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CBSE-XI-Physics

07: Circular Motion

with Solutions -

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  • #
    Section : i
  • Qstn #1
    You are driving a motorcycle on a horizontal road. It is moving with a uniform velocity. Is it possible to accelerate the motorcycle without putting higher petrol input rate into the engine?
    Ans : Yes, it is possible to accelerate the motorcycle without putting higher petrol input rate into the engine by driving the motorcycle on a circular track.
  • Qstn #2
    Some washing machines have cloth driers. It contains a drum in which wet clothes are kept. As the drum rotates, the water particles get separated from the cloth. The general description of this action is that “the centrifugal force throws the water particles away from the drum”. Comment on this statement from the viewpoint of an observer rotating with the drum and the observer who is washing the clothes.
    Ans : The person rotating with the drum will observe that centrifugal force and coriolis force act on the water particles and the person washing the cloth will observe that water particles are thrown outward (away from the drum) and no pseudo force is acting on the particles.
  • Qstn #3
    A small coin is placed on a record rotating at
    3313rev/minute. The coin does not slip on the record. Where does it get the required centripetal force from?
    Ans : The coin gets the required centripetal force from the frictional force between the coin and the record.
  • Qstn #4
    A bird while flying takes a left turn, where does it get the centripetal force from?
    Ans : The bird tilts its body and tail in such a way that the air around offers a dragging force in left direction, perpendicular to its initial direction of motion. This dragging force provides the necessary centripetal force to take the left turn.
  • Qstn #5
    Is it necessary to express all angles in radian while using the equation ω = ω0 + at?
    Ans : No, it is not necessary to express all the angles in radian while using the equation ω = ω0 + at.
    If ω (angular velocity) and α (angular acceleration) are in rad/s and rad/s2, respectively, we will get the angle in radian.
  • Qstn #6
    After a good meal at a party you wash your hands and find that you have forgotten to bring your handkerchief. You shake your hand vigorously to remove the water as much as you can. Why is water removed in this process?
    Ans : While shaking, our hand moves on a curved path with some angular velocity and water on our hand feels centrifugal force in the outward direction. Therefore, water get detached from our hand and leaves it.
  • Qstn #7
    A smooth block loosely fits in a circular tube placed on a horizontal surface. The block moves in a uniform circular motion along the tube. Which wall (inner or outer) will exert a nonzero normal contact force on the block?
    Figure
    Ans : The outer wall will exert a non-zero normal contact force on the block. As the block moves in a uniform circular motion, centrifugal force in radially outward direction acts on it and it comes in contact with the outer wall of the tube.
  • Qstn #8
    Consider the circular motion of the earth around the sun. Which of the following statements is more appropriate?
    -(a) Gravitational attraction of the sun on the earth is equal to the centripetal force.
    (b) Gravitational attraction of the sun on the earth is the centripetal force.
    digAnsr:   a
    Ans : (a) Gravitational attraction of the Sun on the Earth is equal to the centripetal force. This statement is more appropriate.
    Gravitational attraction of the Sun on the Earth provides the necessary centripetal force required for the circular motion of the Earth around the Sun.
  • Qstn #9
    A car driver going at some speed v suddenly finds a wide wall at a distance r. Should he apply brakes or turn the car in a circle of radius r to avoid hitting the wall?
    Ans : As the wall is at a distance r, the driver should either take a circular turn of radius r or apply brakes to avoid hitting the wall.
  • Qstn #10
    A heavy mass m is hanging from a string in equilibrium without breaking it. When this same mass is set into oscillation, the string breaks. Explain.
    Ans : When the same mass is set into oscillation, the tension in the string increases because of the additional centripetal force of the mass oscillating in a curved path.
  • #
    Section : ii
  • Qstn #1
    When a particle moves in a circle with a uniform speed
    (a) its velocity and acceleration are both constant
    (b) its velocity is constant but the acceleration changes
    (c) its acceleration is constant but the velocity changes
    (d) its velocity and acceleration both change.
    digAnsr:   d
    Ans : (d) its velocity and acceleration both change.
    In a circular motion, the direction of particle changes. Therefore, velocity, being a vector quantity, also changes.
    As the velocity changes, acceleration also changes.
  • Qstn #2
    Tow cars having masses m1 and m2 moves in circles of radii r1 and r2 respectively. If they complete the circle in equal time, the ratio of their angular speed ω1/ω2 is
    (a) m1/m2
    (b) r1/r2
    (c) m1r1/m2/r2
    (d) 1.
    digAnsr:   d
    Ans : (d) 1
    Time period (T) is same for both the cars.
    We know that:
    `` \omega \propto \frac{1}{T}``
    `` \,\mathrm{\,So \,},\frac{{\omega }_{1}}{{\omega }_{2}}=1``
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