Mechanical resonance is the tendency of a mechanical system to respond at a greater amplitude when the frequency of its oscillations matches the system's natural frequency of vibration (its resonance frequency or resonant frequency) than it does at other frequencies.

The time interval between application of the force is equal to the period of oscillation.

Think of a swing set.  To go help someone go higher on the swing (increase the amplitude,) you need to apply a force at a certain time, when the swing gets to the highest position closest to you.  If you added a force at any other time, you would not be increasing the amplitude, you would be working against the motion of the swing.

Resonance is adding a force at a certain time interval to increase the amplitude of a wave.

The Tacoma Narrows Bridge is a pair of twin suspension bridges that span the Tacoma Narrows strait of Puget Sound in Pierce County, Washington.  The Tacoma Narrows Bridge opened in July 1940, but collapsed because of aeroelastic flutter four months later.  The bridge was later rebuilt.

Most people think that the bridge collapsed because of resonance.  They thought the wind was hitting the bridge at the same frequency as the natural frequency of the bridge.  When talking about resonance, most people bring up the collapse of the Tacoma Narrows Bridge.

Click on the video to see what happened to the bridge on November 7, 1940.

It wasn't resonance that collapsed the Tacoma Narrows Bridge, it was aeroelastic flutter.  Click on the video to see an explanation on why the bridge collapsed.

The Mackinac Bridge is a suspension bridge just like the Tacoma Narrows bridge.  Don't worry!  The 5 mile suspension bridge that connects the upper peninsula with the lower peninsula is safe.  What do you notice about the Mackinac Bridge that the Tacoma Narrows Bridge did not have?

We talked about reflection in lesson 2.  Reflection is when the wave bounces back into the original medium.  We will be revisiting reflection in chapter 17 when we talk about reflection and mirrors.  

When talking about reflection we need to talk about a normal line.  Remember when we talked about forces in chapter 4, a normal force was perpendicular to the surface.  Normal means perpendicular to.  A normal line is a line that is drawn perpendicular to the surface where the incident wave hits.

The law of reflection states that the angle of incidence is equal to the angle of reflection.  The angle of incidence is the angle measured from the normal line to the incident wave.  The angle of reflection is the angle measured from the normal line to the reflected wave.  Both angles will ALWAYS be equal.

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