What is the braking distance for a small car at 40 km/h during emergency braking?

[Francis]

Braking distance is directly proportional to the square of the speed and inversely proportional to the friction coefficient. The friction coefficient μ is related to various factors, with a general value of around 0.8. It can drop below 0.2 on rainy days and even lower on icy or snowy roads. Assuming a friction coefficient μ of 0.8, the braking distance at a speed of 40 km/h is 7.9 meters. Rust on car brake discs is a normal phenomenon. Here is a detailed introduction about it: 1. Slight rust: Minor rust often occurs, such as after the car is left in the rain overnight or when it doesn't dry properly after washing. In such cases, continuous braking during driving can help remove the rust, as the brake disc rubs against the brake caliper during braking, and the rust will be worn off after multiple applications of the brakes. 2. Severe rust: If the brake disc has severe rust, the entire surface of the brake disc will be covered with a relatively thick layer of rust. Stubborn rust can also accelerate the wear of brake pads, so it's necessary to take the car to a repair shop to have the brake disc polished.