## How to find kinetic energy from velocity

Natalie Phan 3G
Posts: 50
Joined: Wed Sep 30, 2020 9:48 pm

### How to find kinetic energy from velocity

Can someone please explain to me what the steps are to find the kinetic energy of the ejected electron given the velocity of the ejected electron and the work function? I was confused to what formulas I should use/rearrange/plug in.

Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 10^5 m.s^-1. The work function for sodium is 150.6 kJ.mol^-1. What is the kinetic energy of the ejected electron?

Thank you!

Chem_Mod
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### Re: How to find kinetic energy from velocity

The formula for kinetic energy is 1/2mv^2 where m is the mass of the electron in kilograms, and v is velocity in m/s. The question gives you the velocity of the ejected electron, and the mass of the electron will be given on the formula sheet. Therefore, you can plug in those values to find the kinetic energy in Joules (J).

Sera Aintablian 2E
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### Re: How to find kinetic energy from velocity

Kinetic Energy =
(mass of Na = 22.990 amu)
so,...
KE = (1/2)(22.990)((6.61x10^5)^2) = 5.0224 x 10^12 J (if my calculations are correct!)

With this value and the given work function, you can also solve for the Energy(photon) by manipulating the equation,
E(photon) = work function/threshold energy + kinetic energy !

jessicasilverstein1F
Posts: 95
Joined: Wed Sep 30, 2020 9:57 pm

### Re: How to find kinetic energy from velocity

the kinetic energy formula is (1/2)(m)(v^2) where m is mass and v is velocity

Justin Zhang_1A
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Joined: Wed Sep 30, 2020 9:48 pm

### Re: How to find kinetic energy from velocity

The formula to calculate kinetic energy with velocity is 1/2 * m * v^2.

Chance Herbert 3A
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Joined: Wed Sep 30, 2020 9:55 pm

### Re: How to find kinetic energy from velocity

The formula for Kinetic Energy is 1/2(m)(v^2). In this problem, the velocity is provided as 6.61x10^5 m/s. Here you would not use molar mass as you are considering the velocity for a sole electron and this would not provide you with the correct units for Kinetic Energy (keep in mind Joules are kgm^2/s^2). You would use the mass of an electron which is 9.109x10^-31 Kg, which would give you the kinetic energy! 1/2(9.109x10^-31Kg)((6.61x10^5m/s)^2)=1.99x10^-19 Kgm^2/s^2 or 1.99x10^-19 J.

905290504
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### Re: How to find kinetic energy from velocity

to find kinetic energy from velocity use the Ek = 1/2mv^2 equation!