Reaction Time

Calculating Your Reaction Time

• Have your friend hold a metre stick about 1.5 metres above the floor.

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• Hold your fingers opposite the 20cm mark, but don't touch the stick! Without warning your friend should let go of the metre-stick, and you should try to catch it with your fingers.

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• Work out how many cm the stick fell before you caught it. Try it several times to get an average.

Now The Maths…..

• Your reaction time can be determined using the following equation

t = √ (0.2S)

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• Remember to convert the distance into metres!

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Reaction Time Testing On The Web

• There are lots of sites that will test your reaction time on the web.
• Here are the links to a few of them:

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http://www.exploratorium.edu/baseball/index.html

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http://www.fetchfido.co.uk/games/reaction/reaction_test.htm

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http://www.getyourwebsitehere.com/jswb/rttest01.html

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http://www.compfused.com/directlink/827/

Reaction Times and Road Safety

• Your reaction time has serious effects on your ability to avoid a collision or stop if need be.
• Basically while you are thinking about stopping - you keep moving until you actually start braking.
• This distance is called your thinking distance.

Factors affecting your reaction time?

Brainstorm? Think-Pair-Share?

How far do you travel while thinking ?

• Based on the average reaction time of 0.2s, how far would you travel while thinking about stopping at the following common speeds?

2.2

3.3

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5.5

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6.7

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8.9

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Braking Distance

• The braking distance will depend on a number of factors (not exhaustive):
• Speed
• Mass of Vehicle
• Road Conditions (rain etc)
• Tyre Conditions

Braking Distance - Values

• Unlike thinking distances, braking distances are not directly proportional to speed. A slight increase in speed greatly increases braking distances. This is why there is a continuous campaign in the UK to enforce the 30 mph speed limit in built-up areas.

Braking Distances -Values

Looking carefully at the data, we can see that doubling the speed (from say 20 to 40 mph) increases the braking distance by four times (6 to 24 m). Doubling from 30 to 60 mph has the same effect: braking increases 4 times from 14 to 56 m.

Stopping Distance

• Your Stopping Distance is the combination of your Thinking Distance and your Braking Distance.

Example

• A woman drives a car of mass 1300kg when she sees a family of ducks crossing the road in 20m in front of her. She brakes for 1·5 s and took 1·8 s to stop.

OH NO! Poor ducks?! Let’s analyse this!....

• What is her reaction time?

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• If she was travelling at 20m/s how far did she travel while thinking?

• What is the Kinetic energy of the car?

• What acceleration is required to bring her to a stop?

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• What force is required on the brakes to do this?

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• What distance does she travel while breaking?

• Does she hit the ducks?

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Experiment: Try the experiment on Braking distance with the “Hot Wheels” track…

0.3s

6m

260000J

13.3m/s²

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-17290N

15m

Yes (Total stopping distance is 21m. Ducks are only 20m in front of her!)

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