NCERT Solutions for Class 11 Physics Chapter 2 Motion in a Straight Line
Multiple Choice Questions
Q. 1 Among the four graph shown in the figure there is only one graph for which average velocity over the time interval (0, T) can vanish for a suitably chosen T. Which one is it?
Thinking Process
In this problem, we have to locate the graph which is having same displacement for two timings. When there are two timings for same displacement the corresponding velocities will be in opposite directions.
Ans. (b) In graph (b) for one value of displacement there are two different points of time.
Hence, for one time, the average velocity is positive and for other time is equivalent negative.
As there are opposite velocities in the interval 0 to T hence average velocity can vanish in (b). This can be seen in the figure given below.
Here, OA = BT (same displacement) for two different points of time.
Q. 2 A lift is coming from 8th floor and is just about to reach 4th floor. Taking ground floor as origin and positive direction upwards for all quantities, which one of the following is correct?
(a) x < 0, v < 0, a > 0 (b) x > 0, v < 0, a < 0
(c) x >0, v <0, a >0 (d) x >0, v >0, a <0
Ans. (a) As the lift is coming in downward directions displacement will be negative. We have to see whether the motion is accelerating or retarding.
We know that due to downward motion displacement will be negative. When the lift reaches 4th floor is about to stop hence, motion is retarding in nature hence, x < 0;
a > 0.
As displacement is in negative direction, velocity will also be negative i.e., v< 0.
This can be shown on the adjacent graph.
Multiple Choice Questions (More than One Option)
Very Short Answer Type Questions
Short Answer Type Questions
Ans.It is clear from the graph that displacement x is positive throughout. Ball is dropped from a height and its velocity increases in downward direction due to gravity pull. In this condition v is negative but acceleration of the ball is equal to acceleration due to gravity i.e., a = – g. When ball rebounds in upward direction its velocity is positive but acceleration is a = – g.
(a) The velocity-time graph of the ball is shown in fig. (i).
Q. 19 A bird is tossing (flying to and fro) between two cars moving towards each other on a straight road. One car has a speed of 18 km/h while the other has the speed of 27 km/h. The bird starts moving from first car towards the other and is moving with the speed of 36 km/h and when the two cars were separated by 36 km. What is the total distance covered by the bird?
Thinking Process
In this problem we have to use the concept of relative velocity it will be subtracted for velocities in same direction and added for velocities in opposite directions.
Ans. Given, Speed of first car = 18 km/h
Speed of second car = 27 km/h
Relative speed of each car w.r.t. each other = 18 + 27 = 45km/h
Long Answer Type Questions