Question

An object of mass 1 kg is dropped from a height of 20 m. It hits the ground and
rebounds with the same speed. Find the change in momentum.(Take g=10 m/s2)

TNSCERT Class 10 Physics Ch 15 Laws of Motivation and Gravitation

Answer 1

mass of the object, m = 1kg

the object is dropped from the height , h = 20m

so, initial velocity of the object, u = 0

using formula, v² = u² + 2as To find final velocity , v.

here, u = 0, a = g = 10m/s² and s = h = 20m

so, v² = 0² + 2(10)(20)

or, v² = 400 ⇒v = 20m/s

here initial velocity , u' velocity of object when it hits the ground = 20m/s

final velocity , v' = -u' = -20m/s

now, change in momentum = final momentum - initial momentum

= mv'- mu'

= -mu' - mu'

= -2mu'

= -2 ×1kg × 20m/s [ from eq. (1) ]

= 40 Kgm/s

hence, magnitude of change in momentum = 40kgm/s