Answer to Question #144926 in Mechanics | Relativity for Mikkos Joshua Garcia

Question

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 is applied to crate A and the two crates move off to the right. (a) Draw clearly labelled free-body diagrams for crate A and for crate B. Indicate which pairs of forces, if any, are third-law action-reaction pairs. (b) If the magnitude of force is less than the total weight of the two crates, will it cause the crates to move? Explain.

Answer 1

Part (a)

Here, N₁ on both crates are equal and opposite which is due to action and reaction of two crates.

similarly, R_A and R_B both are due to reaction of ground against the respective weight of crates.

Hence:

• N₁ on A and N₁ on B are third-law action-reaction pair
• (R_A,M_Ag) and (R_B,M_Bg) are third-law action-reaction pairs.

Part(b)

Free body diagram of composite system tells there is only one force(F) in horizontal direction and net force in vertical direction is zero: (R_A+R_B)-(M_A+M_B)=0

Hence object will move in horizontal direction irrespective of weight provided surface is frictionless.