Cracking the SAT Physics Subject Test

15. The normal force on the object balances the object’s weight, so FN = mg = (20

kg)(10 m/s^2 ) = 200 N. Therefore, Ff (kinetic) = μk FN = (0.3)(200N) = 60 N.

16. The net horizontal force that acts on the crate is F − Ff = 90 N − 60 N = 30 N,

so the acceleration of the crate is a = Fnet/m = (30 N)/(20 kg) = 1.5 m/s^2.