Ameasure of how consistently a result is determined by repeated evaluations is called

Debbie places two shopping carts in a cart corral she pushes the first cart which then pushes a second cart what force is being exerted

Can anybody me with this? ? (picture included.)

Automobile fuel efficiency is often measured in miles that the car can be driven per gallon of fuel (mpg). heavier cars tend to have lower fuel efficiencies (lower mpg ratings). which variable (weight or fuel efficiency) is most naturally considered the explanatory (predictor) variable in this context?