SolveItNCERT · CBSE Boards

NCERT Exemplar · Class 9 Science Work and Energy

28 questions · 28 still being checked

Multiple Choice Questions 1–9 (part 1 of 3)

  1. Exercise 1

    When a body falls freely towards the earth, then its total energy
    (a)
    increases
    (b)
    decreases
    (c)
    remains constant
    (d)
    first increases and then decreases

    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) Remains constant.\[E_{total} = KE + PE \]In free fall, with no air resistance, mechanical energy is conserved: height lost as \(\displaystyle PE\) reappears exactly as \(\displaystyle KE\). NCERT_Solution_Class9_Science_Exemplar_Ch11_Q1

  2. Exercise 2

    A car is accelerated on a levelled road and attains a velocity 4\displaystyle 4 times of its
    initial velocity. In this process the potential energy of the car
    (a)
    does not change
    (b)
    becomes twice to that of initial
    (c)
    becomes 4\displaystyle 4 times that of initial
    (d)
    becomes 16\displaystyle 16 times that of initial

    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) Does not change.\[PE = mgh \]On a levelled road \(\displaystyle h\) is fixed, so \(\displaystyle PE\) cannot change with speed; only \(\displaystyle KE\propto v^{2}\) rises — here to $\displaystyle 16$ times its initial value.

  3. Exercise 3

    In case of negative work the angle between the force and displacement is
    (a)
    00\displaystyle 0^{0}
    (b)
    450\displaystyle 45^{0}
    (c)
    900\displaystyle 90^{0}
    (d)
    1800\displaystyle 180^{0}

    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
    (d)
    (d) \(\displaystyle 180^{\circ}\).\[W = Fd\cos\theta \] \[\theta = 180^{\circ} \;\Rightarrow\; \cos\theta = -1 \;\Rightarrow\; W = -Fd \]Only an angle past \(\displaystyle 90^{\circ}\) makes work negative, and \(\displaystyle 180^{\circ}\) is the sole such option offered.

  4. Exercise 4

    An iron sphere of mass 10\displaystyle 10 kg has the same diameter as an aluminium
    sphere of mass is 3.5\displaystyle 3.5 kg. Both spheres are dropped simultaneously from a
    tower. When they are 10\displaystyle 10 m above the ground, they have the same
    (a)
    acceleration
    (b)
    momenta
    (c)
    potential energy
    (d)
    kinetic energy

    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) Acceleration.\[a = \frac{F}{m} = \frac{mg}{m} = g \quad (\text{air resistance neglected}) \]\(\displaystyle g\) carries no mass term, so both spheres accelerate identically; their differing masses instead give different momenta, kinetic energies and potential energies at the same height.

  5. Exercise 5

    A girl is carrying a school bag of 3\displaystyle 3 kg mass on her back and moves 200\displaystyle 200
    m on a levelled road. The work done against the gravitational force will
    be (g =10\displaystyle 10 m s−2\displaystyle s^{-2})
    (a)
    6\displaystyle 6 ×103\displaystyle 10^{3} J
    (b)
    6\displaystyle 6 J
    (c)
    0.6\displaystyle 6 J
    (d)
    zero

    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
    (d)
    (d) Zero.\[F = mg = (3\text{ kg})(10\text{ m s}^{-2}) = 30\text{ N} \] \[W = Fs\cos\theta = (30\text{ N})(200\text{ m})\cos 90^{\circ} = 0\text{ J} \]Her weight acts straight down while she walks straight across, so force and displacement are perpendicular and gravity does zero work. NCERT_Solution_Class9_Science_Exemplar_Ch11_Q5

  6. Exercise 6

    Which one of the following is not the unit of energy?
    (a)
    joule
    (b)
    newton metre
    (c)
    kilowatt
    (d)
    kilowatt hour

    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) Kilowatt.\[1\text{ kW} = 1000\text{ J s}^{-1} \]That is a rate of energy transfer — power — not an amount of energy; joule, newton metre and kilowatt hour are all genuine energy units.

  7. Exercise 7

    The work done on an object does not depend upon the
    (a)
    displacement
    (b)
    force applied
    (c)
    angle between force and displacement
    (d)
    initial velocity of the object

    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
    (d)
    (d) Initial velocity of the object.\[W = Fd\cos\theta \]Work depends only on the force, the displacement and the angle between them — velocity, initial or otherwise, appears nowhere in this formula.

  8. Exercise 8

    Water stored in a dam possesses
    (a)
    no energy
    (b)
    electrical energy
    (c)
    kinetic energy
    (d)
    potential energy

    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
    (d)
    (d) Potential energy.\[PE = mgh \]Water held at height \(\displaystyle h\) is at rest, so it has no \(\displaystyle KE\); its height above the base gives it stored \(\displaystyle PE\), released as it falls.

  9. Exercise 9

    A body is falling from a height h. After it has fallen a height
    h 2\displaystyle 2 , it will possess
    (a)
    only potential energy
    (b)
    only kinetic energy
    (c)
    half potential and half kinetic energy
    (d)
    more kinetic and less potential energy

    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) Half potential and half kinetic energy.\[PE_{h/2} = mg\left(\frac{h}{2}\right) = \frac{mgh}{2} \] \[KE_{h/2} = mgh - mg\left(\frac{h}{2}\right) = \frac{mgh}{2} \]Both halves equal \(\displaystyle \tfrac{1}{2}(mgh)\), the total energy — the split is exactly even at the midpoint of the fall.