Chapter 8, Problem 1CQ

The objects shown in Figure 8.31 move together. Identify internal and external forces for each of the following systems: (a) The system consists of only block A. (b) The system consists of only block B. (c) The system consists of both blocks A and B.

Figure 8.31

Question 1.

To determine

To identify the internal and external forces for the system consisting of only block $A$.

Answer to Problem 1CQ

The forces $n_{BA}$, $f_{BA}$, and $w_A$ are the external forces acting on the block $A$ and there is no internal forces on it. The free body diagram of the block $A$ is given in Figure 1.

Explanation of Solution

The arrangement of the blocks are shown in Figure 8.31. There are three different forces acting on the block $A$ externally. The normal force that $B$ exerts on $A$ ($n_{BA}$), the frictional force that $B$ exerts on $A$ ($f_{BA}$), and the gravitational force on the $A$ ($w_A$). There is no forces that acts internally to the block $A$.

The free-body diagram of the block $A$, indicating $n_{BA}$, $f_{BA}$, and $w_A$ is given in Figure 1.

To determine

To identify the internal and external forces for the system consisting of only block $B$.

Answer to Problem 1CQ

The forces $n_{AB}$,$n$, $f_{AB}$, $P$ and $w_B$ are the external forces acting on the block $B$ and there is no internal forces on it. The free body diagram of the block $B$ is given in Figure 2.

Explanation of Solution

The arrangement of the blocks are shown in Figure 8.31. There are five different forces acting on the block $B$ externally. The normal force that $A$ exerts on $B$ ($n_{AB}$), the normal force that the table exerts on $B$ ($n$), the frictional force that $A$ exerts on $B$ ($f_{AB}$), the pulling force ($P$), and the gravitational force on the $B$ ($w_B$). Since the table is frictionless, there is no frictional force that table exerts on $B$. There is no forces that acts internally to the block $B$.

The free-body diagram of the block $A$, indicating $n_{AB}$,$n$, $f_{AB}$, $P$ and $w_B$ is given in Figure 2.

To determine

To identify the internal and external forces for the system consisting of both blocks $A$ and $B$.

Answer to Problem 1CQ

The forces $P$,$n$, $w_A$, and $w_B$ are the external forces acting on the system consisting of both blocks $A$ and $B$ the forces $n_{BA}$, $n_{AB}$, $f_{BA}$, and $f_{AB}$ are the internal forces acting on that system The free body diagram of the system consisting of both blocks $A$ and $B$ is given in Figure 3.

Explanation of Solution

The arrangement of the blocks are shown in Figure 8.31. There are four different forces acting on the system consisting of both blocks $A$ and $B$ externally. the pulling force ($P$), the normal force that the table exerts on $B$ ($n$), the gravitational force on the $A$ ($w_A$), and the gravitational force on the $B$ ($w_B$). Since the table is frictionless, there is no frictional force exerted by the table on the blocks.

The free body diagram of the system consisting of both blocks $A$ and $B$ is given in Figure 3.

There are four different forces acting on the system consisting of both blocks $A$ and $B$ internally. The normal force that $B$ exerts on $A$ ($n_{BA}$), the normal force that $A$ exerts on $B$ ($n_{AB}$), the frictional force that $B$ exerts on $A$ ($f_{BA}$), and the frictional force that $A$ exerts on $B$ ($f_{BA}$).