Time, Speed and Distance
BASIC Formulas for Time and speed calculations:
1. (a) Total Distance 'd' covered by a body traveling at a speed 's' with 't' as the
time of travel is given by
d = s x t
(b). Total time taken by a body to cover a distance 'd' in when traveling at a
speed 's' is given by
t = d/s
(c) Speed of a body that covered a distance of 'd' in time 't' is
s = d/s
TIPS And TRICKS:
1. To convert speed in km/hr to m/s, multiply the given speed by (5/18) to get the speed in m/s.
for example to convert the speed 40km/hr to speed in m/s, multiply 40 by (5/18) to get 40 km/hr = (40)x(5/18) m/s = 11.11 m/s
2. To convert speed in m/s to km/hr, multiply the given speed by (18/5) to get the speed in km/hr.
for example to convert the speed 20 m/s to speed in km/hr multiply 20 by (18/5) to get 20m/s = (20)x(18/5) km/hr = 72 km/hr
3. For the types of questions where we have to find the average speed when two equal distances are traveled with different speeds, say x and y , the formula is
average speed = (2xy) /(x + y)
(a) when there are 3 speeds(x,y,z) involved then, the formula is
average speed = (3xyz)/ (xy + yz +zx )
(b) when there are 4 speeds (w,x,y,z) involved then, the formula is
average speed = (4xyz)/ (xyz + yz +zx )
4. For the type of questions where a distance is traveled at different speeds (x and y )from the usual speed and the difference of time from the usual time taken is given, we use the formula to get the distance as
Distance = [(x x y)( x- y)]x(t1 - t2)
Note: (i) for time t1 and t2, use different signs (+ve, -ve) for someone
being late or early.
(ii) if someone is late while traveling at the given speeds use the
same sign for both the situations. The same is applied when some
one is early in both the situation.
5. If two bodies are traveling opposite to each other at x km/hr and y km/hr respectively,then the relative speed would be (x + y) km/hr
6. If two bodies are traveling opposite to each other at x km/hr and y km/hr respectively,then the relative speed would be (x - y) km/hr
7. Distance covered by a train while crossing a point ( a person or post) is the length of the train itself.
8. Distance covered by a train while crossing a platform ( of length L) is equal to the sum of the length of the platform and the length of the train.
9. Distance covered by a train of length L while crossing another train of length M
is equal to (L + M).
10. If someone travels the same distance with different speeds(s1 and s2) and
hence arrive at different times (t1 and t2), then the formula for getting the
distance is
Distance = {(s1 x s2)/(s1 - s2)}x (t1 - t2)
