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# Mechanics Warmups

Physics is not a discipline, but a way of looking at the world. See if you can use your sense of the world to explain everyday phenomena.

Four beetles, call them John, Paul, George, and Ringo, start off at the corners of a square room 2 meters on a side. Looking down into the room from above, John is in the upper left, Paul in the upper right, George in the lower right, and Ringo in the lower left. The beetles start crawling towards the next beetle, i.e. John crawls directly at Paul, Paul crawls directly at George, George crawls directly at Ringo, and Ringo crawls directly at John. Each beetle's speed is \(0.01~\mbox{m/s}\).

How far has one beetle traveled **in meters** when all the beetles meet in the middle?

Hint: The first **and** second laws of thermodynamics are your friend.

**Details and Assumptions:**

- The pulleys are smooth and massless.
- \(\displaystyle k_1 = 100 N/m\)
- \(\displaystyle k_2 = 200 N/m\)
- \(\displaystyle M = 4 kg\)
- \(\displaystyle g = 9.8 m/s^2\)

Suppose the system is placed into one of the steady states \(r_i\) and is perturbed *very* slightly away to \(r_i+\Delta r\). How does the perturbation change over time?

Toy helicopters with rechargeable batteries fly for a few minutes on a single charge. Manufacturers want to choose the right size of battery to achieve the longest flight time between charges. A larger battery stores more energy, but also increases the mass of the helicopter so it takes more energy to keep it in the air. Our question is, for a helicopter and battery type that behave as specified in the assumptions below, what is the linear size \(l\) of the battery **in mm** that will maximize the time the helicopter can hover in place?

**Details and assumptions**

- The acceleration of gravity is \(-9.8~\mbox{m/s}^2\).
- The base mass \(m_0\) of the helicopter without the battery is \(50~\mbox{g}\).
- The battery is a uniform cube with side length \(l\) and density \(\rho= 4000 ~\mbox{kg/m}^3\).
- The total energy contained in the battery does not quite scale as the volume does, but instead scales as \(l^{2.7}\).
- The emitted voltage from the battery is constant and does not change with \(l\). All that changes is how long the battery lasts. Therefore the rotors spin and push air in the same way no matter what the battery size is.
- Assume the mass of the air pushed by the rotors is a constant value, independent of rotor speed. In an actual helicopter the mass increases with speed, effectively from Bernoulli's equation, but that's more complication than we want to go into.

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