Why does alternate way of solving the problems not work?

  • Thread starter Perseverence
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In summary, the kinetic energy of the object is 14 joules when passed without air resistance and it is only 9 joules when air resistance is taken into account.
  • #1
Perseverence
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Homework Statement



Princeton p 411 #6

A 7 kg ball is dropped from 20m. if the speed just before it hits the ground in 18 meters per second, what is the work done by air resistance?[/B]

Homework Equations


KE= (mv^2)*1/2
PE=mgh[/B]

The Attempt at a Solution


Mgh equals 1400 joules. PE equals KE. using the kinetic energy formula I was able to derive that the velocity of the ball without air resistance would be 20 meters per second. Which would be 2 meters per second less than 18 meters per second. Then I calculated the amount of kinetic energy there would be for 2 meters per second he found that it would be 14 joules. Why isn't this the answer for the amount of air resistance? I know how to find the right answer, but why doesn't this work?[/B]
 
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  • #2
Perseverence said:

Homework Statement



Princeton p 411 #6

A 7 kg ball is dropped from 20m. if the speed just before it hits the ground in 18 meters per second, what is the work done by air resistance?[/B]

Homework Equations


KE= (mv^2)*1/2
PE=mgh[/B]

The Attempt at a Solution


Mgh equals 1400 joules. PE equals KE. using the kinetic energy formula I was able to derive that the velocity of the ball without air resistance would be 20 meters per second. Which would be 2 meters per second less than 18 meters per second. Then I calculated the amount of kinetic energy there would be for 2 meters per second he found that it would be 14 joules. Why isn't this the answer for the amount of air resistance? I know how to find the right answer, but why doesn't this work?[/B]
Kinetic energy varies as the square of velocity. ##(v+2)^2 - v^2 \neq 2^2##.
 
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With no friction/air resistance the total energy is conserved. With friction/air resistance the loss of total mechanical energy is converted to thermal energy. In this case heating of the projectile and the air it passes through.
 
  • #4
Dr Dr news said:
With no friction/air resistance the total energy is conserved. With friction/air resistance the loss of total mechanical energy is converted to thermal energy. In this case heating of the projectile and the air it passes through.
All true, but sadly nothing to do with the question Perseverence asked, and most likely all well understood by the student already. See tnich"s answer.
 

Related to Why does alternate way of solving the problems not work?

1. Why do we need alternate ways of solving problems?

Alternate ways of solving problems allow us to approach a problem from different perspectives and potentially find more efficient or effective solutions. It also helps us to think outside the box and be more creative in our problem-solving processes.

2. What are some common reasons for alternate solutions not working?

Some common reasons for alternate solutions not working include lack of thorough analysis or understanding of the problem, incorrect assumptions, and limited resources or constraints. It's important to carefully evaluate and test any alternate solutions before implementing them.

3. How can we determine if an alternate solution is the best approach?

The best approach is subjective and can vary depending on the problem and context. However, some factors to consider when evaluating an alternate solution include its feasibility, efficiency, effectiveness, and potential impact on stakeholders. It's also important to consider potential risks and drawbacks of the solution.

4. What can we do if an alternate solution is not working?

If an alternate solution is not working, it's important to reassess and identify the root cause of the issue. This may involve going back to the drawing board and considering other possible solutions. It can also be helpful to seek input and feedback from others to gain new insights and perspectives.

5. How can we encourage the use of alternate solutions in problem-solving?

To encourage the use of alternate solutions, it's important to create a culture that values creativity and innovation. This can involve promoting open communication, fostering a growth mindset, and rewarding out-of-the-box thinking. It's also helpful to provide resources and support for exploring and testing alternate solutions.

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