Pendulums - definition and types.
Definition of pendulum
A pendulum is a mass that is tied to a rope that hangs from a fixed point, where the rope must have a fixed longitude, this means that the rope should not be flexible.
The motion of a pendulum can be seen as an oscillatory movement or a periodic motion, this means that the movement of a pendulum will repeat over and over again, in the case of the pendulum, the motion is moving from side to side.
Factors that affects the motion of a pendulum
The main characteristics of the motion of a pendulum will depend on different factors, for example, how high the pendulum will reach will depend on the initial angle where the pendulum is dropped, therefore the height where the pendulum is dropped (or the initial angle), will always be the max height and angle of the pendulum in perfect conditions (without taking into account the air) is physically impossible to a pendulum to reach a higher height than its initial height.
There are many videos in the internet where we can see that somebody hangs a bowling ball from the ceiling, and drop the ball from his face, and when the ball is coming again towards his face, it does not impacts him, this happens because of what we explained before.
Another factor that affects the motion of a pendulum is the length of the rope, the shorter this is, the longer the period of the pendulum will be, reminding that the period is the time that an object takes to repeat a motion.
In a pendulum we can see what a restorative force is, a restorative force happens when a body separates from his equilibrium point (in case of the pendulum it is when it reaches his highest height), then the restorative force makes the body come back to his equilibrium point (that is when the pendulum is still).
A pendulum could be considered a simple harmonic motion in some cases, the higher the initial angle of the pendulum is, the closer the coincidence of the motion with simple harmonic motion will be, and vice versa.
Types of pendulums
Now we are going to present the most known kind of pendulums, with their characteristics and the physics behind them.
Simple pendulum
There are many types of pendulums, each with different and unique characteristics and curiosities, the one we describe above was the simple pendulum, that is the most basic one, and is a mass fixed to a rope.
The simple pendulum has a semi circular trajectory and it goes from left to right over and over again.
Double pendulums
A double pendulum is composed by 2 simple pendulums, the first one is hanging from a fixed point, and the other one is hanging from the mass of the first pendulum.
The trajectory of the first pendulum is limited to a circle which radio is equals to the longitude of the rope of the first pendulum, but the most impressive thing is the trajectory of the second mass, this has a chaotic trajectory, this means that has a no apparent order.
The double pendulum has this trajectory because both masses affect the motion of each other, this means that the first mass affects the motion of the second, and the second mass affects the motion of the first. The motion of a double will depend on multiple conditions, in the following animation both ropes have the same longitude but the first mass is greater than the second, the whole motion and chaos will be different when the masses and the length of the ropes change.
(In this animation we can see that the trajectory of a double pendulum seems to not have any apparent order o patron.)
Click the button to restart in case the double pendulum has already lost much of its energy to watch the animation from the beginning
Newton´s cradle
The newton´s cradle is maybe the most known pendulum even out of the physics, because of its eye-catching motion this is also used as a toy or decoration.
This is composed by 5 simple pendulums aligned horizontally, the behavior of this pendulum is the following: when one of this pendulums is lifted and dropped the velocity it impacts the next pendulum will be transmitted through every central pendulum till the last pendulum, and it will continue with the same velocity the first pendulum impacted.
In this object we can see the law of conservation of energy, this is applied in system isolated from external factors, this means that the energy that a newton´s cradle produces is not going anywhere outside the pendulum, it is just moving from one extreme of the cradle to the other repetitively, and is not going to stop unless an external factor makes it stop.
Click the following buttons to change the number of pendulums lifted at the beginning.