How do grandfather clocks keep going? The pendulum is what makes the clock go. However, the pendulum will slow down due to friction. What energy source keeps the pendulum from eventually stopping?
Answer
The premise of your question is incorrect: in fact, the pendulum is what keeps the clock from running! And the clock keeps the pendulum running!
A clock is essentially a motor: a device that uses energy from some source to drive the hands of the clock around and around. The source of the energy varies; it could be a tightly wound spring, or a weight dropping down after being raised to some height. The energy is dissipated in the friction in the various gears that are used to reduce the speed of the motor for the different hands. The speed of this motor would depend only on the friction in the various gears...
The role of the pendulum is critical. In part of its motion back and forth, it stops the gear train from moving. As the pendulum moves further in its swing, it releases a tooth of the gear, which rotates a little until another part of the pendulum catches another tooth. So each swing of the pendulum allows the clock "motor" to rotate only a fixed number of teeth (usually one tooth exactly)
Here's a simple example of an escapement: 
The next trick is to have the teeth of the gear give a little push to the pendulum as each tooth is released. This compensates for the friction in the pendulum which would otherwise stop the pendulum in a few hours. So the energy source in the clock is keeping the pendulum swinging, as the pendulum regulates the rotation of the gear...
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