84. Boxes A and B in Figure P10.84 have masses of 12.0 kg and 4.0 kg, respectively. The two boxes are released from rest. Use conservation of energy to find the boxes' speed when box B has fallen a distance of 0.50 m. Assume a frictionless upper surface. A B

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### Problem Statement

**84.** Boxes A and B in Figure P10.84 have masses of 12.0 kg and 4.0 kg, respectively. The two boxes are released from rest. Use conservation of energy to find the boxes' speed when box B has fallen a distance of 0.50 m. Assume a frictionless upper surface.

### Diagram Description

**Figure P10.84** shows a horizontal surface on which Box A (mass of 12.0 kg) rests. A pulley system connects Box A to Box B (mass of 4.0 kg) with a rope. Box B hangs vertically off the edge of the surface. When released, Box B falls a distance of 0.50 meters vertically downward, while Box A moves horizontally. The pulley and the surface beneath Box A are assumed to be frictionless.

### Solution and Explanation

The setup involves the use of conservation of mechanical energy to determine the speed of the boxes. Since there is no friction, we only need to consider the conversion of potential energy to kinetic energy.

1. **Initial Energy:**
   - Potential Energy (U) of Box B before it falls:
     \[
     U_{\text{initial}} = m_B \cdot g \cdot h
     \]
     where:
     \[
     m_B = 4.0 \, \text{kg}, \quad g = 9.8 \, \text{m/s}^2, \quad h = 0.50 \, \text{m}
     \]
     Substituting the values:
     \[
     U_{\text{initial}} = 4.0 \, \text{kg} \cdot 9.8 \, \text{m/s}^2 \cdot 0.50 \, \text{m} = 19.6 \, \text{J}
     \]

2. **Final Energy:**
   - When Box B has fallen 0.50 m, it loses its gravitational potential energy, which gets converted into the kinetic energy of both boxes.
   - The kinetic energy (K) of Box A and Box B:
     \[
     K_{\text{final}} = \frac{1}{2} m_A v^2 + \frac{1}{2} m_B v^2
     \]
     Simplifying:
     \[
     K_{\
Transcribed Image Text:### Problem Statement **84.** Boxes A and B in Figure P10.84 have masses of 12.0 kg and 4.0 kg, respectively. The two boxes are released from rest. Use conservation of energy to find the boxes' speed when box B has fallen a distance of 0.50 m. Assume a frictionless upper surface. ### Diagram Description **Figure P10.84** shows a horizontal surface on which Box A (mass of 12.0 kg) rests. A pulley system connects Box A to Box B (mass of 4.0 kg) with a rope. Box B hangs vertically off the edge of the surface. When released, Box B falls a distance of 0.50 meters vertically downward, while Box A moves horizontally. The pulley and the surface beneath Box A are assumed to be frictionless. ### Solution and Explanation The setup involves the use of conservation of mechanical energy to determine the speed of the boxes. Since there is no friction, we only need to consider the conversion of potential energy to kinetic energy. 1. **Initial Energy:** - Potential Energy (U) of Box B before it falls: \[ U_{\text{initial}} = m_B \cdot g \cdot h \] where: \[ m_B = 4.0 \, \text{kg}, \quad g = 9.8 \, \text{m/s}^2, \quad h = 0.50 \, \text{m} \] Substituting the values: \[ U_{\text{initial}} = 4.0 \, \text{kg} \cdot 9.8 \, \text{m/s}^2 \cdot 0.50 \, \text{m} = 19.6 \, \text{J} \] 2. **Final Energy:** - When Box B has fallen 0.50 m, it loses its gravitational potential energy, which gets converted into the kinetic energy of both boxes. - The kinetic energy (K) of Box A and Box B: \[ K_{\text{final}} = \frac{1}{2} m_A v^2 + \frac{1}{2} m_B v^2 \] Simplifying: \[ K_{\
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