Question

there is a crate of mass 4.67 kg on top of another crate of mass 2.29 kg. The coefficient of friction between the lower crate and the floor is μk = 0.280 and the coefficient of static friction between the two crates is μs = 0.820. If the crates start at rest and a force is applied to the right so that both move a distance of 7.85 m, what is the minimal amount of time required in which this can be accomplished without the top crate sliding on the lower crate?