SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science How Forces Affect Motion

26 questions · 19 still being checked

Pause and Ponder 6.3–6.4 (part 5 of 18)

  1. Exercise 6.3

    Hold the rubber band slightly stretched between your forefinger and thumb on the wooden table top (Fig. 6.12a). Mark points A and B at its ends as shown in Fig. 6.12b. Make another mark C up to which you will stretch the rubber band. (a) on the surface, and (c) rubber band stretched back to point C with stack of 4\displaystyle 4 coinsNCERT_Question_Class9_Science_Ch6_PP_Q6-3

    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 — no net force acts on it.
    Constant velocity means the magnitude and direction of the velocity are both unchanging, so the acceleration is zero.
    Newton's first law: an object in motion continues to move with a constant velocity unless a net force acts upon it.
    Individual forces may well be acting (weight, normal force, friction, an applied push), but they cancel out, so the net force is \(\displaystyle 0\ \text{N} \).

  2. Exercise 6.4

    Holding the ends of the rubber band at A and B, place the stack of coins near the middle of A and B. Now, using a finger of your other hand, push back the stack of coins till the rubber band is pulled back to the mark C (Fig. 6.12c). Then, release the stack of coins and observe its motion. Do you find that after losing contact with the rubber band, the velocity of the stack of coins decreases gradually and it comes to rest after travelling some distance? Measure the distance travelled from C and record it. Repeat this step twice.NCERT_Question_Class9_Science_Ch6_PP_Q6-4

    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
    (i)
    ; (ii)
    (i) and (ii) are possible; (iii) is not.
    (i) Possible — with zero net force an object at rest stays at rest (Newton's first law).
    (ii) Possible — an object already moving keeps moving with the same constant velocity.
    (iii) Not possible — constant acceleration \(\displaystyle a \) needs a net force \(\displaystyle F = ma \), and with \(\displaystyle F = 0\ \text{N} \) we get \(\displaystyle a = 0\ \text{m s}^{-2} \), so the object cannot be accelerating.