I don't understand when to use the formule with the constant k, and when to use the formula without it. In the exercise that i sent here, they use the formule with a constant but I don't get why.

Can someone help me out please?
Thanks.

Hydrogen and helium are both members of?

If an astronaut takes an object to the moon the following is true (choose only one). a the mass will change but the weight will be the same. b both the mass and the weight of the object will be the same. c the weight will change but the mass will be the same. d both the mass and the weight of the object will change.

Which of these is constant for all types of electromagnetic radiation in a vacuum? select one: a. wavelength b. frequency c. photon energy d. amplitude e. velocity