SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science How Forces Affect Motion

26 questions · 19 still being checked

Revise, Reflect, Refine 6.5–6.6 (part 8 of 18)

  1. Exercise 6.5

    When a net force acts on an object, we observe that the object accelerates: (i) opposite to the direction of force, with acceleration proportional to the force acting on the object. (ii) opposite to the direction of force, with acceleration proportional to the mass of the object. (iii) in the direction of force, with acceleration inversely proportional to the force acting on the object. (iv) in the direction of force, with acceleration proportional to the force acting on the object.

    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
    (iv)
    Correct option: (iv) in the direction of force, with acceleration proportional to the force acting on the object.
    NCERT_Solution_Class9_Science_Ch6_RRR_Q6-5
    Newton's second law: the object accelerates in the direction of the net force, which rules out (i) and (ii).
    The magnitude of the acceleration is proportional to the net force, \(\displaystyle a \propto F \), which rules out (iii)'s "inversely proportional to the force".
    It is the mass that appears inversely, \(\displaystyle a = \dfrac{F}{m} \).

  2. Exercise 6.6

    The position-time graph for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on: (i) Object A (ii) Object B (iii) Object C (iv) Object D Position 0\displaystyle 0 Time Object ANCERT_Question_Class9_Science_Ch6_RRR_Q6-6

    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
    (iii)
    A net force acts on (iii) Object C.
    On a position-time graph, the slope is the velocity.
    A straight line means the slope, and hence the velocity, is constant — a horizontal line means the object is at rest, a sloping line means constant velocity. In both cases the acceleration is zero, so by Newton's first law the net force is zero (this is exactly Fig. 6.15a and Fig. 6.16a).
    A curved line means the slope keeps changing, so the velocity is changing, the object is accelerating and a net force must be acting.
    In Fig. $\displaystyle 6.37$ only the graph of Object C is curved, so C is the one experiencing a net force.