| Problem statement Solution video |
DISCUSSION THREAD
Ask and answer questions here. You learn both ways.
DISCUSSION and HINTS
Recall the following four-step plan outline in the lecture book and discussed in lecture:
Step 1: FBDs
Draw single free body diagram (FBD) for the entire system (B + slotted block). Do NOT consider A and B in separate FBDs!
Step 2: Kinetics (Work/energy and linear impulse/momentum)
Consider all of the external forces that you included in your FBD above.
- Which forces, if any, do non-conservative work on that system? If there are no such forces, then energy is conserved. Write down the expressions for kinetic and potential energy (of B+block) for the initial and final states of the motion.
- Now, which external forces have components in the horizontal direction (x-direction_? If none, then linear momentum (of B+block) in that direction is conserved.
Step 3: Kinematics
At position 1 the system is at rest. At Position 2, both the block and B have only x-components of velocity; that is, vB = vBx.
Step 4: Solve
Combine your kinetics equation from Step 2 with your kinematics that you found in Step 3, and solve for the velocities of A and B.
so because the velocity of particle b is only in the x direction and there's no other outside forces the linear momentum would be conserved?
Yes, if you consider a system of B + block, then there are no external forces acting in the horizontal direction, and linear momentum in that direction for the system is conserved.
This is true regardless of the final direction of the velocity of B provided that you include only the horizontal component of the velocity in the expression for momentum.
Since we need to draw the FBD for the entire system, does that mean we should not draw the internal forces acting on B (gravity and normal force by the block)?
Gravity, yes. Normal force, no.