(^416) A Textbook of Engineering Mechanics
EXERCISE 19.3
- Two ships A and B, at a given instant, are 5 km away from each other. The ship A is
South-East of the ship B. If the ship B is travelling at 10 km.p.h. due East and the ship A
is travelling at 16 km.p.h. due North, determine (i) velocity of the ship A relative to the
ship B ; (ii) shortest distance between the two ships ; and (iii) the time when the two ships
are nearest to each other. (Ans. 18.9 km.p.h. ; 1.1 km ; 15.5 min.) - At noon, a ship A is 10 kilometers due East of another ship B, and is travelling at 14
km.p.h. in a direction 30° West of North. The ship B is travelling North-East at 10 km.p.h.
Find the velocity of ship A relative to B and the least distance apart and time when this
occurs. (Ans. 15 km.p.h. ; 20° ; 37.6 min.) - At midnight, two ships A and B are 80 km apart, the ship B being East of A. The ship A is
steaming at a speed of 10 km.p.h. towards South, while ship B is steaming at 22 km.p.h. on
South-East course. Find the velocity of ship B relative to A.
If the two ships can exchange signals when they are 30 km apart, find when the two ships
can start signalling and how long they can continue to signal?
(Ans. 16.5 km.p.h. ; 1 hour 36 min.)
QUESTIONS
- What do you understand by the term ‘relative velocity’?
- What are the different methods of finding out the relative velocity of two bodies moving
along inclined directions? Explain one of them. - How would you determine the time, when the two bodies moving along inclined directions,
are closest to each other? How would you find out this distance? - Explain clearly, how would you find out the time for exchange of signals of the two bodies
moving along inclined directions?
OBJECTIVE TYPE QUESTIONS
- The relative velocity of A with respect to B is the velocity with which A appears to move
to an observer sitting in B when it is
(a) at rest (b) in motion (c) either (a) or ( b) - The rain is falling vertically downwards, but it appears to fall at some angle to a man
walking along a road. If the man increases his speed, the inclination of the rain with the
vertical will
(a) increase (b) remain the same (c) decrease - The relative velocity of two bodies moving along inclined directions is found out by
superimposing the actual velocities of both the bodies.
(a) Agree (b) Disagree - When the two ships are moving along inclined directions, then the time when the two
ships will be closest together depends upon
(a) velocity of one of the ships
(b) velocity of both the ships
(c) angle between the two directions
(d) all of the above
ANSWERS
- (c) 2. (a) 3. (b) 4. (d)
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