A pendulum, shown in the photograph below, is lifted to the 30 cm level toward the left of the picture and released.  When it falls down to its minimum level, just below the suspension point, it encounters a peg at the 35 cm level.  What will then happen?  In particular, how high will the pendulum rise as it moves to the right?  When the string hits the peg, how will that encounter effect the angular momentum?  Will it be sufficiently increased such that it will rotate over the pin?

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The pendulum bob will:

  • (a) rise to a height less than 30.
  • (b) rise to a height of 30.
  • (c) rise to a height of greater than 30.
  • (d) loop around the lower peg.

To see the answer, click Read More on 1/23/15


The answer is (b): the pendulum will rise exactly to a height of 30, assuming that no loss in energy occurs, as seen in an mpeg video by clicking your mouse on the photograph below.

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This can be seen from conservation of energy: the initial gravitational potential energy with the pendulum bob held at the 30 level to the left of the photograph is converted into kinetic energy at the bottom of its path, then back to potential energy.With no loss it should again rise to 30.