# Confusion about Instructor's solution of HRK

• phymath7
In summary, the conversation discusses a potential error in the instructor's solution for finding the initial speed of a ball. The error involves the final position of the projectile not being accounted for, but it may not affect the solution in this case. The conversation also mentions the possibility of the interpretation of the horizontal range being different, leading to the discrepancy in the solution. The conversation concludes by acknowledging that it is common for solution manuals to contain errors.

#### phymath7

Homework Statement
This is an excercise from the book 'Physics' by HRK:A batter hits a pitched ball at a height 4.0 ft above the ground so that its angle of projection is 45° and its horizontal range is 350 ft. The ball travels down the left field line where a 24-ft high fence is located 320 ft from home plate. Will the ball clear the fence? If so, by how much?
Relevant Equations
##y-y_0=xtan\theta-\frac {gx^2}{2v_0^2cos^2\theta}##
The instructor's solution goes like this:
The initial speed of the ball is given by ##v_0=\sqrt{gR}## where R is the range.But this is true if the final position of the projectile is y=0 but in this case, y=-4.Though this doesn't affect much in this case,but for higher velocity and extreme cases this certainly would.Am I right?

Yes, you are right.

topsquark
TSny said:
Yes, you are right.
Then it's a pity that this solution manual contains such basic error.

phymath7 said:
Then it's a pity that this solution manual contains such basic error.

Perhaps the solution in the manual interprets the horizontal range R (350 ft) to be the horizontal distance that the ball would travel before it returns to its initial height of 4 ft above the ground (assuming the ball clears the fence). Then, ##v_0 = \sqrt{gR}## is OK. I'm not defending this interpretation, but it might be what was going on in the mind of whoever wrote the solution. Who knows. I'm with you in interpreting the horizontal range as the horizontal distance traveled until landing on the ground.

It's fairly common for solution manuals to have occasional errors.

Last edited:
SolarisOne and topsquark

## 1. What is HRK?

HRK stands for "Halliday, Resnick, and Krane," which is a commonly used physics textbook written by David Halliday, Robert Resnick, and Kenneth Krane.

## 2. Why is there confusion about the instructor's solution of HRK?

The instructor's solution of HRK can sometimes be confusing because it may not always align with the method or approach that the student is familiar with. Additionally, there may be errors or typos in the solution manual that can cause further confusion.

## 3. How can I clarify confusion about the instructor's solution of HRK?

If you are confused about the instructor's solution of HRK, you can seek clarification from your instructor or teaching assistant. You can also consult with other students or refer to online resources for additional explanations and examples.

## 4. What should I do if I find an error in the instructor's solution of HRK?

If you believe you have found an error in the instructor's solution of HRK, it is best to bring it to the attention of your instructor or teaching assistant. They can verify the error and provide a correction for you and your classmates.

## 5. How can I effectively use the instructor's solution of HRK?

The instructor's solution of HRK can be a helpful resource for understanding and practicing physics problems. It is important to carefully follow the steps and explanations provided in the solution manual and to use it as a supplement to your own problem-solving process.

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