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Question
A truck of mass 1000 kg, increases its speed from 36 km h-1 to 72 km h-1. Find the increase is its kinetic energy.
Solution
Mass (m) = 1000 kg
1 km h−1 = `5/18` ms−1
Initial speed u = `36 "km" "h"^-1 = 36 × 5/18 = 10 "ms"^-1`
Final speed v = `72 "km" "h"^-1 = 72 ×5/18 = 20 "ms"^-1`
Increase in kinetic energy = ?
Increase in kinetic energy = `1/2 mv^2 − 1/2 m u^2`
= `1/2 m [(v + u ) (v-u)]`
= `1/2 × 1000 ( 20 + 10 ) ( 20 − 10 )`
= `1/2 × 1000 × 30 × 10`
Increase in kinetic energy = 150000 J = 1.5 × `10^5` J
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