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Potential Energy

Potential energy lets us do work in the present to change things in the future. If energy is currency, then potential energy is money in the bank.

Level 2-3

         

Lukla airport in Nepal is one of the strangest in the world. Built to support tourism to the Himalayas, the airport has a single landing runway. What is more, the runway is only 20 m wide, 450 m long, and a 2,800 m cliff at the runway's end, leaving little room for error. In fact, the airport can only be used by so-called Short Takeoff and Landing planes (STOL). Helping somewhat is a 12% incline in the runway from start to finish, so that planes rise through over the course of their deceleration.

Suppose a STOL plane's landing speed is 45 m/s (\(\approx\) 100 mph). Neglecting any other effects like wind flaps, or drag, how small will the plane's velocity (in m/s) be at the top of the runway?

Details

  • The runway itself is 450 m long, i.e. if you walked from the bottom to the top, you'd walk 450 m along the runway.
  • An \(f\)% incline means that if you walk a distance \(d\) along an incline, your rise is given by \(fd/100\).

Climbing out of bed is sometimes hard in the morning. If you have a mass of \(60~\mbox{kg}\), what is the minimum amount of work in joules you have to do to get out of bed? Assume your center of mass when lying in your bed is \(0.75~\mbox{m}\) above the floor, and your center of mass when standing is \(1.25~\mbox{m}\) above the floor.

Details and assumptions

  • The acceleration due to gravity is \(-9.8~\mbox{m/s}^2\).

Consider a system of \(N\) particles whose coordinates are \(\{r_i\}\), and whose velocities are \(\{\dot{r}_i\}\). Each pair of particles \(\left(i, j\right)\) interacts through a potential \(V_{ij}\left(r_i, r_j\right)\) which has no direct dependence on time. As the system evolves in time, which of the following bulk quantities must be conserved?

A particle is constricted to move along the positive \(x\)-axis under the influence of a potential energy:

\[U(x) = \frac{3}{x} + 7x\]

Find the point of equilibrium for the particle. (Round your answer to three decimal places.)

Consider a system of \(N\) particles whose coordinates are \(\{r_i\}\), and whose velocities are \(\{\dot{r}_i\}\). Each pair of particles \(\left(i, j\right)\) interacts through a potential \(V_{ij}\left(r_i, r_j\right)\) which depends on the coordinates \(\left(r_i, r_j\right)\) only through their difference, i.e. \[V_{ij}\left(r_i, r_j, t\right) = V_{ij}\left(r_i-r_j, t\right)\] As the system evolves in time, which of the following bulk quantities must be conserved?

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