# Can anyone help with this physics problem?

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We cannot figure out how they got their answer.

Three blocks are arranged in a stack on a frictionless horizontal surface. The bottom block has a mass of 37.0 kg. A block of mass 18.0 kg sits on top of it and a 16.0 kg block sits on top of the middle block. A downward vertical force of 170 N is applied to the top block. What is the magnitude of the normal force exerted by the bottom block on the middle block?

Their answer is 425N. We got 503N.

Thank you!!!

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We cannot figure out how they got their answer.

Three blocks are arranged in a stack on a frictionless horizontal surface. The bottom block has a mass of 37.0 kg. A block of mass 18.0 kg sits on top of it and a 16.0 kg block sits on top of the middle block. A downward vertical force of 170 N is applied to the top block. What is the magnitude of the normal force exerted by the bottom block on the middle block?

Their answer is 425N. We got 503N.

Thank you!!!

I'll ask my brother, who is a Dr. in Physics, but perhaps someone else will answer in the meantime. Sometimes college physics books have the wrong answers in the back, so perhaps this does, too. I don't know this myself. It may take a few days to get an answer, depending on when he checks his email, how busy he is at work, etc. He has 4 dc who all do a myriad of activities.

Edited by Karin
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I will ask my husband when he calls who is a PhD physicist. He probably would enjoy doing the problem versus the horrendous airplane problems he has had today.

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Perhaps the book is wrong, as Karin suggested? (An annoying and all-too-frequent occurrence.) Here's what he wrote:

There is no specific mention of gravity, but let's assume we're at sea level and there is gravity.

So the downward force on the bottom block is the 170N plus the force of gravity from the

middle and top block. Their total mass is 34 kg, so their weight is 34 kg * 9.8 m/s^2 = 333.2 N

So the total force down is 503.2 N, and since (apparently) all the blocks are at rest, the bottom block

must be exerting an upward force on the middle block of 503.2N.

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Thank you everyone for your offers of help. Laura, please tell your dh that we appreciate his time in answering the question for us. Ds solved it the same way as your dh. We went over everything several times and could not figure out what he had missed in solving the problem.

Thank you again!!

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I see you figured it out. However, since my db-the-physics-geek ("physics geek" are his words) has kindly sent this to me, I'll post it anyway because I always enjoy how he explains the process (not that I'm biased in any way ;)).

Sure, since we are interested in the force on the middle block, lets consider our "object" to be the top two blocks

What are the forces acting on this "object"?

gravity of course down Fg = (18+16)g

A downward force of 170N is also applied,

and finally an upward force by the bottom block, call this N

Well, if we take upwards as positive and we add up all the forces,

N+ (- (18 + 16)g) + (-170N) = what???

well it should equal 0, so N = 34g + 170

N ~ 340 +170 ~ 510N, so I would say your answer is correct, and 425N is wrong.

I can't even figure out how they get 425N by any arrangement of the numbers, so it is probably a typo, perhaps downward force is 100N?

PS I went crazy on the negative signs and parenthesis to make it clear that we always sum up our forces, but it just happens that some are negative....

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I googled the first sentence of the problem, and found it posted several times online with slightly different numbers for the mass of one of the blocks. Of course, that would change the answer. Interesting .....

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Karin,

Thank your brother for me! I really appreciate that these busy men would take time to answer a question for a complete stranger!

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It is from Kinetic Conceptual Physics. :)

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Karin,

Thank your brother for me! I really appreciate that these busy men would take time to answer a question for a complete stranger!

I thanked him, but hadn't told him that it was for a complete stranger (to him; after all, you and I have met the same person IRL and you're on here.) Nor did I say it was for us, exactly, although he probably thinks that. I don't ask him for help often, although I am going to see if he'll help us more when dd does Conceptual Physics or, more likely, if and when she does a more mathy Physics.

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If you email Kinetic Books about the wrong answer, they are generally very quick to answer back. We were among the first users of their Algebra I and Algebra II programs. My dd found a few wording errors (where they asked for something different than what the answer key actually wanted) and a few just plain wrong answer errors. They reworded the problems that she commented on in the next release. They also fixed the incorrect answers in the answer key.

My degree is in physics and I got the same answer as your ds. If you use 9.8 as the gravitational constant, you should get 503N. If you use 10 as the gravitational constant, you should get 510N.

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