# NCERT Physics Class 11 Exemplar Ch 3 Motion In A Straight Line Part 4

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Q. 20 A man runs across the roof-top of a tall building and jumps horizontally with the hope of landing on the roof of the next building which is of a lower height than the first. If his speed is 9 m/s, the (horizontal) distance between the two buildings is 10 m and the height difference is 9 m, will he be able to land on the next building?

Answer: Suppose that the fall of 9 m will take time t. Hence

Since

In this time, the distance moved horizontally is

Yes-he will land.

Q. 21 A ball is dropped from a building of height 45 m. Simultaneously another ball is thrown up with a speed Calculate the relative speed of the balls as a function of time.

Answer: Both are free falling. Hence, there is no acceleration of one w. r. t. another. Therefore relative speed remains constant

Q. 22 The velocity-displacement graph of a particle is shown in Figure.

(a) Write the relation between v and x.

(b) Obtain the relation between acceleration and displacement and plot it.

Answer:

The variation of a with x is shown in the figure. It is a straight line

with a positive slope and a negative intercept