A train is moving with a velocity 400 m/s. With the application of brakes a retardation of 10 m/s

(i) After how much time it will stop ?

(ii) How much distance will it travel before it stops?

Using equation, v = u + at, we get

0 = 400 + (–10) × t

or t = 40 s

(ii) For calculating the distance travelled, we use equation,

v

(0)

or 20s = 400 × 400

or s = 8000 m = 8 km

A body is thrown vertically upwards with an initial velocity of 19.6 m/s. If g = –9.8 m/s

(i) The maximum height attained by the body.

(ii) After how much time will it come back to the ground ?

Using equation v

(0)

or h = = 19.6 m

(ii) Time taken to reach the maximum height can be calculated by the equation,

v = u + gt

or 0 = 19.6 + (–9.8) × t

ort = 2s

In the same time, it will come back to its original position.

Total time = 2 × 2 = 4s

From the top of a tower of height 490 m, a shell is fired horizontally with a velocity 100 m/s. At what distance from the bottom of the tower, the shell will hit the ground ?

Using equation,h = ut +gt

490 = 0 + × 9.8 × t

or t

or t = 10 s

∴ It takes 10 seconds to reach the ground.

Now, horizontal distance = horizontal velocity × time

= 100 m/s × 10 s = 1000 m

∴ The shell will strike the ground at a distance of 100 m from the bottom of the tower.

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