Question

calculate the amount of work needed to stop a car of 250 kg, moving at a speed of 18 km/hr

Answer 1

Answer on Question 58483, Physics, Other

Question:

Calculate the amount of work needed to stop a car of $250\,kg$, moving at a speed of $18\,km/hr$ ($18\,km/hr = 18 \cdot \frac{1000\,m}{3600\,s} = 5\,ms^{-1}$).

Solution:

Let's apply the Work-Energy theorem. It states that the work done by the net force acting on a car (friction force) is equal to change in the kinetic energy of the car:

$$\begin{array}{l} Work = -F_{fr}d = KE_{f} - KE_{i} = \frac{1}{2} \cdot 250\,kg \cdot (0\,ms^{-1})^2 - \frac{1}{2} \cdot 250\,kg \cdot (5\,ms^{-1})^2 \\ = -3125\,J. \end{array}$$

Sign minus indicates that the friction force is directed in the opposite direction to the motion of the car.

Answer:

$W = 3125\,J$.

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