CBSE Class 9 Answered

Part (a)

Initial total energy when the object starts falling from a height h = 10 m is only potential energy.

Potential energy at a height h =  ( m g h )

where m is mass , g is acceleration due to gravity and h is height

Initial total energy  E when the object is at height h = 10 m is

E = 1 × 9.8 × 10 J = 98 J

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Part (b)

Potential energy U(4) at a height h = 4 m is

U(4) = 1 × 9.8 × 4  J = 39.2 J

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Part (c)

Kinetic energy K(4) at a height h = 4 m is difference of total energy E and potential energy U(4)

K(4) = ( 98 - 39.2 ) J = 58.8 J

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Part (d)

When the object just before reaches ground its initial total energy is converted to kinietic energy K(0)

K(0) = E = 98 J

KInetic energy K(0) = (1/2) m v² = 98 J

where v is speed of object just before reaches ground.

From above expression, speed v is calculated as