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Answer :
Given, mass of the car (m) = 1200 kg
Initial velocity (u) = 90 km/hour = 25 meters/sec
Terminal velocity (v) = 18 km/hour = 5 meters/sec
Time period (t) = 4 seconds
So, the acceleration of the car,
\( \therefore a = \frac{v-u}{t} \ \)
\( \Rightarrow \ a = \frac{5-25}{4} \ \)
\( \Rightarrow \ a = -5 \ m/s^2 \)
Therefore, the acceleration of the car is \( -5 m/s^2 \) .
Initial momentum of the car
= m × u
= (1200kg) × (25m/s)
= 30,000 kg m/s
Final momentum of the car
= m × v
= (1200) × (5)
= 6,000 kg m/s
\(\therefore \) change in momentum (final momentum – initial momentum)
= (6,000 – 30,000)
= -24,000 kg m/s
External force applied
= mass of car × acceleration
= (1200) × (-5) = -6000 N