5.4 Braking and collisions

• Natuurkunde
• Middelbare school
• havo, vwo
• Leerjaar 2

5.4 Braking and collisions

Knowing how to calculate the stopping distance

Knowing influencing factors on reaction distance

Knowing influencing factors on braking distance

Knowing how the use the formula's the right way

Bicycle Experiment

Get out your Handout

Discuss the questions and graph.

In traffic you always have to able to stop in time without having a collision.

What factors will influence the braking distance?

1. Mass of the car
2. Braking force
3. Speed of the car
4. All answers above

The higher the initial speed,
the longer the braking distance.

The greater the mass of the vehicle,
the longer the breaking distance.

If the road is slippery you have to brake slowly, othewise the car will slip.

Braking distance

Depends on:

• Initial speed;
  the faster you are going, the greater the braking distance
  
• Total mass;
  the greater the mass the greater the braking distance
  
• Braking force;
  the harder the brakes are pressed the shorter the braking distance

Stopping distance = reaction distance + braking distance

(Thinking Time)

Reaction distance

Reaction distance  =  reaction time  x  velocity

The reaction time in normal conditions is around 1.0 second.

STOPPING DISTANCE = Reaction Distance + Breaking Distance

Braking distance

Reaction distance

A horse runs to the finishline at 39.6 km/h. The drivers reaction time is 0.2 s. The braking distance is 22.8 m. Determine the stopping distance.

Formulas:
stopping distance = reaction distance + braking distance

reaction distance = reaction time x velocity

A horse runs to the finishline at 39.6 km/h. The drivers reaction time is 0.2 s. The braking distance is 22.8 m. Determine the stopping distance.

  v = 39.6 km/h = 11 m/s (39.6 / 3.6)

  t = 0.2 s (reaction time)

  braking distance = 22.8m 

stopping distance = reaction distance + braking distance

reaction distance = v x t = 11 x 0.2 = 2.2 m

           stopping distance = reaction distance + braking distance
           stopping distance = 22.8 + 2.2 = 25m

initial speed and braking distance

If the speed is n times bigger,&nbsp;<div>the braking distance becomes n² times longer.<br></div>

v = 10 m/s

s = 5 m

v = 20 m/s

s = 20 m

v = 30 m/s

s = 45 m

n^2&nbsp;= 2^2&nbsp;= 4<div>braking distance = 4 * 5 = 20 m</div>

n²&nbsp;= 3² = 9<br>braking distance = 9 * 5 = 45 m

If the initial velocity is 10 m/s the braking distance is 5 m. What is the braking distance with an initial velocity of 90 km/h?

If the initial velocity is 10 m/s the braking distance is 5 m. What is the braking distance with an initial velocity of 90 km/h?

v = 90 km/h = (90/3.6) m/s = 25 m/s

n = 25/10 = 2.5
n² = 2.5² = 6.25
braking distance = n² x old braking distance 
braking distance = 6.25 x 5
braking distance = 31.25 m

If the initial velocity is 10 m/s the braking distance is 5 m. What is the initial velocity with a braking distance of 80 m?

If the initial velocity is 10 m/s the braking distance is 5 m. What is the initial velocity with a braking distance of 80 m?

braking distance = 80 m

braking distance = n² x old braking distance 
80 = n² x 5
n² = 80 / 5 = 16
n = √16 = 4
initial velocity = 4 * 10 = 40 m/s

Which safety features can we find in a car?

Safety features against crashing

- Seatbelt

- Airbag

- Crumple zone

Give a non-traffic example where force being distributed over a larger surface area also protects something/someone
(against weapons for instance)

Give a non-traffic example where slowing down more slowly protects something/someone
(against high speed impact)

Homework

finish chapter 5

Breaking and collisions

Uniform deceleration

• Speed decreases evenly
  
• Speed decrease per second is 

      called deceleration

The deceleration here is 2 m/s²

a = -2 m/s²

Calculating stopping distance

You have a reaction time (of about 0,7 seconds) and then a breaking distance...