Finding acceleration and applied force with two blocks, 28 N and 14 N contact forces

[B-80]

Fig. 5-1a shows two blocks on a smooth surface. A constant horizontal force Fa is applied to block A, which pushes against block B with a 28.0 N force horizontally to the right. In Fig. 5-1b, the same force Fa is applied to block B so that block A pushes on block B with a 14.0 N force horizontally to the left. The blocks have a total mass of 30.0 kg.

Fig. 5-1

(a) What is the magnitude of the acceleration of the blocks in Fig. 5-1a?

what i tried was F=ma so 28=30a so 28/30=a

(b) What is the magnitude of the force Fa?

 

[Doc Al]

(a) What is the magnitude of the acceleration of the blocks in Fig. 5-1a?

what i tried was F=ma so 28=30a so 28/30=a

28.0 N is the force on block B (in case a), not on both blocks. You have to use all the information given to solve this. Hint: The same force pushes the same total mass in each case.