# Building a reaction tester

##### Class practical

To get the class to build a simple reaction tester using a falling sheet of card.

#### Apparatus and materials

For each student pair

A3 card cut into strips

Metre rule

#### Health & Safety and Technical notes

The A3 card should be cut into strips, about 4 cm wide, the length of the short side of the paper.

Alternatively, plastic 30 cm rules can be used instead.

#### Procedure

a The students prepare a reaction timer, a sheet of card.

b The student whose reaction time is being tested places their thumb and first finger around the zero mark (without touching the card), while a partner holds the card vertically.

c When the partner releases the card, the first student closes the finger and thumb to catch it. The distance the card falls before being stopped is used to measure the reaction time.

#### Teaching notes

1 At the simplest level a chart of distances can be used and ranked. At a higher level the teacher could supply a set of distances to be marked on the card for times of 0.1 s, 0.2 s, etc beginning with zero at the lowest level.

2 At a more advanced level still the students individually or as a whole class can be set the problem of putting on lines at times corresponding to the card falling for 0.1 s, 0.2 s, etc. They should use g = 10 m/s2 for the calculations.

3 As students find most reaction times are in the area of 0.1 to 0.2 s they could consider marking lines more densely there. This leads into ideas about how to take data in non-uniform distributions. Students might also be led into the idea of taking averages and class surveys.

4 This links in with reaction time in stopping cars and is good practice with equations involving uniform acceleration. Distance versus time can be found by calculation using s=ut+1/2 at2, u=otherefore s=5t2 or by graphs, with either produced by the student, by computer or by the teacher.

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.