Reaction Time Meter

Overview

Following a discussion about free fall, students will make a reaction timer that uses free fall to test reaction time. They will then test their own reaction time and compare it with the results of other students in the class. They will end with questions on how this relates to free fall.

Standards Addressed

Grade 8, Physical Science

06-08 Benchmark
B. In simple cases, describe the motion of objects and conceptually describe the effects of forces on an object.

Y2003.CSC.S03.G06-08.BB.L08.I01 / Forces and Motion

01. Describe how the change in the position (motion) of an object is always judged and described in comparison to a reference point.

 

Y2003.CSC.S03.G06-08.BB.L08.I02 / Forces and Motion

02. Explain that motion describes the change in the position of an object (characterized by a speed and direction) as time changes.

Y2003.CSC.S03.G06-08.BB.L08.I03 / Forces and Motion

03. Explain that an unbalanced force acting on an object changes that object’s speed and/or direction.

Materials

• Strips of thick paper (construction paper, poster board, card stock) per group, cut to make approximately 2 cm by 30 cm strips

• Rulers

• Calculators

Procedure — Free Fall Formative Assessment

1. Have the students complete the Free Fall Formative Assessment handout.

2. Set up the PowerPoint file titled speed_freefall.ppt found at http://www.WesternReservePublicMedia.org/ubiscience/speed_freefall.htm.

3. Review and discuss all questions from the assessment as you work through the PowerPoint presentation.

Formative Assessment Answers

1. c. Both will hit at the same time. The mass of the ball does not affect the rate at which it falls.

2. c. Both will hit at the same time. The volume of the spheres does not affect the rate at which they fall.

3. c. The shape of the parachute causes more air resistance, or friction, which slows the rate at which he can fall.

4. a. It will continually drop at the same rate.

5. a. Everything on the space station is falling, so in comparison it seems as though everything is floating. The space station is held near the Earth by gravity. It is not far enough away to experience zero gravity.

Procedure — Reaction Time Meter Activity

1. Divide students into groups of two.

2. Give each group a Reaction Time Meter Activity worksheet, paper strip and ruler.

3. Read through the introduction with the class and help them fill out the chart. Depending on the level of your students or time constraints, you may want to give your class the completed chart for them to fill in.

Displacement (cm)	Time (s)	Displacement (cm)	Time (s)	Displacement (cm)	Time (s)
1.225	0.05	7.056	0.12	17.689	0.19
1.764	0.06	8.281	0.13	19.6	0.2
2.401	0.07	9.604	0.14	21.609	0.21
3.136	0.08	11.025	0.15	23.716	0.22
3.969	0.09	12.544	0.16	25.921	0.23
4.9	0.1	14.161	0.17	28.224	0.24
5.929	0.11	15.876	0.18	30.625	0.25




4. After students have filled in their charts, help them create their reaction time meter by measuring out all the distances they have just calculated on their strip of paper. Make certain that all measurements are made from the bottom line, not from the last line measured.

5. Help students use their reaction time meter once they have completed making it. One student holds a hand over the edge of a desk with his or her thumb and forefinger an inch apart. The other student holds the reaction time meter so that his or her thumb is over the “0s” line.

6. Without warning, the first student drops the reaction time meter, while the other student attempts to catch it. By looking at where they caught it, they can see how long it took. They must have a partner for this so that the second student doesn’t know when it will be dropped. You place you hand over the edge of a desk so you don’t drop your hand lower when trying to catch it.

7. Help the students take an average of their times.

8. Help the students with the questions at the end of the worksheet.

Enrichment

This can be done using the dominant hand followed by the weaker hand. Find the average of each. Gather the data from the whole class using the ordered pair (right hand, left hand). Make a scatter plot of class data. Make sure that that there is a title for the graph, that the axes have equal intervals and that each axis is labeled. If there are left-handed students in the class, these points will appear as outliers on the graph. This is an excellent way to get across the concept that outliers are points that need to be examined and explained, not deleted.

Summative Assessment Answers

1. Gravity.

2. Unbalanced, because is causes a change in motion.

3. The lines get further apart. The card is in constant acceleration and each second, it falls faster than the previous second.

4. a. The card is moving downward.
   b. The card is not moving; the hand is moving upward.

Rubric for Lab