20 Physics -- Work, Energy and Power

A cord is used to lower vertically a block of mass M, through a distance "d" at a constant downward acceleration of g/8. The work done by the cord on the block isa. Mgd b. Mgd/8c. 3Mgd/8d. -7Mgd/8

A cord is used to lower vertically a block of mass M, through a distance "d" at a constant downward acceleration of g/8. The work done by the cord on the block is

a. Mgd

b. Mgd/8

c. 3Mgd/8

d. -7Mgd/8

1 Answers

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Here, Ma=Mg-T

or, M(g/8)=Mg-T

or, T= M(g-g/8)

or, T= 7Mg/8

so, work done = T*d cos180⁰

=7Mg/8 * (-d)

= -7Mgd/8

So, correct option is d

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Now, in center of mass frame of reference,

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In lab frame of reference,

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The change in magnitude of the ball is 28 kgm/s.

Solution:

Here,

mass of the ball (m) = 2kg

initial velocity of the ball (u) = 8 m/s (say downward direction is positive)

final velocity of the ball (v) = -6 m/s (because in upward direction)

Hence,

change in linear momentum of the ball (delta P) = m(v-u)

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