All Questions
21 questions
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How to Calculate the Stopping Distance of Train on a Complex Track [closed]
I am attempting to determine when a train should begin braking to reach a certain point at a complete stop. There are several factors that need to be considered that make this more complicated and I ...
- 13
0
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0
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31
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What does the square in kinetic energy formula imply? [duplicate]
I was wondering if the velocity squared imply that you need exceedingly more energy to gain speed. And if so, why is this a thing?
- 340
2
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0
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115
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$E_k = (1/2) mv^2$ doesn't make sense to me [duplicate]
I hope this is the right platform to ask stupid questions like this, but this is driving me nuts right now. Probably embarrassing to say, but I actually have somewhat of a physics background, but also ...
- 21
4
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4
answers
749
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Car stopping times (not distances)
(Apologies if this is not the correct Stack Exchange site; I was unsure where I should post this question)
I was prompted to write this after reading on Reddit the stories told by drivers who had ...
- 143
1
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1
answer
64
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Speedometer Dilemma [closed]
This is what is written in Resnick Halliday Krane Physics Vol.1
The speedometer of a car indicates it's speed, not it's velocity, because it does not specify a direction. Speed is a scalar, since it ...
- 31
2
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1
answer
126
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Is this expert report wrong about basic kinematics? [closed]
This question is about the application of kinematics in an expert report for Eirikson v. Breton, ABQB 2000 798 (archived), a judgement for a case where a woman was driving a car that was accelerating ...
- 183
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1
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86
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Calculate efficiency from discrete power, time and distance
I have a sequence of points from GPS and OBD for some vehicles.
Each point has the instantaneous power of the engine $(P)$, the time elapsed from the last points $(T)$ and the distance traveled from ...
2
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1
answer
189
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Rotational motion from curved slope?
So I've learned for any slope with a sliding object, say a box, the speed at the bottom only depends on the height it was dropped from because of conservation of energy, assuming absence of friction ...
- 133
2
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5
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4k
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Why is it said that speed always remains constant in circular motion?
One can cover circular motion with different speeds at different positions, right? The only aim is to complete his circular motion, right?
1
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1
answer
38
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How force affect the objects that we apply on? [duplicate]
Why do things get accelerated rather than do a moving at constant speed when we apply a force on them? How my force affect the object at atomic level? Does it increase the kinetic energy of atoms ? If ...
0
votes
0
answers
70
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Why do we experience more centripetal force when we make a sharp turn compared to a wide turn? [duplicate]
If $F=m\omega^2r$, why do we experience more force when we decrease the radius of the turn?
- 502
1
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1
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76
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Determining acceleration in order to meet a space-time-velocity reservation
An Example
A train on a single line track is told to be at position $x_{res}$, at time $t_{res}$, and to be moving at exactly $v_{res}$ when it hits that point. Where $_{res}$ denotes the reservation....
- 11
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0
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51
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Why are velocity and kinetic energy not proportional? [duplicate]
In the equation $E_k=\frac12mv^2$ the fact that the kinetic energy of a moving object is not proportional to it's velocity seems counter-intuitive to me.
Put another way, it seems intuitive to me ...
- 101
8
votes
5
answers
794
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What's wrong with this argument that kinetic energy goes as $v$ rather than $v^2$? [duplicate]
Trying to non-technically explain why (kinetic) energy goes as $v^2$ rather then the perhaps-more-intuitive "double the speed, double the energy" $v$, I ended up putting my foot in my mouth (itself a ...
user89220
0
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2
answers
83
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Can a solid object gain speed instantly?
Is it possible for a solid real world object to instantly gain speed without having to accelerate from $v = 0 \, \text{m/s}$ to $v = 1000\,\text{m/s}$, or does it have to accelerate in order to reach ...
- 193
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3
answers
4k
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Does Kinetic Energy assume constant acceleration?
Okay, I've always been someone who doesn't learn anything unless I can prove it. In this case, I set out to prove the Kinetic Energy formula and (wrongly?) found that it was based on constant ...
- 53
2
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0
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736
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How did we arrive that kinetic energy $KE = \frac{1}2 mv^2 $? [duplicate]
There are proofs mathematically how we get kinematics formulae. But i can't understand the proof for kinetic energy equation. I know we can proof $KE$ equation from work energy theorem but what I ...
- 313
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1
answer
2k
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Explanation about friction force directions of two moving bodies in contact
can someone explain me why in this scenario (picture below) we have a friction force Ftr12 in the same direction as movement (left part of the picture, case 1 ) and how these directions of friction ...
- 11
0
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1
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97
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A question about speed [closed]
A object with mass 0.5 kg slides in straight line for 200 m, where the friction act on it is 0.4 times the normal reaction acting on it. Find its initial speed.
I let $u$ be the speed.
$u^2=2as=2*0....
- 129
-3
votes
3
answers
4k
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Speed of an object in air versus in vacuum
Okay, imagine that you are shooting a rocket down a tunnel that is 60 miles long and the rocket is travelling at 60mph, so the rocket should reach the end of the tunnel in a hour, right? Yep, in an ...
310
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18
answers
85k
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Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$.
Why does this not increase linearly with speed? Why does it take so ...
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