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9th Class Science Gravitation and Floatation Question Bank Gravitation

  • question_answer
    On a planet (whose size is the same and mass 4 times as that of the earth), the energy needed to lift a 2 kg mass vertically upwards through 2 m distance on the planet is \[(\text{g }=\text{ 1}0\text{ m}/{{\text{s}}^{\text{2}}}\]on the surface of the earth)

    A)  16 joules         

    B)         160 joules  

    C)  32 joules         

    D)         320 joules

    Correct Answer: B

    Solution :

     On the planet,\[g'=\frac{G\times 4M}{{{R}^{2}}}\] On the earth,       \[g=\frac{GM}{{{R}^{2}}}\] \[\therefore \]                  \[\frac{g'}{g}=4\] \[\Rightarrow \]                               \[g'=4g=\mathbf{40}\,\,\mathbf{m/}{{\mathbf{s}}^{\mathbf{2}}}\] Energy needed to lift \[2\,\,kg\] mass through\[2\,\,m\] distance,                 \[E=mg'h\]                     \[=2\times 40\times 2\]                     \[=160\] joules.


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