Why are metals sometimes added to gold?

There are two-speed cameras, one in an are where the speed limit is 30 km/h and another where the speed limit is 50 km/h. in which will there be a smaller time interval between the photographs? explain your answer.

Infrared radiation has wavelengths that are shorter than visible light is part of the visible light spectrum has wavelengths that are longer than visible light has the same wavelengths as ultraviolet radiation

Consider a system with two masses that are moving away from each other. why will the kinetic energy differ if the frame of reference is a stationary observer or one of the masses?