Question

A 1500 kg car, moving at a speed of 20 m/s comes to a halt. How much work was done by the brakes?

Answer 1

Word done by brakes: -300,000J

Explanation:

Data:

mass of car : m = 1,500 kg

initial velocity of car: v1 = 20 m/s

final velocity of car: v2 = 0 m/s (car at halt)

kinetic energy at v1 : k1 = ?

kinetic energy at v2 : k2 = ?

Formula:

$k\text{ = }\frac{1}{2}\cdot\text{ m }\cdot v^2_1$$\begin{gathered} k1\text{ = }\frac{1}{2}\cdot\text{ 1500}\cdot20 \\ k1\text{ = 300,000 J} \end{gathered}$$\begin{gathered} k2\text{ = }\frac{1}{2}\cdot1500\cdot0 \\ k2\text{ = 0 J} \end{gathered}$$\begin{gathered} \text{Force done by brakes:} \\ k2\text{ - k1 = 0 - 300000 = -300,000 J} \end{gathered}$