2_4 On the Road

Look at the data for the BMW 3 Series below.What is the connection between power and

acceleration?

May 3, 2023

Model Acceleration0 – 62 mph in

s

PowerkW

Masskg

318 9.1 105 1435320 8.2 125 1445325 6.7 160 1535330 6.1 200 1555335 5.6 225 1610

2_4 On the RoadMay 3, 2023

Model Acceleration0 – 62 mph in

s

PowerkW

Power x

Accn

Masskg

318 9.1 105 956 1435320 8.2 125 1025 1445325 6.7 160 1072 1535330 6.1 200 1220 1555335 5.6 225 1260 1610

Any similarities?

2_4 On the Road

• What is the resultant force at constant velocity?

• What does the stopping distance of a vehicle depend on?

• What factors increase stopping distance?

May 3, 2023

The “Bloodhound” supersonic car is aiming to go faster than 1000mph! Not even a plane has been that fast at sea level!

Why do cars have a top speed?

Model Top speed, mph

Power, kW

X3 2.0 D 127 130120D 142 130320D 143 130520D 144 130520D Tourer

139 130

X3

120D

320D

520D

Tourer

What does this table tell you about these cars?What must happen to the forces at top speed for there to be no acceleration?

Forces on cars

• No resultant force means no acceleration.• Top speed is reached when the _________

from the ________ equals the _________ from the air resistance.

Force from engineAir resistance

Stopping distances

What happens if the thrust force reduces?How much distance would this need to stop at 500 m/s?

Car travelling at 1000mph

Drag force

Thrust force

Stopping distances

What happens if the thrust force reduces? Car slows down.

How much distance would this need to stop at 500 m/s? Around 2.5 miles!

Car travelling at 1000mph

Drag force

Thrust force

Highway code stopping distances

• Thinking distance = distance covered before brakes are applied.

• Braking distance = distance covered while braking.

What can increase stopping distance?

Thinking distance

Braking distance

Summary

• Thinking distance is how far a car travels before a driver hits the brakes.

• Braking distance is how far the car travels while braking.

• Stopping distance is thinking distance + braking distance.