# How Do You Find Practical Stopping Distance?

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## Introduction: How Do You Find Practical Stopping Distance?

### Teacher Notes

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## Step 1: What Is Practical Stopping Distance?

Practical stopping distance is something you may come across in class or in the real world. There are multiple engineering classes where this equation is needed. This equation is also useful in a real life. If you are trying to figure out how far you travel before you are stopped this equation is very useful.

So whether you are trying to solve an engineering problem or trying to avoid a deer in the middle of the road these instructions can be very helpful. Depending on how many givens you have given to you in the problem this process should only take at most 15 minutes. If you are trying to solve for your own car it will take you more time because you have to measure deceleration and grade.

## Step 2: Supplies You Need

If you are doing a problem for a class with a given grade and acceleration you will need a pencil, paper, and a calculator.

If you are doing this to find the practical stopping distance of your car you will need everything listed above plus a ruler, level, and stopwatch.

## Step 3: Practical Stopping Distance Equation

Here is what the symbols mean from the equation shown:

d= practical stopping distance ( in feet)

V1= the initial velocity of your vehicle ( In feet per second)

V2= the ending velocity of your vehicle  (In feet per second)

g= gravity constant

a= acceleration or deceleration of the vehicle (in feet per second squared)

## Step 4: Transportation Engineering Problem

This is a common problem you will come across in most civil engineering classes.                                                                                      A car is traveling down a road at 60 Mi/hr. There is a deer that appears on the roadway approximately 750 feet in front of the car. If the deceleration of your vehicle is 6.03 ft/s^2 and the grade of the road is -3% will you stop before or hit the deer?

## Step 5: How to Solve

How to solve the problem

1)      Find all variables:

G= -3/100= -.03

A=6.03 feet per second squared

V1= 60 mph

V2=  0 mph

g= 32.2 (constant)

2)      Convert units:

Your Velocity needs to be in feet per second. Take 60mph*5280/3600 to get 88 feet per second

3)      Last you are going to enter all of this into your equation as shown in the picture. After you do this you are going to get

7774/10.128 = 764.61 feet as your practical stopping distance. So you will not be able to stop before the deer.

## Step 6: Real Life Application

Sometimes in real life it is necessary to know the actual stopping distance of your car. We can use the equation above and relate it to every day life. What if you were driving your dads car and accidentally hit a deer because you couldn’t stop in time? How could you prove to him you did just about everything possible to stop before hitting the deer? You can find the practical stopping distance and show him by the start of your skid marks that you in fact did or didn’t have enough distance between you and the deer to stop.

Just like the picture, you take the level and make it level on the road. Then set the ruler against it and measure the height it takes for the level to be level. Then take the measurement on the ruler divided by the length of the level. That will give you your decimal grade.

## Step 8: How to Find Deceleration of Your Car

The equation to find the deceleration of your vehicle is a=(V1-V2)/time it takes to stop

a=V1-V2/ time

Say it takes you 15.3 seconds to stop. That is the number you will use for your time. Your velocity 1 is your starting velocity. You will want to use 0 as velocity 2.

## Step 9: Finishing the Real World Problem

After you get your grade of your roadway and the deceleration of your car, you can enter them back into the original equation shown above. While doing this you can find your practical stopping distance and compare it to the distance you had to stop before you hit the deer. You can refer back to slide 5 for the equation and the steps on how to use it.

## Step 10: Wrapping Up

As you can see this equation is not only useful in a class room but also in a real world sense. Whether you are trying to ace an exam or find the stopping distance of your car these instructions should have really helped. So tell your friends to use this to help them if they ever need any help finding practical stopping distance.

## Step 11: Troubleshooting

Sometimes in the book problems they will give you the distance and ask you the velocity of the vehicle before it comes to a complete stop. There are many variations to the equation that can be found by rearranging the equation. For example the equation for solving for velocity 1 if velocity 2 is 0 is shown.                                                                                                                                                         Sources:                                                                                                                                                                                                  Techradar                               Toledoblade                                                                                                                                                                                          Bikesatwork