Work, Energy, Power & Collision / कार्य, ऊर्जा और शक्ति
JEE Main & Advanced
Physics
Work, Energy, Power & Collision / कार्य, ऊर्जा और शक्ति
Question Bank
Conservation of Energy and Momentum
question_answer
A bullet moving with a speed of 100 \[m{{s}^{-1}}\]can just penetrate two planks of equal thickness. Then the number of such planks penetrated by the same bullet when the speed is doubled will be [KCET 2004]
A) 4
B) 8
C) 6
D) 10
Correct Answer:
B
Solution :
Let the thickness of each plank is s. If the initial speed of bullet is 100 m/s then it stops by covering a distance 2s By applying \[{{v}^{2}}={{u}^{2}}-2as\]Þ \[0={{u}^{2}}-2as\] \[s=\frac{{{u}^{2}}}{2a}\] \[s\propto {{u}^{2}}\] [If retardation is constant] If the speed of the bullet is double then bullet will cover four times distance before coming to rest i.e. \[{{s}_{2}}=4({{s}_{1}})=4(2s)\] Þ \[{{s}_{2}}=8s\] So number of planks required = 8