 #### Distance measurement using HY-SR04 Piezo sensor

 HY-SR04 is a widely available ultrasound ranging module. It consists of two 40 kHz Piezo crystals, one acts as a transmitter and other as a receiver, and some electronics circuitry to measure the time interval between a burst of sound transmitted and the echo received. The four pins are the power supply, ground, trigger and Echo. This module can be connected to ExpEYES-17  as shown in the schematic, to measure distance between the module and the reflecting surface, with an error less than 3mm. This can be used to design several experiments in mechanics.  #### The Mass and Spring problem

When a mass suspended on a spring is made to oscillate, the period of oscillation is given by  T = 2 * pi * sqrt( m/k), where m is the mass and k is the spring constant. We can measure the period of oscillation by measuring the distance to the moving mass as a function of time. The distance versus time graph is fitted with a sine function to calculate the frequency. The photograph below shows a metal plate suspended on a spring and the SR04 facing it from the bottom. The distance to the oscillating plate is measured for 5 seconds and the data is fitted to a sine function. A video of the experiment is HERE.

#### Simple Pendulum

The period of oscillations of a pendulum also can be measured using SR04 connected to ExpEYES. The figure shows a pendulum with a rectangular bob, but one can use a spherical bob with a piece of paper (to provide a flat reflecting surface) pasted to it. The measured frequency is 1.05 Hz for a pendulum having a length of 22cm. One can calculate the value of acceleration due to gravity from this data. A video is HERE.

#### Free Fall, Displacement as a function of Time

It is possible to measure the distance to a body falling under gravity. The value of 'g' can be calculated by fitting the data with the polynomial
S(t)= a * t^2 + b * t + c
'g' is given by 2*a