SolveItNCERT · CBSE Boards

NCERT Exemplar · Class 9 Science Force and Laws of Motion

20 questions · 20 still being checked

Multiple Choice Questions 1–8 (part 1 of 3)

  1. Exercise 1

    Which of the following statement is not correct for an object moving along
    a straight path in an accelerated motion?
    (a)
    Its speed keeps changing
    (b)
    Its velocity always changes
    (c)
    It always goes away from the earth
    (d)
    A force is always acting on it

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (c)
    (c) It always goes away from the earth.Accelerated motion only requires velocity to change, and direction of travel relative to Earth is unrestricted: \[a \neq 0 \;\Rightarrow\; F_{net} \neq 0 \] so (a), (b), (d) hold; only (c) is false.

  2. Exercise 2

    According to the third law of motion, action and reaction
    (a)
    always act on the same body
    (b)
    always act on different bodies in opposite directions
    (c)
    have same magnitude and directions
    (d)
    act on either body at normal to each other

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (b)
    (b) Always act on different bodies, in opposite directions.\[\vec F_{AB} = -\vec F_{BA} \] Equal magnitude, opposite direction, each acting on the other body — never both on the same object.

  3. Exercise 3

    A goalkeeper in a game of football pulls his hands backwards after holding
    the ball shot at the goal. This enables the goal keeper to
    (a)
    exert larger force on the ball
    (b)
    reduce the force exerted by the ball on hands
    (c)
    increase the rate of change of momentum
    (d)
    decrease the rate of change of momentum

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (b)
    (b) Reduces the force exerted by the ball on hands.\[F=\dfrac{\Delta p}{\Delta t} \] Longer \(\displaystyle \Delta t\) lowers \(\displaystyle \Delta p/\Delta t\), so the force on the hands falls. NCERT_Solution_Class9_Science_Exemplar_Ch9_Q3

  4. Exercise 4

    The inertia of an object tends to cause the object
    (a)
    to increase its speed
    (b)
    to decrease its speed
    (c)
    to resist any change in its state of motion
    (d)
    to decelerate due to friction

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (c)
    (c) To resist any change in its state of motion.An object continues at rest or in uniform motion unless a net external force acts on it — that resistance to change is inertia.

  5. Exercise 5

    A passenger in a moving train tosses a coin which falls behind him. It
    means that motion of the train is
    (a)
    accelerated
    (b)
    uniform
    (c)
    retarded
    (d)
    along circular tracks

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (a)
    (a) Accelerated.\[a_{\text{train}} > 0\ (\text{forward}) \;\Rightarrow\; v_{\text{coin}}\ (\text{unchanged}) \;\Rightarrow\; \text{coin falls behind} \] If the train moved at uniform velocity the tossed coin would return to the same point in the passenger's hand.

  6. Exercise 6

    An object of mass 2\displaystyle 2 kg is sliding with a constant velocity of 4\displaystyle 4 m s−1\displaystyle s^{-1} on a
    frictionless horizontal table. The force required to keep the object moving
    with the same velocity is
    (a)
    32\displaystyle 32 N
    (b)
    0\displaystyle 0 N
    (c)
    2\displaystyle 2 N
    (d)
    8\displaystyle 8 N

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (b)
    (b) $\displaystyle 0$ N.\[v = \text{constant} \;\Rightarrow\; a = 0 \] \[F = ma = 2\ \text{kg} \times 0\ \text{m s}^{-2} = 0\ \text{N} \] A frictionless table offers no opposing force, so none is needed to sustain the motion.

  7. Exercise 7

    Rocket works on the principle of conservation of
    (a)
    mass
    (b)
    energy
    (c)
    momentum
    (d)
    velocity

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (c)
    (c) Momentum.\[\vec p_i = \vec p_f \;\Rightarrow\; 0 = m_{\text{rocket}}\vec v_{\text{rocket}} + m_{\text{gas}}\vec v_{\text{gas}} \] Hot gas is ejected backward at high speed; the rocket gains an equal, opposite momentum and moves forward.

  8. Exercise 8

    A water tanker filled up to 23\displaystyle \frac{2}{3} of its height is moving with a uniform speed.
    On sudden application of the brake, the water in the tank would
    (a)
    move backward
    (b)
    move forward
    (c)
    be unaffected
    (d)
    rise upwards

    Matches the book, not yet reviewed

    This working reaches the answer NCERT prints. It has not yet been read through by hand.

    NCERT’s answer
    (b)
    Short Answer Questions
    (b) Move forward.Sudden braking decelerates the tank, but the water's inertia keeps it moving at the old speed, so it surges toward the front wall relative to the tank.