Road science

Activity 1 – Speed and Acceleration

Speed

Average speed    =     Distance traveled

                      Time taken

v_av =   _d

           ^t

Three methods to calculate the average speed

1.       Use graph 1 to calculate average speed (the car travels 20 m in 4½ s)

2. You can also use the slope of the graph: rise/run (40 m / 10 s)
3. Or add initial speed to the final speed and divide by two (10 m/2.24 s + 40 m/10 s divided by 2 = (4.4 + 4.0) = 4.2 m/s

 

d = vt                             

t = d/v                   d

v = d/t

                              v       t

 

 

Note: to convert from km/h to m/s divide by 3.6

 

Instantaneous speed can be calculated from the slope of the graph – Calculate the instantaneous speeds of Car C at 1 s, 5 s, and 9 s,

 

Exercises

 

 

Acceleration occurs when an object changes speed.

 

Average acceleration = change in speed

time taken

 

When an object gets faster it accelerates. If the speed is increasing steadily there is constant acceleration.

When it gets slower it decelerates.

 

Ticker timers can be used to measure acceleration. As a paper tape is pulled through the timer, the timer makes a dot on it every 0.1 s.

• If the dots are evenly spaced there is constant speed.
• If the dots move further apart, there is acceleration.
• If the dots become closer together, there is deceleration.

 

Activity 2

 

Exercises.

 

 

 

Stopping distances depend on reaction time, speed of the vehicle, conditions of the brakes, tyres and road surface.

 

Activity 3 – reaction times.

 

List all of the steps between seeing an object on the road and your pushing the brake pedal.

The distance that the car moves during this sequence is the reaction distance. It increases with increased vehicle speed.

 

The braking distance is the distance the car travels from when the brakes are applied until the car stops. It can be affected by the condition of the brakes, the tyres, whether the road is wet, dry, gravel, etc.

 

The stopping distance is the combination of reaction distance with braking distance.

 

Exercises

 

 

Tyres grip the road by friction.

There are two types of friction – static friction and sliding friction.