Physics · Kinematics · chapter 2 of 14

Class 11 Physics Chapter 2: Motion in a Straight Line Assertion-reason questions with answers

2 assertion-reason questions on Motion in a Straight Line, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): The velocity of a body at an instant is equal to the slope of the tangent to its position-time graph at that instant. Reason (R): Instantaneous velocity is the limit of the average velocity Δx/Δt as Δt → 0, and in this limit the chord joining two points of the graph becomes the tangent.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true, and R explains A: the average velocity over an interval is the slope of the chord, and shrinking the interval turns the chord into the tangent at the instant.

Question 2
Assertion (A): For a body falling freely near the earth's surface, its velocity increases by equal amounts in equal intervals of time. Reason (R): In free fall, the change of velocity per metre fallen is the same throughout the fall.
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Answer: (c) A is true, but R is false.

A is true: free fall has a constant acceleration g. R is false: Galileo found that the change of velocity with distance is not constant; it decreases as the distance of fall increases. It is the rate of change with time that is constant.

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