What is conserved when two objects collide in a closed system? a)momentum b)acceleration c)speed d)velocity

In which of these cases are the two wave points considered to be in phase with each other?

Aball of mass 0.3kg is released from rest at a height of 8m.how fast is it going when it hits the ground? accleration due to gravity is g=9.8m/s

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?