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A ball of mass 2 kg and another of mass 4 kg are dropped together from a 60 feet tall building. After a fall of 30 feet each towards earth, their respective kinetic energies will be in the ratio of
  • a)
    1 : √2
  • b)
    √2 : 1
  • c)
    1 : 4
  • d)
    1 : 2
Correct answer is option 'D'. Can you explain this answer?
Most Upvoted Answer
A ball of mass 2 kg and another of mass 4 kg are dropped together fro...
Given:
Mass of first ball, m1 = 2 kg
Mass of second ball, m2 = 4 kg
Height of building, h = 60 feet
Height of fall, h1 = h2 = 30 feet

To find:
Ratio of kinetic energies of the two balls after falling through 30 feet each towards the earth.

Solution:
1. Potential energy at the top of the building = mgh
Potential energy at the top of the building = (2+4) x 9.8 x 60 = 3528 J

2. Potential energy after falling through 30 feet = mgh
Potential energy after falling through 30 feet = (2+4) x 9.8 x 30 = 1764 J

3. Kinetic energy after falling through 30 feet = Potential energy at the top - Potential energy after falling through 30 feet
Kinetic energy of first ball after falling through 30 feet = 3528 - 1764 = 1764 J
Kinetic energy of second ball after falling through 30 feet = 3528 - 1764 = 1764 J

4. Ratio of kinetic energies of the two balls after falling through 30 feet each towards the earth
Ratio of kinetic energies = KE1/KE2
KE1/KE2 = (1/2) m1v1^2 / (1/2) m2v2^2
KE1/KE2 = m1v1^2 / m2v2^2
Velocity v of a ball after falling through a height h can be calculated using the formula v = sqrt(2gh)
v1 = sqrt(2gh1) and v2 = sqrt(2gh2)
KE1/KE2 = m1(2gh1) / m2(2gh2)
KE1/KE2 = m1/m2 x h1/h2
KE1/KE2 = 2/4 x 30/30
KE1/KE2 = 1/2

Therefore, the ratio of kinetic energies of the two balls after falling through 30 feet each towards the earth is 1:2. Hence, option D is the correct answer.
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Community Answer
A ball of mass 2 kg and another of mass 4 kg are dropped together fro...
Since height is same for both balls, their velocities on reaching the ground will be same
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A ball of mass 2 kg and another of mass 4 kg are dropped together from a 60 feet tall building. After a fall of 30 feet each towards earth, their respective kinetic energies will be in the ratio ofa)1 : √2b)√2 : 1c)1 : 4d)1 : 2Correct answer is option 'D'. Can you explain this answer?
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