These ______ .

We cannot feel speed. But our senses let us know that we are moving. We see things moving past us and feel that we are, being shaken.
   We can feel acceleration, an increase in speed. But we notice it for only a short time. For example, we feel it during the takeoff of an airplane.
   We feel the plane’s acceleration because our bodies do not get speed as fast as the plane does. It seems that something is pushing us back against the seat. Actually, our bodies are trying to stay in the same place, while the plane is carrying us forward.
   Soon the plane reaches a steady speed. Then, because there is no longer any change in speed, the feeling forward motion stops.
8. What lets us know that we are moving?