How is the visible light spectrum used in the doppler effect to give us the approximate age of the universe

Atrain departs from its station at a constant acceleration of 5 m/s. what is the speed of the train at the end of 20s?

Mutt and jeff are running along a path each pushing a cart loaded with rocks. if mutt's wagon has a mass two times greater than jeff's and they want to stay together, arriving at their destination at the same time, what must happen? a) jeff should slow down so mutt can catch up. b) mutt must use half as much force to push his cart. c) mutt must use twice as much force to push his cart. d) mutt and jeff will use the same force, but mutt will take bigger steps.

How is zig zagging a form of acceleration?