (a) $$\frac{\text d\vert\vec v\vert}{\text dt}$$ (b) $$\vert\frac{\...
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0answers
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Forces on Colliding Blocks [closed]

A block of mass $m_1$ travelling at velocity $v_{1\ pre}$ collides with a block of mass $m_2$ travelling at velocity $v_{2\ pre}$. Both travel along the same axis and experience no external forces (...
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1answer
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At what time will the two points meet (uniform circular motion) [closed]

Consider points (A) and (B) that trace the same circle starting at the same time from the same coordinates and moving to the same direction. Point (A) has a period of T=25(sec) and point (B) has a ...
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2answers
34 views

What is the direction of linear motion after this impact?

I've been thinking about a situation where I am not sure what the resulting motion would be. This is what I am thinking, 1) P is the point of application of force. If there are two force components ...
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0answers
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Which bench will be hit a ball projected at an angle (benches are at different heights)? [closed]

Question: The benches of a gallery in a cricket stadium are $1$ $m$ wide and $1$ $m$ high. A batsman strikes the ball at a level $1$ $m$ above the ground and hits a mammoth sixer. The ball starts ...
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2answers
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Do objects move in 2 directions at once?

If a velocity vector of an object can be divided into an x and y component relative to a second object's position, and both objects have gravity that attracts both objects to each other. We then know ...
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1answer
29 views

Finding tangential and normal velocity from a curve [closed]

How do you find tangential and normal velocity from a curve? I know how to find dy/dx, but I have no idea how to obtain ut and un and dv/dt.
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Special Relativity; 3 Momentum Conservation

In SR, I understand you can use 4 momentum conservation, but what are the special cases where you can use 3 momentum/energy conservation? An example I have seen is with $$P_1=(M_1, 0) \\ P_2=(M_2,0) \...
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2answers
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Factorising a 4D Dirac delta function in a rest frame

I'm working through a QFT problem and at one stage in the solutions we have this step: $$\delta^{(4)}(p - q_1 - q_2) = \delta(E_1 +E_2 - M)\delta^{(3)}(\bf{q_1} - \bf{q_2}).$$ We are working in the ...
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4answers
121 views

Can we do a thought experiment to prove that the horizontal and vertical motion of a projectile are independent of each other?

It seems really hard for me to grasp the idea that horizontal and vertical motion of a projectile are independent of each other. Intuitively I feel that they should affect each other. How can it be ...
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1answer
68 views

Projectiles with unequal initial velocities, purely horizontal

If projectiles $A$ and $B$ are projected horizontally with different initial velocity from the same height, then both $A$ and $B$ possess the same vertical component of velocity, at any instant. ...
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What is the rate of change of speed called? [duplicate]

Magnitude of acceleration is definitely not the answer in case of rate of change of speed unless it it is moving in a straight path. So is there any name for rate of change of speed?
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3answers
78 views

How is final velocity negative when acceleration is in positive direction?

When a body starts from rest and starts accelerating in positive direction and moves certain positive distance, its final velocity is given by : $$v^2 = u^2 + 2as$$ Solving it gives two values of $v$, ...
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0answers
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Why don't we define time derivative of acceleration? [duplicate]

When we started the study of kinematics we defined position and its change with respect to time. After that we defined time derivative of velocity which gave us acceleration. These 3 concepts really ...
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2answers
42 views

Can displacement be negative after calculation?

Regardless of the positive or negative, doesn't the number determine the total displacement and not the sign in front of the numbers?
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86 views

Why does a corner slow a car down?

Lets say we have a car, with power being given to maintain it at 100 km/h and about to negotiate a slight bend. As the car turns the corner, there is no change to the accelerator pedal, and no braking....
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0answers
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Physics of a catenary with sinusoidal motion at one end

If we have a catenary line with a mass per unit length of m with one end fixed and the other end moving in sinusoidal motion like $A.sin(\omega.t)$, how would I ...
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2answers
32 views

Find the resultant force in a motion problem with a loop

In the approximate diagram, the block with mass $m$ starts without movement in the position $P$ in the top of a hill with height $5R$. After that, the block falls down through the hill, and reaches a ...
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7answers
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Is there centripetal acceleration with velocity of 5 m/s?

So if an object is moving at $5\ \mathrm{m\ s^{-1}}$ in a circle, is it accelerating? Me and a friend are having a conversation about this.
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3answers
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How to derive kinematics equations using calculus? [closed]

I read derivation of kinematics equations using calculus: $$a=\frac{\text dv}{\text dt}$$ $$\implies \text dv=a\text dt$$ $$\implies \int_{v_0}^v\text dv=\int_0^t a\text dt$$ $$\implies v-v_0=at$$ $$\...
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1answer
37 views

SUVAT applied to circular motion

My teacher set me this problem: A ball is suspended from a ceiling by a string of length $7.5\ m$. The ball is kicked horizontally and rises to a maximum height of $6.0\ m$. Assuming that air ...
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1answer
55 views

Is velocity the derivative of position, distance, or displacement?

All over the internet, I am seeing different people defining velocity as the derivative of either position, distance, or displacement and it is really confusing me. I can understand how the derivative ...
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0answers
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Bouncing of tennis ball [duplicate]

Some one posted this question but didn't give the entire details. I request that any one with the question post it for me. Iam interested in the detailed question, in other words the first part of the ...
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4answers
321 views

Is there a scalar acceleration?

Distance is paired with Displacement and it seems to be a bigger idea than just the magnitude of Displacement. Speed is paired with Velocity. I have always thought that there is not such pairing with ...
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1answer
29 views

Velocity of approach which changes continuously [closed]

There are two particles A and B which are moving with constant speed $v$ and $u$ such that $v$ is always directed towards B. At $t = 0$, the separation between A and B is $l$ and $u$ is perpendicular ...
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Single number to quantify difference between two local reference frames?

I'm working with some satellite data that defines the spacecraft reference frame from two star cameras and two different measurements of orientation configuration of the cameras, and I'm trying to ...
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1answer
25 views

Will sound be generated by the capture of air from a vacume?

I have a thought experiment. If I had a box sized vacuum sitting on the floor in front of me and I were to instantaneously remove all the walls of my box. Would the capturing of air from that vacuum ...
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0answers
25 views

Calculating time taken for acceleration [closed]

about tickertape timer,when we measure the time for a particular object's acceleration, we would take 7 ticks multiply 0.020s(for example), instead of taking 8 ticks. Why do we exclude the other tick, ...
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2answers
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Calculating the charge density from a form factor

For an atomic form factor $F(\textbf q)$, the corresponding charge density distribution is given by $$ \rho(\textbf r) = \frac{1}{(2\pi)^3}\int\text{d}^3 \textbf q \,F(\textbf q)\,\text{e}^{-\text{i}\...
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1answer
51 views

Linear and angular speeds of a train

I am using Radar sensor in a train. Sensor is in the front part of the train to detect object and avoid collision. It needs vehicle motion data to calculate objects longitudional and lateral speeds....
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7answers
179 views

What happens to velocity when Time equals zero?

I am not formally educated in Science but natural questions have always intrigued me.The way I put it is that I am married to Commerce but Science has been a childhood love. Now I have this very basic ...
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1answer
50 views

Why is the kinetic energy a fixpoint of the Legendre transformation?

Question: Why is (from an intuitive standpoint) the kinetic energy $T$ a fixpoint of the Legendre transformation, i.e. $\frac{\partial T}{\partial \dot q}\dot q-T = T$ for any general coordinate $q$? ...
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2answers
35 views

Speed and tangential acceleration in pendulum motion

Speed is a scalar quantity which corresponds to the magnitude of velocity, so it must always be nonnegative. In a general 2d motion, the tangential component of acceleration is given by the time ...
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1answer
52 views

Impossibility of perpetual motion as a mathematical theorem

Can the impossibility of perpetual motion be expressed as a mathematical theorem? What are some relevant books or other sources?
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0answers
37 views

A body moving along a circular path covers a distance of 3 m in 1 s uniformly [closed]

A body moving along a circular path covers a distance of 3 m in 1 s uniformly, such that the body returns to its starting point. (i) Is the body moving with uniform speed? Explain. (ii) Is the body ...
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1answer
53 views

Finding the mass of a relativistic particle resulting from decay

I'm studying relativistic kinematics for the first time. I’m working with a problem from a book but I’m stuck: A relativistic particle of rest mass $m_1$ decays at rest into a particle of rest mass $...
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1answer
50 views

how does a car move? and how do the brakes affect its velocity? [duplicate]

im doing a project that requires me to understand how cars move. from what I understand , its the friction that is creates in the tires that enables the tire to rotate, which then push the car forward....
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2answers
77 views

There are two velocities on the sphere. How to sum them on the sphere?

As shown in the figure, there are two velocities on the sphere, one is the velocity along the meridian direction, the arc length is its size, the other is the velocity along the equatorial direction, ...
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0answers
41 views

Significance of differentiation of the equation of trajectory of a projectile with respect to different variables [closed]

The original question was: A fighter plane is flying horizontally at a height of 250 m from ground with constant velocity of 500m/s.It passes exacily over a cannon which can fire a shell at any ...
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3answers
46 views

Why is the normal reaction zero in this question?

A skier rolling down a frictionless hill where she should fly off tangentially and lose contact with ground. In answer of this question, solution states that $mg\cos x = mv^2 / R$ at that point (to ...
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1answer
47 views

circular(ish) motion?

how would one describe as a function of time the position vector of an object with initial position $\vec{x_0}$ and velocity $\vec{ v_0}$ experiencing a central force of constant magnitude around the ...
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1answer
33 views

Momentum and changed a changing mass

let there be a spaceship which is floating in space with a surface size of A. The spaceship floats in a constant speed of $v_0$, dust with density of $\rho$ gets stuck on the (only) surface of the ...
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2answers
63 views

If an object in free-fall is moving upward, the object's velocity is decreasing. Does this mean that the acceleration is -9.8 m/s/s?

I have a problem (that I am not asking you to solve) where it takes $4s$ for an object to travel upward to its maximum height, and we have to determine how far the object travelled. If the ...
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0answers
51 views

Why there is $1/2$ in $KE=(1/2)mv^2$ and Etc [duplicate]

Hello I’m a high school student, my teacher asked the class for why and how there is a 1/2 in (KE=1/2mv^2)&(PE=1/2 k Δx^2)&(PE=1/2 c(Δv)^2)&(KE=1/2 I W). This is my first time on this ...