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NCERT Exemplar · Class 9 Science Gravitation

26 questions · 26 still being checked

Multiple Choice Questions 11–15 (part 2 of 4)

  1. Exercise 11

    The force of attraction between two unit point masses separated by a unit
    distance is called
    (a)
    gravitational potential
    (b)
    acceleration due to gravity
    (c)
    gravitational field
    (d)
    universal gravitational constant

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    NCERT’s answer
    (d)
    (d) Universal gravitational constant.\[F = \frac{G\,m_1 m_2}{d^{2}}, \qquad m_1 = m_2 = 1,\ d = 1 \;\Rightarrow\; F = G \]By definition, the force between two unit masses at unit separation equals \(\displaystyle G\) itself.

  2. Exercise 12

    The weight of an object at the centre of the earth of radius R is
    (a)
    zero
    (b)
    infinite
    (c)
    R times the weight at the surface of the earth
    (d)
    1\displaystyle 1/R2\displaystyle R^{2} times the weight at surface of the earth

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    NCERT’s answer
    (a)
    (a) Zero.\[W = mg, \qquad g_{centre} = 0 \]At the earth's centre the pull of surrounding mass is symmetric in every direction and cancels exactly, so the net gravitational force, and the weight, is zero.

  3. Exercise 13

    An object weighs 10\displaystyle 10 N in air. When immersed fully in water, it weighs only
    8\displaystyle 8 N. The weight of the liquid displaced by the object will be
    (a)
    2\displaystyle 2 N
    (b)
    8\displaystyle 8 N
    (c)
    10\displaystyle 10 N
    (d)
    12\displaystyle 12 N

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    NCERT’s answer
    (a)
    (a) $\displaystyle 2$ N.\[\text{Loss of weight} = W_{\text{air}} - W_{\text{water}} \] \[= 10\ \text{N} - 8\ \text{N} = 2\ \text{N} \]By Archimedes' principle, this loss of weight equals the weight of the liquid the object displaces.

  4. Exercise 14

    A girl stands on a box having 60\displaystyle 60 cm length, 40\displaystyle 40 cm breadth and 20\displaystyle 20 cm
    width in three ways. In which of the following cases, pressure exerted by
    the brick will be
    (a)
    maximum when length and breadth form the base
    (b)
    maximum when breadth and width form the base
    (c)
    maximum when width and length form the base
    (d)
    the same in all the above three cases

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    NCERT’s answer
    (b)
    (b) Maximum when breadth and width form the base.\[P = \dfrac{F}{A} \]Weight \(\displaystyle F\) is fixed, so \(\displaystyle P\) is largest where the base area \(\displaystyle A\) is smallest.\[A_{L\times B} = 60\times40 = 2400\ \text{cm}^2 \] \[A_{B\times W} = 40\times20 = 800\ \text{cm}^2 \] \[A_{W\times L} = 20\times60 = 1200\ \text{cm}^2 \]\(\displaystyle A_{B\times W}\) is smallest, so that base gives the maximum pressure.

  5. Exercise 15

    An apple falls from a tree because of gravitational attraction between the
    earth and apple. If F1\displaystyle F_{1} is the magnitude of force exerted by the earth on the
    apple and F2\displaystyle F_{2} is the magnitude of force exerted by apple on earth, then
    (a)
    F1\displaystyle F_{1} is very much greater than F2\displaystyle F_{2}
    (b)
    F2\displaystyle F_{2} is very much greater than F1\displaystyle F_{1}
    (c)
    F1\displaystyle F_{1} is only a little greater than F2\displaystyle F_{2}
    (d)
    F1\displaystyle F_{1} and F2\displaystyle F_{2} are equal

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    (d)
    Short Answer Questions
    (d) \(\displaystyle F_1\) and \(\displaystyle F_2\) are equal.\[F = G\dfrac{M_{\text{earth}}\,m_{\text{apple}}}{r^2} \]By Newton's third law the force on the apple and the reaction force on the earth are equal and opposite.\[\vec{F}_1 = -\vec{F}_2 \]