18. When a ball falls freely from rest under gravity then the distance travelled by it in last two seconds is 6 times the distance travelled by it in first teo seconds. The total time of journey
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Motion In A Straight Line
1.
Here,
t = 6 s
u = 0
Distance traversed in 6 s
s1 = ut + ½gt2
= ½ × 9.8 × 62
= 176.4 m
Let t1 be the time elapsed before last two seconds
t1 = 6 – 2 = 4 s
Distance traversed in 4 s
s2 = ut + ½gt2
= ½ × 9.8 × 42
= 78.4 m
Distance covered in last two seconds
= s1 – s2
= 176.4 – 78.4
= 98 m
2.
Here,
u = 20 ms-1
g = 10 ms-2
t = 3 s
s = ut + ½ (-g)t2
=> s = 20×3 – ½×10×32
=> s = 60 – 45
=> s = 15 m
Distance traversed = 15 m
1.
s = 0.5 g t 2
Distance travelled in first 4 seconds = 0.5 * 10 * 42 = 80 m
Distance travelled in 6 seconds = 0.5 * 10 * 62 = 180 m
Distance travelled in last 2 seconds = 180 - 80 = 100m
2.
s = ut - 0.5 g t2
= 20*3 - 0.5*10*32
= 60 - 45
= 15 m