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Work Power and Energy question

2021 · 20 Jul · Shift 1 · Q65
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Work Power and Energy question

2021 · 20 Jul · Shift 1 · Q65

JEE MainPhysicsWork Power and EnergyNumerical+4 / −1
In a spring gun having spring constant 100 N/m a small ball 'B' of mass 100 g is put in its barrel (as shown in figure) by compressing the spring through 0.05 m. There should be a box placed at a distance 'd' on the ground so that the ball falls in it. If the ball leaves the gun horizontally at a height of 2 m above the ground. The value of d is ‾\underline{\hspace{2cm}}​ m. (g = 10 m/s2). JEE Main 2021 (Online) 20th July Morning Shift Physics - Work Power & Energy Question 84 English
Numerical answer
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Correct answer: 1

  1. Given data
  • Spring constant: k=100 N/mk = 100\,\text{N/m}k=100N/m
  • Mass of ball: m=100 g=0.1 kgm = 100\,\text{g} = 0.1\,\text{kg}m=100g=0.1kg
  • Compression of spring: x=0.05 mx = 0.05\,\text{m}x=0.05m
  • Height from ground: h=2 mh = 2\,\text{m}h=2m
  • Acceleration due to gravity: g=10 m/s2g = 10\,\text{m/s}^2g=10m/s2

The ball leaves the gun horizontally.


  1. Find the speed of the ball as it leaves the spring gun

Using conservation of energy:

12kx2=12mv2\frac{1}{2}kx^2 = \frac{1}{2}mv^221​kx2=21​mv2

Substitute the values:

12(100)(0.05)2=12(0.1)v2\frac{1}{2}(100)(0.05)^2 = \frac{1}{2}(0.1)v^221​(100)(0.05)2=21​(0.1)v2

50×0.0025=0.05v250 \times 0.0025 = 0.05v^250×0.0025=0.05v2

0.125=0.05v20.125 = 0.05v^20.125=0.05v2

v2=0.1250.05=2.5v^2 = \frac{0.125}{0.05} = 2.5v2=0.050.125​=2.5

v=2.5 m/sv = \sqrt{2.5}\,\text{m/s}v=2.5​m/s


  1. Find the time of fall from height 2 m2\,\text{m}2m

Since the ball is projected horizontally, initial vertical velocity is zero.

h=12gt2h = \frac{1}{2}gt^2h=21​gt2

2=12(10)t22 = \frac{1}{2}(10)t^22=21​(10)t2

2=5t22 = 5t^22=5t2

t2=0.4t^2 = 0.4t2=0.4

t=0.4 st = \sqrt{0.4}\,\text{s}t=0.4​s


  1. Find horizontal distance ddd

d=vtd = vtd=vt

d=2.5⋅0.4d = \sqrt{2.5} \cdot \sqrt{0.4}d=2.5​⋅0.4​

d=2.5×0.4=1=1 md = \sqrt{2.5 \times 0.4} = \sqrt{1} = 1\,\text{m}d=2.5×0.4​=1​=1m


  1. Final answer

d=1 m\boxed{d = 1\,\text{m}}d=1m​

This matches the stored correct answer.

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