# Simple motion experiments with a datalogger

##### Demonstration

An introduction to motion using ultrasound position sensors.

#### Apparatus and materials

Ultrasound position sensor

Trolley, ramp

Lab jack

Buffer to rebound trolley

#### Health & Safety and Technical notes

Most position sensors will return the distance from the sensor. Often they can also produce automatically a value for velocity and acceleration. They should be able to produce 'live' graphs and a data collection rate of 10 Hz is adequate.

#### Procedure

There is a series of participative experiments you can perform but the details will depend upon the datalogging system you have.

a Get a few students to produce live position-time graphs of themselves as they walk towards and away from the sensor. They should be able to identify which parts of each graph correspond to what they were doing.

They can then predict the speed-time graph, which should include positive and negative parts. Then this can be compared to what the datalogging software produces.

b Record the position-time graph of a trolley rolling on a friction-compensated ramp. Place the sensor at the top of the ramp.

Again predict and check the velocity-time graph.

c Repeat b but with the ramp steeper so the trolley accelerates. Record the velocity-time graph and predict the position-time graph and acceleration-time. Again, check with the computer output.

5 Repeat b with a buffer at the bottom of the ramp so the trolley rebounds.

6 This last step may confuse even the best students - have them roll the trolley up the ramp towards the motion sensor so it comes to a halt and then starts to roll back. Again compare velocity-time and acceleration-time graphs.

#### Teaching notes

The aim is to get the students understanding the relationships between distance-time and velocity-time graphs.

This is best done through getting them to predict what graphs will look like before they get the computer to produce them.

With a high-level group it would be sensible to discuss how the computer produces a velocity-time graph from position data. It simply takes differences in position and divides them by the time interval (not the total time). This helps to emphasize the difference between instantaneous and average speed.

The direction of motion helps to emphasize the difference between velocity and speed.

The last experiment will help to emphasize the idea that velocity and acceleration can be in opposite directions.

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.