SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Work, Energy, and Simple Machines

28 questions · 20 still being checked

Pause and Ponder 7.1–7.2 (part 2 of 21)

  1. Exercise 7.1

    In the previous chapter, a weightlifter is shown holding a barbell steady in her hands (Fig. 6.8\displaystyle 6.8). Is she doing any work on the barbell while holding it steady?

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    NCERT’s answer
    No
    No — she does no work on the barbell while holding it steady.
    Work needs displacement: \(\displaystyle W = F \times s \). Here the barbell does not move, so \(\displaystyle s = 0 \) and \(\displaystyle W = 0 \), however large the force she applies.
    She still feels tired because her muscles keep contracting and expanding and use up the internal energy of her body — but that is not work done on the barbell.

  2. Exercise 7.2

    Is the work done by friction on the stack of coins that travels on a rough surface (Fig. 6.13c) — positive, negative or zero?

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    NCERT’s answer
    Negative
    Negative.
    Friction on the sliding stack of coins acts opposite to the direction of its displacement.
    When the force and the displacement point in opposite directions, the work done by that force is negative.
    That is why the coins lose energy, slow down and finally stop on the rough surface.