always acts in the same direction as the motion of the oscillating object, with the result that the amplitude of the oscillation increases indefinitely, as it's shown in the adjacent diagram. Away from the value of resonant frequency, either greater or lesser, the amplitude of the corresponding frequency is smaller. In a set of driving pendulums with different length of strings hanging objects, the one pendulum with the same length of string as the driver gets the biggest amplitude of swinging. Examples Parts of a car may vibrate if you drive over a bumpy road at a speed where the vibrations transmitted to the body are at the resonant frequency of that part (though most cars are designed with parts with natural frequencies that are not likely to be produced by driving). Bass frequencies from stereo speakers can make a room resonate, particularly annoying if you live next door and your living room resonates due to your neighbour's music a man walks across a field carrying a long plank on his shoulder. At each step the plank flexes a little (a) and the ends move up and down. He then starts to trot and as a result bounces up and down (b). At one particular speed resonance will occur between the motion of the man and the plank and the ends of the plank then oscillate with large amplitude. When using a microwave oven to cook food, the micro wave travels through the food, causing the water molecules vibrate in the same frequency, which is similar to resonance, so that the food as a whole, gets hot fast. Some of the helicopter crashes are caused by resonance too. The eyeballs of the pilot resonate because of excessive pressure in the upper air, making the pilot unable to see overhead power lines. As a result, the helicopter is out of control. Resonance of two identical tune forks See video: https://www.youtube.com/watch?v=aCocQa2Bcuc Double pendulum Introduction A double pendulum is a simple pendulum hanging under another one; the epitome of the compound pendulum system. It shows abundant dynamic behavior. The motion of a double pendulum seems chaotic. We can hardly see a regulated routine that it is following, making it complicated. Varying lengths and masses of the two arms can make it hard to identify the centers of the two rods. Moreover, a double pendulum may exert motion without the restriction of only a