in this video, it shows a guy falling down the stairs. this is the concept of gravity. Gravity is an invisible force holing us to the ground. then he missed a step, gravity pulled him down causing him to fall. he is being pulled down at a speed of 9.8m/s because that is the speed of what all things fall at.

this video demonstrates the concept of uniform motion. where things move at the same speed, and never speed up or slow down. they just keep going like this forever. a place that has things moving at uniform motion is in space.

in this video