# 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 applying a force to keep a body in uniform motion not considered as a force because there is no acceleration. (Friction included) [closed]

Because I have studied that if there is no acceleration then there is no force.
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### Is the relation “slope=velocity” mathematically valid?

$\text{Slope= tan(angle with respect to positive X-axis)= scalar output}$ $\text{velocity= a vector }$ Source: Hugh D Young_ Roger A Freedman - University Physics with Modern Physics In SI Units (...
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### Why the formula of kinetic energy assumes that the object has started from an initial velocity of zero?

According to my physics textbook, the formula of kinetic energy is: $$W = \frac{1}{2}mv^2$$ Where $m$ is is mass of the object and $v$ is the velocity of the object. The equation is calculated from ...
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### What is the magnitude and direction of the change in velocity if the inital velocity is 30m/s south and the final velocity is 40m/s west? [closed]

I got the magnitude correct which is 50 m/s however I don't know how to solve for the direction of change. Any help?
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### How to calculate the time it takes two trains to stop after decelerating? [closed]

The problem statement is as follows: Two trains, each traveling at $30\,\mathrm{ft}/\mathrm{sec}$, approach each other on a straight track. When they are $1000$ feet apart, a bee begins flying from ...
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### Direction of acceleration in elliptical path

The $x$ and $y$ coordinates of a particle moving in a plane are given by x(t)=acos(pt) and y(t)=bsin(pt) where a, b(<a) and p are positive constants of appropriate dimensions and t is time. I know ...
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### Trajectory of a charge in a electrostatic field which is generated by two charges

Consider the following case: We have a negative charge with position $$q = - 1, \quad \mathbf{r}(t=0) = \binom{9}{5}$$ which is placed into a electrostatic field. This Field is generated by two ...
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### What is the difference between m/s and m/s/s?

Can I subtract, say, for example, 600 m/s - 20 m/s/s? I am confused about this topic and there is no good information on the web.
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### Is there a difference between instantaneous speed and the magnitude of instantaneous velocity?

Consider a particle that moves around the coordinate grid. After $t$ seconds, it has the position $$S(t)=(\cos t, \sin t) \quad 0 \leq t \leq \pi/2 \, .$$ The particle traces a quarter arc of ...
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### Time Dependent Spiral motion

I am trying to model a particle traveling through a high efficiency dynamic separator (which allows me to separate solid particles by their size) The problem boils down to spiral motion, in the radial ...
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### How does $\langle 1/t\rangle$ denote a rate?

As a preface I should mention I am a mathematician and not a physicist, so I apologize if this question is elementary. I am going through an old paper of mathematical physics that I have been assigned ...
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### Harmonic motion: displacement

On my textbook the displacement in a simple harmonic motion is given as $$x = x(t)= A \cos(\omega t) \tag 1$$ It is true that $$x = x(t)= A \cos(\omega t)=A \sin(\pi/2-\omega t ) \tag 2$$ but why in ...
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### Is velocity real?

This sounds like a stupid question but I am do not grasp physics concepts easily. "Velocity" is just the change in displacement over the change in time. I can see displacement and time as ...
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### Need some help understanding velocity

I have just started learning kinematics and this is just a small question, that I wish for to clarified as it feels a little bit confusing. Below, the difference between speed and velocity is given. I ...
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### What is the meaning of word 'rate' in physics?

Often, I have seen in physics the rate of change of velocity or something like that in kinematics. And in question based on speed, time and distance. I would like to know the meaning of the word rate ...
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### Two cars are moving away from each other. what is the frequency heard by observer in one of the car if the other blows a horn? [closed]

The cars are moving with same velocity in the opposite direction (one going south and the other is going north)
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### Static Analysis of a Diagram

It is very clear $M_A$ can not be zeros under general loading conditions, so this structure must be able to rotate about A. but the geometrical constraints will not allow it to undergo this rotatory ...
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### Range of Iterated Projectile Motion [closed]

This is the question : Superman is throw inside one end of a room whose floor is made of kryptonite, his only weakness. He tries to fly using his mighty powers with an initial velocity $V_0$ at an ...
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### Why distance equals initial velocity times time Plus acceleration over two times time Squared? [duplicate]

i am a beginner in physics and I do not understand why is the d=vi(t)+(1/2)a(t^2).
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### Problem in instantaneous acceleration and instantaneous velocity

Recently i came accross a problem that said An object is dropped straight down from helicopter the object falls faster and faster but its acceleration decreases over time becoz of air resistance. the ...
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### Time taken to reach the ground, curved frictionless path vs. free fall

An object is released from rest in two conditions, one in which the object follows a straight path to the ground and one in which the object slides down a frictionless curved path. Will the time taken ...
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### Query regarding instantaneous velocity and instantaneous acceleration

Suppose an object's velocity is $5 \ \text{m/s}$ at $t = 1$ seconds and $8 \ \text{m/sec}$ at $t = 2$ seconds then the acceleration here is $3 \ \text{m/sec$^2$}$ i.e at $t = 1$ seconds the ...
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### Would it be easier to break a stack of planks joined together or when they have gaps between them?

If there is a stack of n identical planks (say n=5) and we need to break them (like in karate), would it be easier (i.e. would it require less force) if they are stacked together (figure on the left) ...
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### What is the difference between two types of velocity?

What is the difference between $v=\frac{s}{\Delta t}$ and $\bar{v} =\frac{\Delta\bar{x}}{\Delta t}$, are they the same?
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### Can we explain velocity without time? [closed]

I'm trying to wrap my head around inertia/velocity, or just change in general, I guess. I'm unsure exactly sure were my gap is, or what question to ask, so I'll explain with a scenario. Lets say I'm ...
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### How exactly does motion produce thermal energy?

How exactly does motion produce heat? An electron that is moving from a higher energy state to a lower energy state can dissipate this energy as heat. Conversely, if the atom is provided with heat, ...
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### What is ground speed when falling vertically? [closed]

Ground speed as apposed to air speed. Assume a vacuum around the earth for simplicity. Just wondering if it's 0 or the absolute velocity versus the point of impact on the ground?
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### Can there exist a motion which cannot be described as a function of time?

I know there exist systems for which we cannot solve its differential equation, But I was wondering if there could be a motion that cannot be represented as a differential equation with respect to ...
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### Elasticity of spring and conservation of energy [closed]

I have a problem of one of my students mechanics homework that I am unsure on and could do with resolving by tomorrow is possible! A spring of natural length $0.5m$ is attached to the ceiling and a ...
A train 1 moves from a point $A$ in straight line at $38 km / h$ and after $2.5$ hours is followed by a train 2 which moves from the same point $A$ in straight line at $80$ km per hour. At which time ...