![]() In physical situations in which the forces acting on an object are not in the same, opposite or perpendicular directions, it is customary to resolve one or more of the forces into components. The diagram below depicts the direction of these two forces at five different positions over the course of the pendulum's path. And as the bob swings to the right of its equilibrium position, the tension is directed upwards and to the left. So as the bob swings to the left of its equilibrium position, the tension force is at an angle - directed upwards and to the right. The direction of the tension force is always towards the pivot point. Both its direction and its magnitude change as the bob swings to and fro. The tension force is considerably less predictable. The gravity force is highly predictable it is always in the same direction (down) and always of the same magnitude - mass*9.8 N/kg. The air resistance force is relatively weak compared to the two dominant forces. In our discussion, we will ignore the influence of air resistance - a third force that always opposes the motion of the bob as it swings to and fro. The tension force results from the string pulling upon the bob of the pendulum. And there is a tension force acting upward and towards the pivot point of the pendulum. It results from the Earth's mass attracting the mass of the bob. There is the force of gravity that acts downward upon the bob. So what forces act upon a pendulum bob? And what is the restoring force for a pendulum? There are two dominant forces acting upon a pendulum bob at all times during the course of its motion. It is this restoring force that is responsible for the vibration. The restoring force causes the vibrating object to slow down as it moves away from the equilibrium position and to speed up as it approaches the equilibrium position. Such quantities will include forces, position, velocity and energy - both kinetic and potential energy.Įarlier in this lesson we learned that an object that is vibrating is acted upon by a restoring force. Here we will investigate pendulum motion in even greater detail as we focus upon how a variety of quantities change over the course of time. Pendulum motion was discussed again as we looked at the mathematical properties of objects that are in periodic motion. Pendulum motion was introduced earlier in this lesson as we made an attempt to understand the nature of vibrating objects. The motion is regular and repeating, an example of periodic motion. When the bob is displaced from equilibrium and then released, it begins its back and forth vibration about its fixed equilibrium position. The massive object is affectionately referred to as the pendulum bob. It typically hangs vertically in its equilibrium position. ![]() A simple pendulum consists of a relatively massive object hung by a string from a fixed support.
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