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Question

You throw a ball straight up with an initial velocity of 15.0 m/s. It passes a tree branch on the way up at a height of 7.00 m. How much additional time will pass before the ball passes the tree branch on the way back down?

$1.91 \textrm{ s}$

Note: At 3:23 in the video, there's a mistake in the quadratic formula, in that I should have written the 7.00 factor as −7.00. The calculation is nevertheless correct since I plugged the numbers in correctly, although I took what in retrospect was a confusing shortcut by not writing negatives in front of either -4.9 and -7.00.

Solution Video

OpenStax College Physics Solution, Chapter 2, Problem 49 (Problems & Exercises) (4:06)

Rating

4 votes with an average rating of 4.

Calculator Screenshots

Video Transcript

Submitted by tyler707 on Mon, 09/16/2019 - 16:56

How come the - sign was left off of the 4.9 in the -4ac in the quadratic equation. When the a equals -4.9 not 4.9?

Submitted by ShaunDychko on Fri, 09/20/2019 - 09:56

Hi tyler707, thanks for the question. At 3:22 in the video I'm seeing that the negative is indeed included in the "-4ac" term. If you're still seeing something different, let me know.
All the best,
Shaun

Submitted by ShaunDychko on Wed, 09/09/2020 - 21:58

Thanks for the question here oliviafabre1, you've actually clarified what tyler707 was asking. To clarify the factors: $a = -4.9$, $b = 15.0$, and $c = -7.00$. The calculator screenshot is correct after considering the three negative signs involved in the $-4ac$ term. There is the minus from the formula, and both $a$ and $c$ are negative. To be more clear in the calculator screenshot I could have also written $-4.9$ and $-7.00$, but the result is the same. At 3:23 in the video, there's a mistake however, in that I should have written the $7.00$ factor as $-7.00$.
Thank you for helping straighten this out. I'll put a note in the final answer section about the error in the video.

Submitted by bindangvu on Mon, 02/22/2021 - 16:05

if they ask what is b. What is the maximum height the ball reaches?
c. Find total time of flight

Submitted by ShaunDychko on Tue, 02/23/2021 - 11:15

Hello bindangvu,
I think you can find other questions here asking questions like that. Try the search with "maximum height" and a filter for Chapter 3. You'll find 8 matches, and hopefully some of them will help.
All the best,
Shaun