A double-concave (thickest at the edges, thinnest in the middle) thin lens is made of glass with an index of refraction of 1.6. The radius of curvature of the left and right faces of the lens are 65 cm and 75 cm, respectively. The object lies to the left of the lens. What is the focal length of the lens

Idon’t quite understand this problem

Amass attached to a spring is displaced from its equilibrium position by 5cm and released. the system then oscillates in simple harmonic motion with a period of 1s. if that same mass–spring system is displaced from equilibrium by 10cm instead, what will its period be in this case? a mass attached to a spring is displaced from its equilibrium position by and released. the system then oscillates in simple harmonic motion with a period of . if that same mass–spring system is displaced from equilibrium by instead, what will its period be in this case? a) 0.5sb) 2sc) 1sd) 1.4s

Debris produced by mining is called