Workshop on Motion

A workshop on Motion was organized on 30^th June at Mudaliarpet ERC. This article summarizes the key things learned from that workshop.

Early Understanding of Motion

The discussion began on how the understanding of motion evolved over time. A reading of Aristotle’s and Galileo’s ideas on Motion was shared with the participants.

Human beings have always been curious about the universe. Since early days, they have observed various phenomena and have tried to rationalize how they operate. Among them, one of the first phenomenon that was studied was Motion. The Greek scholars were among the first to try to systematically gather knowledge.  However, it was only through reasoning. Aristotle, a prominent philosopher and scientist of his time, explained motion as how bodies move and what causes them to move. However, his views on motion are very different from our current understanding.

According to Aristotle, objects were made of fundamental elements that had their natural place in the universe. Objects moved to come to back to their natural state. Therefore, a stone that is made of earth, when held in mid-air would fall down as it is eager to come back to the centre of the earth, its natural place. Smoke had fire, and always moved up. Extending this idea, it meant that heavier objects would fall faster to the ground than lighter objects.

However, Galileo disproved this 2000 year old belief through his famous experiments at the leaning tower of Pisa. He dropped a heavy and a light object from the top of the tower and proved that objects fall at the same rate of motion.

Do objects fall at the same rate of motion?

Our experiences have led us to believe that if we drop two objects from the same height, the heavier object will hit the ground faster than the lighter object. Therefore, we associate free fall with mass. However, on closer examination, we come to understand that it is not true.

Then why does a piece of paper fall slower than a cork ball?  Let us also consider a second scenario.  Drop two sheets of paper, one folded and another flat. Here the mass is the same, but the surface area is varied. How has this affected the time taken for both objects to fall to the ground? Here, air resistance comes into play.

A video clip of an experiment was screened, where scientists wanted to find out what happens when a bowling ball and a feather are dropped together in vacuum. The video showed that in the presence of air, the ball reaches the ground first. However, in the absence of air, both the objects hit the ground at the same time. The iron ball, having a greater mass creates a greater impact on the surface of the box and breaks it.

Crime Scene Investigation

The focus of this activity is to represent and interpret motion through distance-time graphs. Designed as a crime scene investigation, it presents a problem-solving approach to interpret distance – time graphs and understand some key features of the graph.

The case - A speeding car inside a tunnel killed a man. The accident happened in an area, which was in the blind spot of the cameras. The information available is the position of the close circuit television (CCTV) cameras inside the tunnel and the time at which four cars passed their view. Participants are required to plot the graph, reconstruct the incident and identify the car responsible for that act.

In the discussions that followed, teachers presented their investigation and argued their case. This also led to the discussion on the importance of graphs and using them to interpret the motion of objects. The straight line represents what happened and a curved line plots and tells us about the motion of the object.

Investigating Motion – Experiments on an inclined plane.

It was time for some action and hands on investigation. The inclined plane set up was ready. Participants were given stopwatches, balls of different masses and data collection sheet.

They were involved in testing the following hypotheses:

1. Balls of different masses roll down at the same time.
2. Increasing the slope of the plane increases the acceleration of the ball.
3. The acceleration of different balls remains constant for a constant slope.

From the experiment, the above-mentioned hypotheses were verified. This discussion was extended to how the motion on an inclined plane can be compared to a free-fall. A free-fall is a special case of motion on an inclined plane, where the angle of inclination is 90⁰. Galileo therefore used an inclined plane to calculate the time taken by objects to fall. To calculate the acceleration of the balls rolling down the inclined plane, the formula S = ut + 0.5at²   is used.

Teachers discussed how friction comes into play when investigating the motion on an incline plane and affects the results. Some considerations to be kept in mind in this experiment are to use balls that have the same shape but larger difference in masses, so that the results offer conclusive evidence to test the hypotheses.

ICT:  The PhET interactive simulation tool was introduced and it was used to differentiate the movement of a person travelling in constant velocity and constant acceleration. The simulation depicts a man running and the three graphs: distance – time, velocity – time and acceleration – time. This would make it one-step easier for students to relate the physical phenomena with the graphs. It helps us visualize how a person travelling in constant velocity will maintain the same speed throughout the journey. A person travelling in constant acceleration, however, has to keep increasing speed every second and therefore will cover larger distance as each second passes.