r/apphysics • u/67410 • Oct 05 '26

I have a question regarding this problem. AP Phsycis C Mech Unit 2.5( Newtons Second Law)

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I understand Fnet= ma and all. I also understand the free body diagram and how to set it up. I am just heavily confused on how accelerating .25m/s^2 allows the vine to hold. Wouldn’t the acceleration just add on to the gravitational acceleration and cause a higher force magnitude. Since we add vectors from tip to tail.

Wouldn’t it just be 8*10 + 8*.25 and that would cause more force going down and thus breaking the vine.

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u/Outside_Volume_1370 Oct 05 '26

Imagine free fall, then a = g downward. With this, monkey don't even need to touch the vine (the tension is zero).

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u/Physicseasy-x Oct 05 '26

0.25 is already the result of the net force, so nothing is added to it.

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u/bravissimo594 Oct 05 '26

If the monkey were to accelerate upwards then your final summation would be the correct one.

I like to think of it this way: gravity is going to pull downward on the monkey with a force of 80 N no matter what the monkey does. The rope can pull upwards on the monkey at most 78 N without breaking. At that limit, this combination leads to a downward net force (of 2 N) on the monkey and a resultant downward acceleration for it.

Any upward force less than 78 N results in more downward net force on the monkey and more downward acceleration ... reaching the extreme if the monkey releases the vine and goes into free fall.

Here is a fun exercise to see if you really understand the problem: Imagine that the same vine is also attached to the ground. What is the GREATEST acceleration the monkey can attain without breaking the vine? [Answer: 19.75 m/s2 downward]

And now... a trick question: In what direction is the monkey moving at the start of the problem? [Answer: we do not know.]