This activity can help develop students’ understanding by addressing the sticking-points revealed by the following diagnostic questions:

• Diagnostic question: Plot story

• Diagnostic question: Line story

• Diagnostic question: Two slopes

• Diagnostic question: Where’s Sally?

• Diagnostic question: The speed of Dwight

When interpreting graphs students must be able to determine which features of a graph correspond to particular physical concepts. With distance-time graphs there is often confusion between the slope (speed) and height (distance) of a graph (Clement, 1986; McDermott, Rosenquist and van Zee, 1987). One strategy to overcome this is to use motion sensors and data-loggers to plot real-time graphs of motion. Students can practise predicting the shape of graph for different descriptions of motion and also reproduce the shape of given graphs by moving in front of a motion sensor. A study of 17- to 18-year-olds (n=75) showed this approach significantly improved students ability to interpret distance-time graphs (Brasell, 1987).

There are several possible explanations that may explain why real-time graphing of motion improves learning: it allows students to process information about the event and the graph simultaneously; the dynamic display of the data-logger focuses attention on one feature at a time; and frequent repetition of graphing events supports consolidation. Further investigation by Brasell (1987) suggests even a short delay of 20-30 seconds between observing a motion and seeing a graph limited students ability to link aspects of motion to features on a graph.

This practical activity gives students the opportunity to practise applying their understanding and to clarify their thinking through discussion. To support this, students should complete the practical in pairs or small groups if there are sufficient motion sensors and data-loggers to allow this.

An alternative strategy is to set up one motion sensor connected to a projector so that the whole class can observe. The motion sensor could be set to allow students to walk up and down in front of it to produce graphs.

The practical work divides into three distinct phases.

1. Students familiarise themselves with the equipment.
   
2. Students move the trolley in prescribed ways and observe the lines that are produced on a graph that represents the motion.
   
3. Students move the trolley to reproduce given graphs. It is helpful to include thinking time to plan the motion before trying it out. This can encourage social construction of new ideas through dialogue.
   

Listening to individual groups as they work often highlights any difficulties they might have. These can often be overcome, through a whole class clarification or redirection part way through the activity.

Asking students to report their findings at end of the practical work is a useful check. After a group has fed back, it might be helpful to model an even better answer. You could do this, for example, by asking another group to add to, or clarify, the first observation. Then ask another group to sum up the important part of the observation, and so on.

Equipment

For each student/pair/group:

1. Motion sensor
   
2. Data-logger
   
3. Connecting cables
   
4. Dynamics trolley
   

For the class (optional):

1. Motion sensor
   
2. Data-logger or computer
   
3. Projector
   
4. Connecting cables
   

Technician notes

There is a wide variety of motion sensors and each type needs to be tested in order that settings are adjusted to obtain the best results.

It can sometimes help the motion sensor detect the position of a trolley if a flat piece of cardboard is taped to the front of it.

Health and safety

There is likely to be a significant amount of movement during this practical.

Dynamics trolleys can easily be damaged if they fall onto the floor.

Practical work should be carried out in accordance with local health and safety requirements, guidance from manufacturers and suppliers, and guidance available from CLEAPSS.

Students should be able to achieve graphs that are approximately the same as those shown.

It is not expected that exact times or gradients will be achieved.

In graphs 3-6 there is an instantaneous transition between the different motions represented. The curves in graph 7 more accurately represent how the changes are more gradual and happen over a period of time.