Derivations from Pendulum (mathematics).
To begin, we shall make three assumptions about the simple pendulum:
Consider Figure 1, showing the forces acting on a simple pendulum. Note that the path of the pendulum sweeps out an arc of a circle. The angle θ is measured in radians, and this is crucial for this formula. The blue arrow is the gravitational force acting on the bob, and the violet arrows are that same force resolved into components parallel and perpendicular to the bob's instantaneous motion. The direction of the bob's instantaneous velocity always points along the red axis, which is considered the tangential axis because its direction is always tangent to the circle. Consider Newton's second law,
where F is the sum of forces on the object, m is mass, and a is the instantaneous acceleration. Because we are only concerned with changes in speed, and because the bob is forced to stay in a circular path, we apply Newton's equation to the tangential axis only. The short violet arrow represents the component of the gravitational force in the tangential axis, and trigonometry can be used to determine its magnitude. Thus,
where
This linear acceleration a along the red axis can be related to the change in angle θ by the arc length formulas; s is arc length:
thus:
This is the differential equation which, when solved for θ(t), will yield the motion of the pendulum. It can also be obtained via the conservation of mechanical energy principle: any given object which fell a vertical distance h would have acquired kinetic energy equal to that which it lost to the fall. In other words, gravitational potential energy is converted into kinetic energy. Change in potential energy is given by
change in kinetic energy (body started from rest) is given by
Since no energy is lost, those two must be equal
Using the arc length formula above, this equation can be rewritten in favor of
h is the vertical distance the pendulum fell. Consider Figure 2, which shows the trigonometry of a simple pendulum. If the pendulum starts its swing from some initial angle θ_{0}, then y_{0}, the vertical distance from the screw, is given by
similarly, for y_{1}, we have
then h is the difference of the two
substituting this into the equation for gives
This equation is known as the first integral of motion, it gives the velocity in terms of the location and includes an integration constant related to the initial displacement (θ_{0}). We can differentiate, by applying the chain rule, with respect to time to get the acceleration
which is the same result as obtained through force analysis.
