Giancoli's Physics: Principles with Applications, 7th Edition
2
Describing Motion: Kinematics in One Dimension
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2-1 to 2-3: Speed and Velocity
2-4: Acceleration
2-5 and 2-6: Motion at Constant Acceleration
2-7: Freely Falling Objects
2-8: Graphical Analysis

Question by Giancoli, Douglas C., Physics: Principles with Applications, 7th Ed., ©2014, Reprinted by permission of Pearson Education Inc., New York.
Problem 22
Q

# A car slows down from 28 m/s to rest in a distance of 88 m. What was its acceleration, assumed constant?

A
$-4.5\textrm{ m/s}^2$

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VIDEO TRANSCRIPT

This is Giancoli Answers with Mr. Dychko. This car has a final velocity of zero meters per second because it comes to rest; it has initial velocity of 28 meters per second and it covers a distance of 88 meters as it's coming to a stop. So what is its acceleration? Well it's assumed to be a constant acceleration which just means that these formulas are a fair game that we have in this chapter and we'll use this one which contains everything that we know with only a single unknown. So we know that the final velocity is zero, the initial velocity we are given and we'll solve this equation for a. Subtract initial velocity squared from both sides and then also divide by 2d that will cancel on this side and end up over here and then switch the sides around so we have the unknown on the left because that's more conventional and we have acceleration is the negative of initial velocity squared divided by 2 times the distance it travels. So we have negative 28 meters per second squared and notice that the negative is outside the brackets so we square this number and then make that result negative and divide by 2 and divide by 88 and we get negative 4.5 meters per second squared is the car's acceleration.

By mcgracia2008 on Fri, 8/25/2017 - 1:50 AM

I think the result is incorrect. Instead ot multiply 2(88), the tutor divided 2/88.

By Mr. Dychko on Fri, 8/25/2017 - 4:53 AM

Hi mcgracia2008, thanks for looking at the working. I think what you're commenting on is the calculator display in which you see $-28^2/2/88$, but this is the same as $-28^2/(2*88)$. I just like the division signs since it's one less character to press instead of using parentheses for the multiplication in the denominator, but that's just a personal preference.

All the best,
Mr. Dychko

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