Posted on - 03-05-2017

Bank PO

Distance covered per unit time is called speed. .

i.e., Speed = Distance/time or .

Distance = Speed x time or

Time = Distance/speed

- If two bodies travel with the same speed, Distance covered m Time (Direct Variation). .
- If two bodies travel for the same period of time, Distance covered m Speed (Direct Variation). .
- If two bodies travel in the same distance, Time m (Inverse Variation)

Distance is normally measured in km, metres or miles; Time in hours or seconds and Speed in Km/hr (also denoted by kmph), miles/hr (also denoted by mph) or metres/second (denoted by m/s). .

To convert speed in kmph to m/sec, multiply it with (5/18). .

To convert speed in m/sec to kmph, multiply it with (18/5). .

The speed of one (moving) body in relation to another moving body is called the relative speed for these two bodies. If two bodies are moving in the same direction, the relative speed is equal to the difference of the speeds of the two bodies. If two bodies are moving in opposite direction, the relative speed is equal to the sum of the speeds of the two bodies. .

Average speed of a body travelling at different speeds is NOT EQUAL to the average of the speeds but is defined as follows:

Average Speed =

A person can cover certain distance in 8 hours at a speed of 50 kmph. At what speed must he travel to cover the distance in 5 hours? .

Distance = Speed x time = 50 x 8

= 400 km

400 = speed x 5

speed = 80 kmph

A train travelled for 9 hours. It travelled the first half of the distance at 12 kmph and remaining distance at 15 kmph. Find the distance. .

Let the distance be x km. .

Time taken =

9x = 120 x 9

&⇒ x = 120 km

A train, 300 m long is moving at 54 kmph. How long does it take .

(a) to cross an electric pole?

(b) to cross a platform 150 in long?

(c) to cross a person moving in the same direction at 18 kmph?

(d) to cross a person moving in opposite direction at 18 kmph?

(e) to cross another train, 300 m long, running at 36 kmph in opposite direction?

(f) to cross another train, 300 m long, running at 36 kmph in the same direction?

Speed of the train = 54 x = 15m/s

(a) speed = 15 m/s; distance = 300 m

so time = = 20 sec.

(b) Distance = 300 + 150 = 450

Time = = 30 sec

(c) Speed of the train relative to the man

= 54 - 18 = 36 kmph

= 36 x m/s = 10 m/s

Time = = 30 sec.

(d) Speed of the train relative to the man

= 54 + 18 = 72 kmph

= 72 x m/s = 20 m/s

Time = = 15 sec

(e) Total distance = 300 + 300

= 600 m

relative speed = 54 + 36 = 90 kmph

= 90 x = 25 m/s

Time = = 24 sec.

(f) Total distance = 300 + 300 = 600 m

relative speed = 54 - 36 = 18 kmph

= 18 x = 5 m/s

Time = = 120 sec .

A person covered 280 km. He covered of the distance at 70 kmph and the remaining distance at 35 kmph. What is the average speed of the person?

of 280 = x 280 = 210 km

Time taken to cover 210 km is

= 3 hours.

Time taken to cover remaining

70 km = = 2 hours

Average speed =

= = 56 kmph

Train T crosses a bridge 100 m long in 21 seconds. It crosses another bridge 220 m long in 33 seconds. Find its speed. .

Let the speed of T be S m/sec. Let its length be L m. .

= 21 and = 33.

&⇒ L = 21S - 100 and L = 33S - 220

&⇒ L= 21S - 100 = 33S - 220

&⇒ 12S = 120

&⇒ S = 10. .