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Fu-Kwun Hwang
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 « Embed this message on: January 29, 2004, 08:33:20 am » posted from:,,Satellite Provider

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## Reaction time : the lapse of time between stimulation and the beginning of response.

Would you like to measure your reation time?
Would you like to estimate how fast you can drive safely on the highway?

You are driving on the high way and listening to the music you like most.

Suddently, you see the brake light of the car in front of you just turned on.

You will try to hit the brake and slow down your car.

But, there is a small time delay before you really do that--- your reaction time.

During that period of time, your car is still moving at the same HIGH speed!

If you do not want something VERY BAD happened,

What is minimum diatance between front of your car and the rear of the car before yours?

If both cars need the same distance to fully stopped, the miniuum distance = ( the velocity of your car )* ( your reaction time )

New version of this simulation: Reaction time measurement

• Click Start to start the animation. Click Brake to stop the car.
• The time delay between the red light is turned on and you click the brake button will be shown in the textfield. (plus the time needed to stop the car after the brake is started. The corresponds distances which the car moved are also shown on the graph)
• Paramenters you can change(Hit ENTER key after you change value in the text field)
• Initial Velocity of the car : initial value is 72 km/h = 20 m/s
You can select the unit for the velocity ( km/h, mile/h or m/s)
• Friction coefficient of tires on the road : initial value is 0.8

•  Typical value of tires static coefficient kinetic coefficient auto tires on dry concrete 1.0 0.7-0.8 auto tires on wet concrete 0.7 0.5 auto tires on icyconcrete 0.3 0.02
• The Y-coordinate the the small dots are proportional to the velocity of the car
Notice that it is not a straight line after the brake turned on! Because ...
• The mouseXV textfield shows X-coordinate and velocity of the car at the mouse position.
If you drag the mouse, it will change to values of the relative distance and the relative velocity.
(relative to the point where you started to drag the mouse)
• Press Reset button to restart the testing.
• Close the window to Quit.

Let's Assume: you are driving a car with speed v, and your reaction time is Treact.
When you find out something happened, you need time Treact to react (including time to hit the brake and time for the brake system to work).
Before the brake really start, the car already move another distanace Dreact=v*T;
Assume the reaction is 0.8s (it is 0.6-1.0 for ordinary person. The reaction time will be longer if someone has been drinking or who is sick or tired).
For a car moving with speed 72 km/hr (which is equal to 20 m/s), it means that the car move another distance Dreaction=20*0.8=16m before the brake is activated. And it will take more time to stop the car (from v to 0).

Now. I am going to show you how to calculate the distance required for the car to be fully stopped.
The friction force between tries and the road is the only external force to slow down the car.
All the energy of the car goes to tires during the process of stopping the car (so the temperature of tires goes up).
Auusume the friction coefficient is μ, then the friction force fr=-μ*N =-μ*m*g where m is the mass of the car and g is the gravity. (μ is between 1-1.2 for ordinary tire and normal road condition. If it is raining, the friction coefficient can be reduced to less than 0.6, and it could be reduced to 0.2-0.4 for ice road condition).

From Newton's law F=m*a = -μ*m*g so a=-μ*g is the de-acceleration.
So the time for the car to stop Tbrake=v/|a| = v/(g* μ).
During the above time interval, the speed reduced from v to zero linearly.
the average speed = (0+v)/2 =v/2
So, the braking distance Dbrake=average speed * Tbrake= (v/2) * v/(g* μ)=v2/(2*g* μ)

What is the meaning of the above equation?
It tell us that the stopping distance is proportional to square of the driving speed.
If the speed is changed from 100 km/hr to 120 km/hr, the distance to fully stop the car change from d100 to (144/100)*d50=1.44 *d100.

For the above case, v=72km/hr (45 mile/h)=20m/s. (g=9.8 m/s2 and I wil use g=10 m/s2 to simply the estimation, μ=1 is used)
Dbrake=v2/(2*g* μ)= 202/(2*10*1)=20 m

The driver need Treact to react and the car need Tbrake to be fully stopped, which imply the total distance traveled is equal to Dreact + Dbrake = v*Treact + v2/(2*g*u).

For car at v=108 km/hr (67 mile/h) = 30 m/s :
Dreact= 30*0.8=24 m
Dbrake= 30*30/ (2*10*1)=45m
So, the car need 24+45=69m to fully stopped.
This is a very long distance (assume the length of the car is 3m, it is a distance of 23 cars in a line).

If several cars with the same speed are in line in high way, the minimum distance between cars is Dreact. Because all the car need the same distance to fully. (recommend: multiple Dreact by 1.5 as safty factor).

However, if you are driving on a road, and someone jump into the road from the sideway.
The car need Dstop=Dreact+Dbrake to fully stopped.
It is the same if a car accident happened in front of you.
All the cars within Dstop from the accident will bump into each other.

Please estimate your own reaction time and calculate Dreact+Dbrake for the maximum speed you normally drive in the high way. May be you will drive slower after you have calculated the value.

The police might not be there to give you a ticket when you drive too fast.
Nature law require Dreact+Dbrake=v*Treact+v2/(2*g*u) for a car to fully stopped.
You will alway receive ticket if you violate the NATURE LAW.
(You might be lucky if you were sent to hospital instead of ...).

Registed user can get files related to this applet for offline access.
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There are 17 translations,
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 « Embed this message Reply #1 on: February 10, 2004, 06:41:23 am » posted from:Taipei,T'ai-pei,Taiwan

Dr. Hwang,

I appreciate your reaction time testing device. Is there any data about how the normal population reacts with your device? To measure reaction time, in seconds, do you divide by 20? Is the 20 meters per second?

Steve sbw@dscience.com
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Fu-Kwun Hwang
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 « Embed this message Reply #2 on: February 10, 2004, 06:42:59 am » posted from:Taipei,T'ai-pei,Taiwan

The first textfield (under label V=) is the velocity of the car. It is not the reaction time.
The reaction time is shown in the textfield under the label : [b:039696da07]Delay +Brake[/b:039696da07]
where "Delay" is the reaction time and "Brake" is the time for the car to fully stopped.
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 « Embed this message Reply #3 on: May 17, 2004, 04:29:10 pm »

Dr Hwang

Enjoy your site and use it all the time with my Physics classes.

I think your calculation of braking distance is wrong on the Reaction Time Measurement applet.

It always displays as 25.5 m regardless of speed.

It should increase as the square of speed. Can you fix this?

Paul
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Fu-Kwun Hwang
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 « Embed this message Reply #4 on: May 24, 2004, 01:05:41 pm »

I think you forgot to hit [b:b664fce8e0]ENTER[/b:b664fce8e0] after you have changed the number in the textFields.

Please try it again and it should work fine.
You can enter reaction time,driving speed,friction coeffcient and select the speed unit.
 reaction time s driving speed km/hmile/hm/s Dreact friction coeffcient Dbrake Dstop

Dutch version web page

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 « Embed this message Reply #5 on: August 04, 2004, 07:04:26 am »

Hi Hawg, I think this is the coolest site and I shall tell all my friends about this site!
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mugluck
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 « Embed this message Reply #6 on: February 21, 2006, 11:03:20 am »

this was the best thing i have ever done in my life, i am telling my kids to do it every day till they r 20.

mugluck signing off, *ZOLTAN*
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Fu-Kwun Hwang
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 « Embed this message Reply #7 on: April 19, 2006, 01:15:04 pm » posted from:Taipei,T'ai-pei,Taiwan

Someone send me video which shown car accident happened inside a tunnel in Russia.
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Overactive Imagination
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 « Embed this message Reply #8 on: May 09, 2006, 04:26:23 pm »

Hey I have a question
(I'm doing a school project on stopping dist. and whatnot)

Which formulaes are implemented in that program?
Thanks

- Luke
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Fu-Kwun Hwang
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 10 topic137 « Embed this message Reply #9 on: May 09, 2006, 05:15:55 pm » posted from:Taipei,T'ai-pei,Taiwan

The friction force = u * m*g = -m*a (u is the friction coefficient )
So a = -g*u
Then you can calculate the stopping distance d= v*v/(2*a)= v*v/(2*g*u)

Due to the reaction time dt , the actua stopping distance will be d+ v*dt;
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Overactive Imagination
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 11 topic137 « Embed this message Reply #10 on: May 09, 2006, 05:24:27 pm »

In the first forumula i'm assuming m stands for mass. What did you use as the mass?

Sorry if this is a stupid question

- Luke
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Fu-Kwun Hwang
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 12 topic137 « Embed this message Reply #11 on: May 10, 2006, 07:52:12 am » posted from:Taipei,T'ai-pei,Taiwan

The mass is the mass for the car.
As you can see in this case, the result has nothing to do with mass.
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sanjeevgogna
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 13 topic137 « Embed this message Reply #12 on: May 17, 2006, 04:44:40 am »

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Fu-Kwun Hwang
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 14 topic137 « Embed this message Reply #13 on: May 17, 2006, 08:55:55 am »

Due to hacker attack. We added some checking for browser. May be your browser did not support those functions we were checking. I just turn off the checking. Please try it again!
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Felsager
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 « Embed this message Reply #14 on: January 17, 2007, 03:45:58 am »

Dear professor Hwang

I downloaded the applet as a zip-file, unpacked it and tried to use it offline, but it does not show the full story: The images are not shown and there are no buttons, so I cannot run the simulation offline. Have you some suggestion as to what might be wrong?

Yours sincerely

Bjørn Felsager

Lecture in physics?/p>

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Felsager
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 « Embed this message Reply #15 on: March 05, 2007, 12:05:54 am »

Dear Professor Hwang,
I found out what was wrong. I was the problem or rather my name. It contains an unusual Danish character (ø) and this means that when I save your applet to a folder in my documents all path-names will contain this unusual character. But the applet cannot handle unusual characters in path names. I moved the applet to the root-directory (C:\) and all problems disappeared. I post the answer because other users may experience similar problems.
Yours sincerely
Bjørn Felsager
Lecturer in physics
Haslev Gymnasium & HF
Denmark
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Fu-Kwun Hwang
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 « Embed this message Reply #16 on: March 05, 2007, 10:41:30 am » posted from:Taipei,T'ai-pei,Taiwan

Thank you for your posting. Yes. Some one else might have the same problem.
If the directory name contain space or some other special characters which might be allowed in Microsoft Windows system, would not work in java.
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i h8 chinks 2
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 « Embed this message Reply #17 on: April 18, 2007, 04:30:41 pm »

ite m8  i really like your website
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i h8 chinks 2
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 « Embed this message Reply #18 on: April 18, 2007, 04:32:17 pm »

Dear Professor Hwang,
I found out what was wrong. I was the problem or rather my name. It contains an unusual Danish character (ø) and this means that when I save your applet to a folder in my documents all path-names will contain this unusual character. But the applet cannot handle unusual characters in path names. I moved the applet to the root-directory (C:\) and all problems disappeared. I post the answer because other users may experience similar problems.
Yours sincerely
Bjørn Felsager
Lecturer in physics
Haslev Gymnasium & HF
Denmark
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roma100
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 « Embed this message Reply #19 on: May 26, 2007, 06:58:31 pm »

Hi,

I would like to get the source code of this reaction time measurement applet. The applet is really great.

Thanks
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Fu-Kwun Hwang
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 « Embed this message Reply #20 on: May 27, 2007, 11:39:23 pm »

This applet was created 10 years ago with JDK1.0.2.
There is an EJS version of the same applet. You will be able to find the EJS source from that page.
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roma100
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 « Embed this message Reply #21 on: May 30, 2007, 04:53:32 pm »

Dear Professor Hwang,

Do you also have the java source code of this reaction time measurement applet? I have problems with the EJS one...

Thanks
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g3mini
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 « Embed this message Reply #22 on: August 08, 2007, 10:16:46 pm »

Hi Professor

I am just a lay person who has a questions about driving and physics. My boyfriend and I disagree about what would happen in the below scenario:

You are driving on a major highway in a Toyota Corolla, it's pouring rain, there is a transport truck in front of you (he is hauling a trailer), you are both doing 110 km/hr and you are about 1 1/2 car lengths behind him (somewhat tailgating). The truck driver slams on his brakes suddenly - given normal reaction time (or even super-human reaction time), would you be able to stop in time to not hit him?
I understand that transports have a greater braking distance, but once he's touched his brakes, at the rate you are going, and your reaction time, would you not hit him being so close? At what distance would you have to be in order to not hit him?
Would you even have time to swerve around and avoid him?

I do not think that you would be able to have enough time to react and avoid hitting the back of the truck, however he thinks you do.

Any help would be greatly appreciated!
Kind Regards
Tracy
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Fu-Kwun Hwang
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 « Embed this message Reply #23 on: August 08, 2007, 11:03:21 pm »

The speed is 110km/hr, it is about 30m/s (=108km/hr).
The distance between cars is about 1 1/2 car lengths (assume to be 5m).

The best reaction time for person driving car is 0.6 s (up to 1s for ordinary person).
So the minimum distance required will be 0.6s* 30 m/s=18m (30m max.)

The reaction time is the time between you saw what happened, and the time you really react to it.
During that time interval, your car will move with the same velocity.
Yes. the car in front of you also moving at the same time, However, the car in front of you starts to slowing down while you are moving at the same speed.
Without consider the thermal effect and assume the brake work perfectly, the stopping distance for both car will be the same.
Due to the thermal effect, larger car will have longer stopping distance (ABS braking system can help to reduce the braking distance to theoretical value).
If you think the difference between the braking distances are more than 18-5=13m (25m max.)
You can try to risk you life. But I would not bet on it.

I would suggest:
Count one second and see what is the distance you car has move.
It is the same minimum distance you should keep between your car and the car in front of you.

Please try to visit Reaction time and car accident to test it with a simulation.

If you think your reaction time is better than 1 second. You can reduce the distance a little bit. (I would not suggest you did that).
But if something happened, you can try to argue the physics with God if you meet with him.
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g3mini
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 « Embed this message Reply #24 on: August 08, 2007, 11:22:42 pm »

So realistically, there is no chance that you could get your car stopped in time without hitting the back of the truck, right?

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Fu-Kwun Hwang
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 « Embed this message Reply #25 on: August 08, 2007, 11:40:37 pm »

Yes. I would say more than 95% that your car will hit the truck in front of you. Unless the braking system of the truck is not function normally(less than 5%).
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g3mini
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 « Embed this message Reply #26 on: August 08, 2007, 11:57:02 pm »

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Fu-Kwun Hwang
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 « Embed this message Reply #27 on: August 09, 2007, 09:28:02 pm »

For 0.8 s reaction time, driving at 30 m/s (108km/hr),  the car will move 30*0.8=24m before you hit the brake.
And the car will fully stopped after moving 30*30/(2*9.8*1)m,  about 45m.
(Police will you the length of the tire trace on the road to estimate the speed of the car).
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John256
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 « Embed this message Reply #28 on: December 18, 2007, 11:40:08 am »

Dr Hwang, would the friction coefficients you posted (rubber on dry pavement = 0.8, etc) equate to the stopping distance if you braked hard enough to lock the wheels and came to a skidding stop?  Would braking distance without locking the wheels be significantly better?

Thank you!
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Fu-Kwun Hwang
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 « Embed this message Reply #29 on: December 18, 2007, 11:11:17 pm »

The table at the top message,shows both static coefficient and kinetic coefficient (One for static friction and another one for sliding friction).
The static coefficient is always slightly larger than the kinetic coefficient (if you lock the wheels).
If you brake too hard and lock the wheel, the temperature of the wheel at the contact point will rise very quickly, which will reduce the coefficient a lot and make things even .. .
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To live close to great minds is the best kind of education. ..."John Buchan (1875~1940 Scotticsh historian, Governor General of Canada)"