02 Kinematics

Measuring Difference in Drop Time Using PhyPhox

In a recent class on Kinematics, I prepared a string of 4 pendulum balls, each separated about 20 cm apart and dropped them from a height. Before that, I got students to predict whether the intervals in time between drops will be constant, increasing or decreasing.

Most students are able to predict rightly that the intervals will be decreasing and explain their reasoning.

What challenged me was this: previously, we had to listen to the intervals of sound to verify the answer. I had tried using laptop software such as Audacity to record the sound before. However, I wanted students to be involved in this verification process. PhyPhox enabled that.

With each student being able to download the mobile app into their phones, all I needed to do was to ensure everyone uses the correct setting: the Audio Scope setting and to change their range to the maximum duration (500 ms). They then had to be familiar with the play and pause button so they can stop the measurement in time to see the waveform.

I then did a countdown before dropping the balls. This is an example of the graph obtained.

Through this graph, you can see that:

  1. the time interval between drops decreases as the balls dropping over a larger height had gained more velocity by the time they reach the table.
  2. the amplitude of sound increases as the balls drop with increasing velocity, therefore hitting the table with larger force.

Relationship between displacement-time and velocity-time graphs

Through this GeoGebra app, students can observe how the gradient of the displacement-time graph gives the instantaneous velocity and how the area under the velocity-time graph gives the change in displacement.

In the GeoGebra app below, you will see a displacement-time graph on the left and its corresponding velocity-time graph on the right. These graphs will be referring to the same motion occuring in a straight line. Instructions

  1. Click “Play” and observe the values of displacement and velocity change in each graph over time.
  2. Note the relationship between the gradient in the displacement-time graph and the value of velocity.
  3. Note the relationship between the area under the velocity-time graph and the value of displacement.

Geogebra App for Kinematics

As one of the first topics in A-level physics, kinematics introduces JC students to the variation of velocity and displacement with acceleration. Very often, they struggle with the graphical representations of the 3 variables.

This Geogebra app allows students to vary acceleration (keeping it to a linear function for simplicity) while observing changes to velocity and displacement. Students can also change the initial conditions of velocity and displacement.

The default setting shows an object being thrown upwards with downward gravitational acceleration of 10 m s-2.

The movement of the particle with time is shown on the left with a reference line showing the position on the displacement graph.

GeoGebra link: https://www.geogebra.org/m/qpxcs6vb