A researcher asks the same group of kids to define "happiness" when they were 5 years old, 10 years old, and again at 20 years old. They then
analyzed how the kids' idea of "happiness" changed over the years. This is
an example of which type of study?
case study
cross-sectional study
longitudinal study
naturalistic observation
double blind study

What object a collides with object b and bounces back its final momentum is?

Atomic nuclei of almost all elements consist of

In a simplified model of the human eye, the aqueous and vitreous humors and the lens all have a refractive index of 1.40, and all the refraction occurs at the cornea, whose vertex is 2.60 cm from the retina. what should be the radius of curvature of the cornea such that the image of an object 40.0 cm from the cornea’s vertex is focused on the retina?