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Question
A ball with a mass of 0.25 kg hits a gym ceiling with a force of 78.0 N. What happens next?
1. The ball accelerates downward with a force of 80.5 N.
2. The ball accelerates downward with a force of 78.0 N.
3. The ball accelerates downward with a force of 2.45 N.
4. It depends on the height of the ceiling.
(a)
Note: Thank you kampei for your comment below flagging the error in the video. The final answer (a) above is correct, but the video incorrectly concludes that it's (b). The ball will accelerate downward with a total force of 80.5 N since there are two forces directed down: 1) the reaction force applied on the ball due to the ceiling (78.0 N down) 2) the force of gravity on the ball ($mg = 0.25\textrm{ kg} \times 9.8\textrm{ N/kg} = 2.45\textrm{ N}$). The total is 80.5 N downward.
Solution Video

# OpenStax College Physics for AP® Courses Solution, Chapter 4, Problem 6 (Test Prep for AP® Courses) (0:55)

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Video Transcript
This is College Physics Answers with Shaun Dychko. This ball is hitting the ceiling of a gymnasium and it applies a force on the ceiling due to the ball of 78.0 newtons. It's good to have subscripts like this to keep track of what is applying the force and I should say, what the force is being applied on and what is being applied by. This is a force on the ceiling due to the ball. The Newton's third law counterpart to this is the force on the ball due to the ceiling which is the subscripts switched around, and it's going to be in the opposite direction. This force has to be of equal magnitude 78.0 newtons and the answer to this is B, this is a Newton's third law question.

Submitted by kampei on Sat, 10/30/2021 - 18:20

wouldn't the net force of acting on the ball be (mg) + the third law reaction? Why is the answer not (a)?

Submitted by ShaunDychko on Mon, 11/01/2021 - 11:42

Hi kampei,
Thank you very much for this great comment - you're quite right! I have updated the final answer with this correction and some explanation.
All the best,
Shaun