Mass as a measure of inertia, and inertia resists a change in velocity, i.e. resists acceleration. More mass means more resistance and smaller acceleration: a goes as 1/m.
ETYMOLOGY: the word mass comes from Greek maza, a barley cake made by kneading into a lump or mass.
The force is what causes the acceleration. Twice the force gives twice the acceleration: a goes as F.
Newton’s Second Law: 
, where the FNET is the sum of all the forces acting.
free-body diagrams vs. vector diagrams
ACTIVITY: have students try to "pick themselves up by their bootstraps."
DEMO: standing still, crouching down, and standing up while on the force plate, graphing force in real time.
ETYMOLOGY: the word tension comes from Latin tendere, 'to stretch'.
terminal velocity
elevators