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# ​A bullet loses 1/20 of its velocity in passing through a plank.The least number of planks needed to stop the bullet is?

3 years ago

Arun
24742 Points

Let the thickness of one plank = d
and the acceleration provided by the plank = a

If n planks are required to stop the bullet, then
0^2 = vo^2 + 2a*nd
2and = -vo^2

v = vo - vo/20 = 19 vo/20 in passing through one plank
(19 vo/20)^2 = vo^2 + 2ad
361/400 * vo^2 = vo^2 + 2ad

Substituting this value of -2ad into equation (1):
n = vo^2/(vo^2 * 39/400) = 400/39
The minimum number of planks needed = smallest integer greater than 400/39 = 11

4 months ago
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• 731 Video Lectures
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• Test paper with Video Solution
• Mind Map
• Study Planner
• NCERT Solutions
• Discussion Forum
• Previous Year Exam Questions