Assertion and Reason Questions for Class 11 Physics Chapter 3 Motion in a Straight Line

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Directions : Each of these questions contain two statements, Assertion and Reason. Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Q.1. Assertion : A body may be accelerated even when it is moving uniformly.
Reason : When direction of motion of the body is changing, the body must have acceleration.

Answer Answer: (a) In uniform circular motion, there is acceleration of constant magnitude.

Q.2. Assertion : Displacement of a body may be zero when distance travelled by it is not zero.
Reason : The displacement is the longest distance between initial and final position.

Answer Answer: (c) The displacement is the shortest distance between initial and final position. When final position of a body coincides with its initial position, displacement is zero, but the distance travelled is not zero.

Q.3. Assertion : For one dimensional motion the angle between acceleration and velocity must be zero.
Reason : One dimensional motion is always on a straight line.

Answer Answer: (d) One dimensional motion is always along straight line. But acceleration may be opposite of velocity and so angle between them will be 180°.

Q.4. Assertion : Displacement of a body is vector sum of the area under velocity– time graph.
Reason : Displacement is a vector quantity.

Answer Answer: (a) According to definition, displacement = velocity × time. Since displacement is a vector quantity so its value is equal to the vector sum of the area under velocity-time graph.

Q.5. Assertion : The position-time graph of a uniform motion, in one dimension of a body cannot have negative slope.
Reason : In one – dimensional motion the position does not reverse, so it cannot have a negative slope.

Answer Answer: (c)

Q.6. Assertion : Position-time graph of a stationary object is a straight line parallel to time axis.
Reason : For a stationary object, position does not change with time.

Answer Answer: (a) Position-time graph for a stationary object is a straight line parallel to time axis showing that not change in position with time.

Q.7. Assertion : Velocity-time graph for an object in uniform motion along a straight path is a straight
line parallel to the time axis.
Reason : In uniform motion of an object velocity increases as the square of time elapsed.

Answer Answer: (c) In uniform motion the object moves with uniform velocity, the magnitude of its velocity at different instance i.e., at t = 0, t =1, sec, t = 2sec ….. will always be constant. Thus velocity-time graph for an object in uniform motion along a straight path is a straight line parallel to time axis.

Q.8. Assertion : The average and instantaneous velocities have same value in a uniform motion.
Reason : In uniform motion, the velocity of an object increases uniformly.

Answer Answer: (c) In uniform motion the speed is same at each instant of motion.

Q.9. Assertion: The average velocity of the object over an interval of time is either smaller than or equal to the average speed of the object over the same interval.
Reason: Velocity is a vector quantity and speed is a scalar quantity.

Answer Answer: (a) Because displacement ≤ distance and so average velocity ≤ average speed.

Q.10. Assertion : The speedometer of an automobile measure the average speed of the automobile.
Reason : Average velocity is equal to total displacement per total time taken.

Answer Answer: (d) Speedometer measures instantaneous speed of automobile.

Q.11. Assertion : An object can have constant speed but variable velocity.
Reason : Speed is a scalar but velocity is a vector quantity.

Answer Answer: (a) Since velocity is a vector quantity, hence as its direction changes keeping magnitude constant, velocity is said to be changed. But for constant speed in equal time interval distance travelled should be equal.

Q.12. Assertion : position-time graph of a body moving uniformly in a straight line parallel to position axis. Says body is at rest.
Reason : The slope of position-time graph in a uniform motion gives the velocity of an object.

Answer Answer: (c) If the position-time graph of a body moving uniformly in a straight line parallel to position axis, it means that the position of body is changing at constant time. The statement is abrupt and shows that the velocity of body is infinite.

Q.13. Assertion : A particle starting from rest and moving with uniform acceleration travels’ a length of x and 3x in first two and next two-seconds.
Reason : Displacement is directly proportional to velocity.

Answer Answer: (c)

Q.14. Assertion : A body is momentarily at rest when it reverses the direction.
Reason : A body cannot have acceleration if its velocity is zero at a given instant of time.

Answer Answer: (c)

Q.15. Assertion : The position-time graph of a uniform motion in one dimension of a body can have negative slope.
Reason : When the speed of body decreases with time, the position-time graph of the moving body has negative slope.

Answer Answer: (c) Negative slope of position time graph represents that the body is moving towards the negative direction and if the slope of the graph decrease with time then it represents the decrease in speed i.e. retardation in motion.

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