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Question
Find the average frictional force needed to stop a car weighing 500 kg at a distance of 25 m if the initial speed is 72 km/h.
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Solution
\[\text{ Given } : \]
\[\text{ Mass of the car, m = 500 kg } \]
\[\text{ Distance covered by the car, s = 25 m }\]
\[\text{ Initial speed of the car, u = 72 km/h = 20 m/s } \]
\[\text{ Final speed of the car, } \nu = \text{ 0 m/s } \]
Retardation of the car,
\[a = \frac{\left( \nu^2 - u^2 \right)}{2s}\]
\[ \Rightarrow a = - \frac{400}{50} = - 8 \text{ m/ s}^2 \]
\[\text{ Frictional force, F = ma } = 500 \times 8 = 4000 N\]
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