Confusion about Instructor's solution of HRK

AI Thread Summary
The instructor's solution for the initial speed of the ball, given by v_0=√(gR), assumes the final position of the projectile is y=0, which is incorrect since the actual final position is y=-4. This discrepancy may not significantly impact lower velocities but could lead to errors in extreme cases. The solution manual likely interprets the horizontal range R as the distance the ball travels before returning to its initial height of 4 ft, which complicates the accuracy of the formula used. Errors in solution manuals are not uncommon, highlighting the importance of careful interpretation in physics problems. Overall, the discussion emphasizes the need for clarity in understanding projectile motion and its implications.
phymath7
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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?
 
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Yes, you are right.
 
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.
 
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