# What is Kinetic: Definition and 1000 Discussions

In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest.
In classical mechanics, the kinetic energy of a non-rotating object of mass m traveling at a speed v is

1
2

m

v

2

{\displaystyle {\begin{smallmatrix}{\frac {1}{2}}mv^{2}\end{smallmatrix}}}
. In relativistic mechanics, this is a good approximation only when v is much less than the speed of light.
The standard unit of kinetic energy is the joule, while the English unit of kinetic energy is the foot-pound.

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1. ### Internal energy of a gas and kinetic energy, "typical velocity"

Source: Shankar Yale OCW physics I have three questions here: 1. K_avg is 3/2kT, sure. But isn't this the kinetic energy of one particle only? So why isn't the answer multiplied by avogadro's number (because one mole). 2. When doing the "typical velocity" derivation, I noticed that they used...
2. ### B The relationship between mc^2 and mc^2 x 1/2

Kinetic energy = 1/2 m V^2 I was thinking about this and thought another formula… E = mc^2 These look very similar except for the multiplication by 1/2. Let’s say you take a kilogram ball of uranium and accelerate it to the speed of light. I know, I know. You can’t. But let’s say you did...
3. ### Classic Incline problem with cylinder

FOr this, Use alternate coordinate system With ##ȳ##-axis parallel to incline and ##x̄##-axis parallel to the x-axis. Kinetic energy using this alternate coordinate system is ##T = \frac{1}{2}M\dot x_p^2 + \frac{1}{2}mR^2\dot \phi^2 + \frac{1}{2}m(\dot x̄^2 + \dot ȳ^2 + 2\dot x̄ \dot ȳ...
4. ### The value of the spring constant k that I calculate seems too high

I expanded ET1=ET2 to get (Total energy at top) 1/2mv^2+mgh = 1/2kx^2 (Total energy at bottom) Rearanged i got k = (mv^2+2mgh)/x^2 so [(73)(20)^2+2(73)(9.8)(52)]/0.465^2 =479137.945N/m
5. ### Rolling Sphere On Incline

I did some algebra and got the final equation mgh=1/2MR^2omega^2+1/2(2/5MR^2)omega^2, then plugged in the numbers and got an omega value of 111.09. Using 1.7 as h instead of cos(17)*1.7=1.63 I got 114.09. Both answers were wrong. Am I missing something, or did I just screw up the math?
6. ### Change in the kinetic energy of a car driving up a hill

part i) i did 1/2 * 1700 * v^2 i dont know what v is... so how do i solve it? part ii) i calculated it correctly by 440*25 please explain in detail why i used 440? and part d) i did 1.7*10^4 = 48000/t my t= 2.82 s but correct answer is 3.5s

8. ### B I have a question regarding temperature and the kinetic energy of molecules

Does the high temperature increase the kinetic energy of molecules or atoms, or does the high kinetic energy of atoms or molecules increase the temperature ? I'm so curious about this. Which concept is more accurate between the two
9. ### Equations of motion of an electron emitted from a surface

Homework Statement: Real world application of freshman physics Relevant Equations: TBD This is not a homework question, this is relevant to my work. It seems simple enough (introductory) but I keep running into problems. An electron is emitted from an surface (material is irrelevant, could...
10. ### Frames of Reference: Using tie-downs to hold a load on a flatbed truck

I calculated the net force. I got 6500. I determined this is bigger than the force of static friction so the force of friction acting on the object must be kinetic. From there I got kinda lost. I know for the component to not slip Fnetx = 0 and Fnety = 0. But I'm not sure what to do from there...
11. ### I I'm calculating more energy out than I put in

Hello everyone, I'm currently working on a physics problem involving the rotation of a 5 kilogram ##M=5## solid sphere subjected to a force of 5 newtons ##F=5##, and I've encountered an inconsistency in my calculations. I'm seeking guidance or insights into where I might have gone wrong. My...
12. ### Find out kinetic Energy using Rutherford Formula

Hello everyone, I am working on this problem and I think I almost solved it, but then I noticed, that I do not know what values I have for dn, n and dθ. Can anyone help me with this?
13. ### Why kinetic energy of system of particles is not equal to p^2/2m

It is good for the people who wants to know physics and it's beauty
14. ### I Spacecraft With Solar Mass Energy Equivalent Kinetic Energy

Suppose in a different star system, a space shuttle sized spacecraft acquired a solar mass energy equivalent amount of kinetic energy, then passed through our solar system. While it was passing through the solar system would the craft’s gravitational effects be more similar to the space shuttle...
15. ### Work done by gravity on a car rolling down a hill

I tried E =Fxcos0 but only ended up with 243kJ
16. ### Gibbs free energy of activation and activation energy

Let's consider a reaction A (reactant) -> B(product) and activated complex is denoted by C. This graph ( potential energy vs reaction coordinate ) tells us that reactant need some amount of activation energy (Ea) to convert in product, which has low potential energy which is shown here in...
17. ### Work energy KE theorem for a book being lifted up in a gravitional field

For this, From the work kinetic energy theorem, if we assume that the book and the earth is the system, and that the finial and inital speed of the system is zero, then is the work KE theorem there is no net work done on the system. However, clearly there is work done on the system is shown by...
18. ### Kinetic Energy of a Cylinder Rolling Without Slipping

Given that there is a cylinder rolling without slipping down an incline, the method I was taught to represent the KE of the cylinder was: ##KE_{total} = KE_{translational} + KE_{rotational}## ##KE_{total} = \frac {1} {2} mv_{cm}^2 + \frac1 2 I \omega^2## Where "cm" is the center of mass, and...

20. ### Kinetic energy transfer from a rotating body in an inelastic collision

The cylinder in question would have a moment of inertia of ~1.67kg*m² and rotational KE of 2.058J. At the point of impact also, assuming the body hits the sphere at a 90deg angle after traversing 90deg of displacement, it should(?) exert a force of 1.31N - enough to give an acceleration of...
21. ### Average Kinetic Energy of Electron in the Conduction Band

Hello, I've seen in a few books on solid state physics that one can deduce an expression for average K.E.: $$<\:K.E.>\:=E_c+3/2\:k_B\:T$$ from the following: $$<\:K.E.>\:=\:\frac{\int \:\left(E-E_c\right)g\left(E\right)f\left(E\right)dE}{\int \:g\left(E\right)f\left(E\right)dE}$$ I can't...
22. ### I The kinetic energy of proton-electron for a black body

I'm watching a video about " What is a black body?". That video said when the light interacts with the surface of a body, the electron and proton start oscillating. The electrons gain more transferred energy from the light that became its kinetic energy, rather than the proton because its mass...
23. ### Deriving the kinetic energy flux in an effusion process

I could not find any derivations in the litterature, except for the expected value of the energy flux expression itself: $$\overline{\Phi_{effusion,\epsilon}} = \overline{\dot{N_{ef}}}\overline{\epsilon_{ef}}=\frac{3Nl}{2A}\sqrt{\frac{(k_BT)^3}{2\pi m}}$$ I've started off by calculating the...
24. ### Kinetic energy transfer from shockwave to secondary body

I would guess that by multiplying the pressure exerted by the shockwave on the body, and then the resulting force - here ~69 Newtons - per the distance the shockwave passed through when traversing body A, I could get the work done but I’m not sure if it’s that easy and whether or not I should...
25. ### Kinetic Energy / Potential Energy / Total Energy question

W_ext is the external work done on B and C, which is 12 J Delta K_tot is the internal work, which is the work done by A on B plus the work done by A on C Delta K_tot = 5 Solving for \Delta U, we find that the change in potential energy is 7 J This answer says otherwise...
26. ### Expectation of Kinetic Energy for Deuteron

Dear Forum, I am solving for the expectation value of the kinetic energy for the deuteron (Krane problem 4.3). I must be missing something since this has become far more complicated than I remember. The problem is as follows: ## <T> = \frac{\hbar^{2}}{2m} \int_{0}^{\infty}...
27. ### B Conflicting definition of degree of freedom in Kinetic Theory of Gases

I am seeing conflicting definitions of degree of freedom in my textbook. If I look at the definition given as per screenshot below then it is the number of independent terms/variables/coordinates used to define the energy of a molecule. But, if I look at the statement of Equipartition of energy...
28. ### Finding the final speed of a space probe using work and kinetic energy

First I found work: W=(3.85x10^5)(2.45x10^8) W= 9.43x10^13 Then used that for difference of kinetic energy: 9.43x10^13 = (1/2) (4.55x10^4)v2^2 - (1/2)(4.55x10^4)(1.22x10^4)^2 9.43x10^13 = (22750)v2^2 - 3.386x10^12 9.43x10^13 + 3.386x10^12 = (22750)v2^2 9.77x10^13 = 22750v2^2 9.77x10^13/22750...
29. ### Net work and kinetic energy (pushing a wagon to accelerate it)

I'm a little confused because my teacher used Bill's 500J of work for the kinetic energy equation and I don't understand why. I used the net work, so 300J, to find the speed and I'm not sure why that's wrong. Wouldn't friction make the wagon move slower than if there was no friction? So why...
30. ### Rotating Rod in Plane: Kinetic Energy & Moment of Inertia

hello guys, I wanted to ask whether I can just consider/think about this as being rotation around a fixed axis in a plane representing it as if it was 'just' a rod. This is mainly so that for the kinetic energy in the second position is where if we think about it in just a plane. Is this...
31. ### Kinetic energy of an alpha particle

Kindly help me solve this question. The only thing so far that I know in this question is that energy is conserved and the momentum of Alpha particle will equal momentum of Thorium.
32. ### I Hubble flow kinetic energy into other types of energy?

Spacetime expands at an accelerated rate and the particles with movement associated to this expansion are coupled to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy associated with it.However, can...