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ICSE Class 8 Physics Question 8 of 25

Energy — Question 9

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Question

Question 9

A truck of mass 1000 kg increases its speed from 36 km h-1 to 72 km h-1. Find the increase in its kinetic energy.

Answer

Given:

Mass (m) = 1000 kg

1 km h-1 = 518\dfrac{\text{5}}{\text{18}} m s-1

36 km h-1 = 518\dfrac{\text{5}}{\text{18}} x 36 = 10 m s-1

So, initial speed (v1) = 10 m s-1

1 km h-1 = 518\dfrac{\text{5}}{\text{18}} m s-1

72 km h-1 = 518\dfrac{\text{5}}{\text{18}} x 72 = 20 m s-1

So, final speed (v2) = 72 km h-1 = 20 m s-1

Increase in its kinetic energy = ?

Increase in kinetic energy = 12\dfrac{\text{1}}{\text{2}} m[(v2)2 - (v1)2]

= 12\dfrac{\text{1}}{\text{2}} x 1000 x [(20)2 - (10)2]

= 500 x [400 - 100]

= 500 x 300

= 150000 J or 1.5 x 105 J

So increase in kinetic energy = 1.5 x 105 J.