All Questions
Tagged with kinematics speed
23 questions
310
votes
18
answers
85k
views
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 ...
- 3,235
3
votes
6
answers
3k
views
Why is speed defined like it is?
I have a rather basic, maybe even dumb, question. I was wondering why speed is defined as it is:
$s = d/t$
Of course, what the equation means is not too difficult to understand. However, there are ...
- 964
1
vote
3
answers
595
views
Is the derivative of the magnitude of a position vector the speed? Is the integral of the speed the magnitude of a position vector?
I'm reading a book titled "Introduction to Mechanics" by Kleppner, and I came across an example: A bead moves along the spoke of a wheel at constant speed $u$ meters per second. The wheel ...
- 113
2
votes
5
answers
865
views
Is there an agreed upon physics definition of the term 'speed'? For example, can it be negative?
The term speed is commonly defined as follows:
https://www.physicsclassroom.com/class/1DKin/Lesson-1/Scalars-and-Vectors
Speed, being a scalar quantity, is the rate at which an object covers ...
- 3,116
9
votes
6
answers
250k
views
Why is this calculation of average speed wrong? [closed]
A person drove 120 miles at 40 mph, then drove back the same 120 miles at 60 mph. What was their average speed?
The average of the speeds is
$$\frac{40\ \text{mph} +60\ \text{mph}}{2} = 50\ \text{...
- 349
-1
votes
5
answers
4k
views
2 balls falling hit the ground at the same time
There are 2 balls in a vacuum, next to each other but not touch. They are on the edge of a surface they will both leave the table at exactly the same time. One gets pushed harder than the other. The ...
- 6,987
5
votes
8
answers
7k
views
Why isn't average speed defined as the magnitude of average velocity?
Speed is usually defined as the magnitude of (instantaneous) velocity. So one could assume that average speed would be defined as the magnitude of average velocity. But instead it is defined as
$$s_{...
- 223
3
votes
2
answers
43k
views
Difference between Speed and Velocity
What is the difference between Speed, Velocity and Acceleration? Could any one describe it pictorially?. I am more over confused even after investigating many times. I am unable to relate myself ...
- 133
2
votes
2
answers
15k
views
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 ...
- 21
2
votes
3
answers
521
views
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 ...
- 317
0
votes
4
answers
672
views
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 ...
- 1,106
6
votes
3
answers
474
views
If an electric car were to drive without having to stop, would the range be greatly affected by the speed at which the vehicle is moving?
Of course aerodynamics factors into this question, and the faster you are moving the more air you have to push out of your way, the more energy you use. But would the difference be only a small ...
- 189
4
votes
2
answers
13k
views
Ascent rate and size of balloon
I am part of a school project, Project Stratos to send a balloon to the edge of space (the closer side :P) and was wondering how you would work out the accent rate of a large balloon (roughly 1m^3 of ...
- 473
3
votes
4
answers
9k
views
What is the difference between uniform velocity and constant velocity? [closed]
I think that uniform velocity implies constant speed but not constant direction. while constant velocity implies constant speed without any changes in direction.
Both tell us that there's no ...
- 41
2
votes
5
answers
4k
views
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?
2
votes
0
answers
736
views
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
2
votes
1
answer
267
views
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 ...
- 131
2
votes
1
answer
189
views
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
votes
3
answers
2k
views
Velocity of a ball rebounding off a moving train
At the bottom of this page says (my emphasis):
It's interesting to note the speeds in the cartoon. The
propeller-beanie kid sees his tennis ball moving away from him at 30
miles per hour. So ...
- 25
1
vote
6
answers
3k
views
Why do we care about the difference between speed and velocity?
I teach 7th grade students about the difference between speed and velocity. One of them ask me why do physicists create the concept of velocity. I cannot answer. I don't know precisely why do we care ...
- 2,747
1
vote
1
answer
7k
views
Distinguish between instantaneous speed and instantaneous velocity
I encountered a line in my text book of physics that:
Average speed over a finite interval of time is greater or equal to the magnitude of the average velocity. But instantaneous speed at an ...
- 285
-4
votes
2
answers
383
views
Another view on mass energy equivalence? $ E = m_0 c^2 [1 - \sqrt{1 - (v/c)^2}]$? [closed]
(this is old thread, but i have edited this question to be more brief)
In the paper, http://arxiv.org/pdf/physics/0206061.pdf "Fundamental Disagreement of Wave Mechanics with Relativity", ...
- 385
-5
votes
1
answer
2k
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Is the term "deceleration" ambiguous? Does it refer to negative acceleration, or is it something else entirely? [closed]
What does that term truly mean? Sometimes it confuses me what exactly it refers to.
- 107