Motion in a Straight Line
- A body is thrown vertically upward from the ground. It reaches a maximum height of 20 m in 5 sec. After what time, it will reach the ground from its maximum height position?
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hmax = 20 m and t = 5 sec. Time taken by the body to reach the ground from some height is the same as taken to reach that height. Therefore, time to reach the ground from its maximum height is 5 sec.
Correct Option: B
hmax = 20 m and t = 5 sec. Time taken by the body to reach the ground from some height is the same as taken to reach that height. Therefore, time to reach the ground from its maximum height is 5 sec.
- A body dropped from top of a tower fall through 40 m during the last two seconds of its fall. The height of tower is (g = 10 m/s²)
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Let the body fall through the height of tower in t seconds. From,
Dn = u + (a/2) 2n - 1 we have, total distance travelled in last 2 seconds of fall is
D = Dt + D(t-1)
0 + g (2t - 1) 
+ 0 + g {2(t - 1) - 1} 2 2 = g (2t - 1) + g (2t - 3) = g (4t - 4) 2 2 2 = 10 × 4(t - 1) 2
or, 40 = 20 (t – 1) or t = 2 + 1 = 3s
Distance travelled in t seconds iss = ut + 1 at² = 0 + 1 × 10 × 3² = 45m 2 2 Correct Option: B
Let the body fall through the height of tower in t seconds. From,
Dn = u + (a/2) 2n - 1 we have, total distance travelled in last 2 seconds of fall is
D = Dt + D(t-1)0 + g (2t - 1) + 0 + g {2(t - 1) - 1} 2 2 = g (2t - 1) + g (2t - 3) = g (4t - 4) 2 2 2 = 10 × 4(t - 1) 2
or, 40 = 20 (t – 1) or t = 2 + 1 = 3s
Distance travelled in t seconds iss = ut + 1 at² = 0 + 1 × 10 × 3² = 45m 2 2
