Car Braking Distance

Peter Horváth

Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava,

Slovakia

C. S. Lewis Bilingual High School, Bratislava

This presentation says about • We have developed an activity for students (classroom idea),

which help students to make their idea about car braking distance correct.

• Students measure car braking distance from variable velocity using video-measurement with software Tracker (Coach).

• By doing this, students also develop their skills to make, read, and analyze data from graph.

• We applied this activity with 17 year olds students since 2010, approximately 150 high school students.

• This activity was also used in teacher training workshops, we had more than 250 teachers.

First Motivation (2009 - 2010)

• Some of our 18-19 years old high school students become

from their parents their first car

• Concretely 8 students had car – it happened 4 seriously car

crash

• In 2010 it was impossible in one our insurance company to

insure the cars driving by young people under 24 years old

• Young people frequently drive cars with high speed...

What are student´s ideas about braking distance?

We prepared for students two very similar test questions.

First question:

What is the car braking distance in dry surface? Fill this table.

Initial velocity Car braking

distance

20 km/h m

40 km/h m

60 km/h m

80 km/h m

100 km/h m

50 km/h m

Results of first question (pilot testing in 2010)

• Year 2010, 17 students, 17-19 years old

• 9 students idea about braking distance from 80

km/h was markedly les than it is in reality

• they claimed that braking distance from 80

km/h is 10 m – 20 m.

Second test question – now we tell students the real number of braking distance from 40 km/h velocity

Initial velocity Car braking

distance

20 km/h m

40 km/h 8,0 m

60 km/h m

80 km/h m

100 km/h m

50 km/h m

The car braking distance from 40 km/h initial velocity is 8,0 meters. What are the braking distances from other initial velocities? Fill this table.

Results of second question, pilot testing

• Year 2010, 17 students 18-19 years old

• 14 students were thinking, that the function between braking distance and initial velocity is linear

• 13 students reported, that the braking distance is more shortly than it is in reality (80 km/h ... 16 m)

Our next experience • 2010 – 2013, 150 high school students, 17 years old

• the results of both questions were very similar as in pilot test (2009-2010) in 17-18 years old students in secondary school.

• We gave the test questions also to faculty students, about 100 students, future physicist and physics teachers in the beginning of their studies - the results were also practically similar.

• More than 70 % students means, they are able to stop his car from 80 km/h in les than 20 meters (2nd Question)

Activity for students

• 5 video sequences of a car, which is braking from variable velocities, were prepared by our faculty student (A. Karlubík). Videos were made in a small airport.

One of our video sequences

Video sequence, the highest speed of the car

Videos of braking cars

• We have 5 with variable car velocity.

• Each video was recorded from one place (one point of view), vertical to the car motion.

• At the beginning of every record, car is starting to break and we can see car until it stop – so we can see (and measure too) the car braking distance.

• The angle of recording was designed according to the highest initial speed of the car.

Activity for students

• The students have to measure the initial velocity and car braking distance and analyse the results.

• They use software Tracker (Coach)

Video measurement with Tracker, step by step

1. Start frame and end frame for analyse

2. Number of frames per step = Step sise

3. Position and orientation of coordinate axies

4. Kalibration

5. Definition (create) object for analyse (point mass)

6. Measure, mark position of object (car)

One video - measurement

Go to Tracker

Initial velocity

Braking distance

Result of one concretely measurement of braking car

Video measurement continues

• Now the students (maybe in groups) analyze the same way all other video sequences with car.

• They become 5 different pair of results.

Here are the all video sequences in one video (this is not for my students, my students have to measure the initial velocity)

Measurement results

How the braking distance depends on initial velocity

Initial velocity Initial velocity Braking distance

m.s-1 km.h-1 m

0 0 0

5 18 2,5

9,6 35 10

14 51 25

18 66 44

23 82 72

Measurement results How the braking distance depends on initial velocity

Discusion

The function between braking distance and initial velocity is (may be) quadratics (we see)

Discusion

• the braking distance from initial velocity 60 km/h is more than 2-times bigger than braking distance from initial velocity 40 km/h

Discusion

• the braking distance from initial velocity 70 km/h is almost 2-times bigger than braking distance from initial velocity 50 km/h.

Activity in lesson – step by step 1. Students have to answer the question number 1

2. Students give the test paper with answer 1 to teacher

3. Students have to answer the question number 2

4. Students give the test paper with answer 2 to teacher

5. Students in groups discus about their answers to question 2

6. Students in groups make video-measurement, they measure initial velocity and car braking distance

7. Repeat the point 6 with other video sequence... They have to measure all 5 video sequences.

Activity in lesson – step by step 8. then students make graph of function between initial velocity

and braking distance

9. Students discuss results – for example: the braking distance from initial velocity 60 km/h is more than 2-times bigger than braking distance from initial velocity 40 km/h.

10. The function between braking distance and initial velocity is (may be) quadratics (we see)

11. Discussion about driving rules

12. (Not necessarily in this lesson) Why is the function between braking distance and initial velocity is quadratics? We discuses about car kinetic energy and the force between car rubber and asphalt - it is almost constant.

Why do we think, this is suitable and good activity for students?

Students make predictions

Students learn, how to:

• find the information from text

• find data from graph

• analyze the data from the graph

• synthesize data

• explain graph

• disuse about the final result – why there is quadratics function

Why we think, this is suitable and good activity for students?

Very important results:

• the braking distance from initial velocity 60 km/h is more than 2-times bigger than braking distance from initial velocity 40 km/h.

• the braking distance from initial velocity 70 km/h is almost 2-times bigger than braking distance from initial velocity 50 km/h.

keeping driving rules is very necessary.

Another ideas about this theme

• Activity for students – braking distance of bicycle

• Measurement of car braking distance under different conditions:

Wet road – Dry road

car with ABS – car without ABS

• We have started to cooperate in this theme with colleagues from University of Žilina (Peter Hockicko)

Literature: • [1] Lepil O.:K vývoji didaktické komunikace ve výuce fyziky. In: Zelenický Ľ. (ed.): Zborník z

konferencie Didfyz 2004. Nitra, FPV UKF a JSMF, 2005, s. 5-10.

• [2] Ješková Z., Kíreš M.: Videomerania fyzikálnych javov v prostredí IP COACH. In: Zelenický, Ľ. (ed.): Zborník z konferencie DIDFYZ 2004 Nitra : FPV UKF a JSMF, 2005, s. 202-207.

• [3] Jilek, M.: Několik nápadu nejen z kroužku fyziky. In Svododová, J., Sládek, P. (ed.): Sborník z konference Veletrh nápadu učitelu fyziky 9, sv. 2. Brno, Paido, 2004, s. 50-51.

• [4] Demkanin, P. 2006. Počítačom podporované prírodovedné laboratórium. Bratislava: FMFI UK, 2006.

• [5] Horváth P., Šedivý M.: Analýza mechanického pohybu videomeraním. In: Horváth, P. (ed.): Zborník príspevkov „Aktivity vo vyučovaní fyziky“, Smrekovica. Bratislava, FMFI UK, 2006. s. 69-77.

• [6] Horváth P., Šedivý M.: Videomeranie tiažového zrýchlenia. In: Horváth, P. (ed.): Zborník príspevkov „Šoltésove dni 2006“, Bratislava, FMFI UK, 2007, s. 31-38.

• [7] Duľa I. Možnosti využitia programu Tracker na hodinách fyziky. In: Zborník z konferencie Tvorivý učiteľ fyziky, Smolenice 2009. s. 35-39, dostupné na http://sfs.sav.sk/smolenice/prispevky.htm

• [8] https://poistenie.csob.sk/pzp/Strana2a.aspx

• [9] http://fyzikus.fmph.uniba.sk/typo/index.php?id=575

• [10] Karlubík A.: Videomerania vo vyučovaní fyziky, diplomová práca. Bratislava, FMFI UK 2010.

Thank you for attention

horvath@fmph.uniba.sk