Two crates, a and b, sit at rest side by side on a frictionless horizontal surface. the crates have masses ma and mb. a horizontal force f is applied to crate a and the two crates move off to the right. draw the free-body diagram for crates a and b.

i have tried < --|> ; < --|--> ; < |--> to no avail, they are all incorrect. normal force up is equal to weight (gravity) force down so sum of vertical forces f = 0n. i know i am doing something wrong but cannot figure out what and the system does not point me in the write direction with each incorrect response.