# 31.7 Worked Example - Two Blocks and a Pulley Using Energy | Summary and Q&A

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June 2, 2017
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31.7 Worked Example - Two Blocks and a Pulley Using Energy

## TL;DR

An analysis of the energy principle and how it applies to rotational kinetic energy in a system consisting of a pulley and two blocks.

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### Q: What is the energy principle and how does it apply to an object falling?

The energy principle states that the change in kinetic energy plus the change in potential energy is zero in the absence of air resistance. When an object is dropped from a height h0, its final velocity can be calculated using the formula square root of 2gh0.

### Q: How can rotational kinetic energy be included in the energy principle?

Rotational kinetic energy can be included in the energy principle by considering the moment of inertia of a rotating object. In the example of a pulley and two blocks, the rotational kinetic energy of the pulley is calculated using 1/2IĻ^2, where I is the moment of inertia and Ļ is the angular speed.

### Q: What is the constraint condition in the system with the pulley and blocks?

The constraint condition in this system is that the rope is fixed in length, so as block 1 moves, the pulley rotates and block 2 moves. This means that the final velocities of block 1 and block 2 are equal.

### Q: How can the final velocity of block 2 be calculated in the given system?

By applying the energy principle and considering both the translational and rotational kinetic energy, the final velocity of block 2 can be found using the equation: Vfinal = square root of (2m2ghfinal) / (1 + m2 + (I/r^2)).

## Summary & Key Takeaways

• The energy principle states that the change in kinetic energy plus the change in potential energy is equal to zero in the absence of air resistance.

• The kinetic energy of an object falling can be calculated using the formula square root of 2gh0, where g is the acceleration due to gravity and h0 is the initial height.

• In a system with a pulley, blocks, and a frictionless surface, the final velocity of one of the blocks can be found by considering the rotational and translational kinetic energy, as well as potential energy.