Assertion and Reason Questions for Class 9 Science Chapter 8 Motion

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Directions: In each of the following questions, a statement of Assertion is given and a corresponding statement of Reason is given just below it. Of the statements, given below, mark the correct answer as:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.

Q.1. Assertion : An object may acquire acceleration even if it is moving at a constant speed.
Reason : With change in the direction of motion, an object can acquire acceleration.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
This is the equation of a parabola. A parabola results when one quantity is proportional to the square of the other. When an object is moving with a constant velocity (zero acceleration), the displacement versus time graph is a straight line and its slope is velocity. When an object is moving at a constant speed but its direction of motion changes, its velocity changes ant thus acceleration is produced. Motion of an object in a circular path is such an example. In a uniform circular motion, the direction of motion of the object changes continuously and hence the velocity changes continuously even though the speed is constant.

Q.2. Assertion : Displacement of an object may be zero even if the distance covered by it is not zero.
Reason : Displacement is the shortest distance between the initial and final position.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
When the final position of an object coincides with its initial position, displacement is zero, but the distance travelled is not zero.

Q.3. Assertion : The graph between two physical quantities P and Q is straight line, when P/Q is constant.
Reason : The straight line graph means that P is proportional to Q or P is equal to constant multiplied by Q.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Q.4. Assertion : Velocity versus time graph of a particle in uniform motion along a straight path is a line parallel to the time axis.
Reason : In uniform motion the velocity of a particle increases as the square of the time elapsed.

Answer Answer: (c) Assertion is true but reason is false.

Q.5. Assertion : the speedometer of a car measures the instantaneous speed of the car.
Reason : Average speed is equal to the total distance covered by an object divided by the total time taken.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.
The speedometer of a car measures the instantaneous speed of the car.

Q.6. Assertion : An object may have acceleration even if it is moving with uniform speed.
Reason : An object may be moving with uniform speed but it may be changing its direction of motion.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
The uniform motion only means that the object is moving at a constant speed but its direction of motion may be changing at in the case of uniform circular motion. Hence, acceleration is produced in uniform motion due to changes in velocity.

Q.7. Assertion : Motion with uniform velocity is always along a straight line path.
Reason : In uniform velocity, speed is the magnitude of the velocity and is equal to the instantaneous velocity.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.
Uniform velocity means that speed and direction remain unchanged.

Q.8. Assertion : If a particle is moving with constant velocity, then average velocity for any time interval is equal to instantaneous velocity.
Reason : If average velocity of a particle moving on a straight line is zero for a given time interval, then instantaneous velocity at some instant within this interval may be zero.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Q.9. Assertion : The displacement of an object can be either positive, negative or zero.
Reason : Displacement has both the magnitude and direction.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.
Displacement may be positive, negative or zero. Displacement is a vector quantity.

Q.10. Assertion : When the displacement of a body is directly proportional to the square of the time. Then the body is moving with uniform acceleration.
Reason : The slope of velocity-time graph with time axis gives acceleration.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

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