Finding kinetic energy

[bgiorgi_3A]

Hey guys! So lately I've been understanding the material, but whenever I do a problem using the Kinetic Energy = [.5 (m)(v)^2] I can never get the right answer. For example in 1.B.3- this example is given:

Suppose you are developing a radiation detector to be used on a spacecraft and decide to use a thin layer of metallic potassium to detect certain ranges of electromagnetic radiation. You need to make some estimates of the physical properties involved. The speed of an electron emitted from the surface of a sample of potassium by a photon is 668 km ⋅ s −1. (a) What is the kinetic energy of the ejected electron?

How do I know what mass to use? Seems like a silly question, but I'm really struggling.

[Sabina House 2A]

You use the mass of an electron, 9.109 * 10^-31 kg, which can be found in the back of the book. You know to use this mass because you use the mass of the object you need to find the kinetic energy of. Since in this situation you are finding the kinetic energy of an electron, you use the mass of an electron. Then, you can use the Ke = 1/2 mv^2 equation to solve for the kinetic energy.

[David Chibukhchian 2G]

Hey, one thing to make sure is that you're using the right units. The mass of the electron is usually given and it is written in kilograms, but if the velocity isn't written in meters/second then you will have to perform unit conversion to get it into that form. Besides that, one thing that sometimes messes me up is squaring the velocity after entering it into my calculator. Parentheses have to be put around the entire velocity before squaring it so that it comes up with the right value. I hope that helps or gives you an idea of how to go about solving!!

[Kayla Booker 1F]

Hi, for the mass you use the mass of an electron which is 9.109 x 10^-31. Your issue might be the speed, since it is in km/s you want to make sure that you convert it to m/s.