What is the Reaction to the Force of Gravity on a Suspended Lead Block?

[2ndyrengneer]

Homework Statement

A lead block is suspended from your hand by a string. The reaction to the force of gravity on the block is the force exerted by:
(a) the string on the block (b) the block on the string (c) the string on the hand
(d) the hand on the string (e) the block on Earth

Homework Equations

Newtons Third Law

The Attempt at a Solution

Ok,

When i saw this question in the exam I thought it was essentially a no brainer; "The reaction to the force of gravity on the block is the force..."- I thought Newtons third law. I could see kids answering b, c or maybe d (definitely not e) but I selected a. The block experiences Fg and the string Tension is the reaction to that force.

I got 89% on this midterm and would have gotten 94% if the answer to this question would have been a. It was e. Yes e. Does anyone here (preferably someone with a Phd in physics) have an explanation as to why the answer is e. I am rather frustrated as you can probably infer from my post. Earth isn't even mentioned in the question. E was in my oppinion an almost joke option because they couldn't think of a 5th option for letter e (all the rest of the questions had answers a-e). Why is it e!?

 

[Quinzio]

We can't say the e) is wrong.
To understand why will be your next task. :)

Now forget it and go on. It's tricky question.

I could see kids answering b, c or maybe d

The mass (of people) is usually 50% right and 50% wrong.
Never look at what the mass is doing unless you HAVE to do so.

I am rather frustrated as you can probably infer from my post.

Giving a bad answer when you got 89% is not a real problem.
Having problems with e.g. your wife is A real problem.

 

[2ndyrengneer]

Ok well that was almost no help to me whatsoever :S I am looking for a solid reply. Someone that can give me an answer. Its not a question for a homework assignment. Its a question from a midterm and I am just looking for answer. An intelligient answer. I am passionate about my school so yes to me this is a real problem. Thanks though.

 

[Dadface]

Remember that the forces referred to in the third law act on different bodies.The force of gravity on the block,in other words the weight of the block,is due to its gravitational attraction to the earth.The Earth pulls the block down with a certain force and the block pulls the Earth up with an equal sized force.

 

[kreil]

Newton's Third Law, which I am stating solely for clarity, is

$$F_{12}=-F_{21}$$

Now, notice that the force exerted by the block on the string is equal in magnitude and opposite in direction to the force of the string on the block. Therefore, (a) and (b) form a "force pair" and they cannot be answers.

Next, notice that the force exerted by the string on the hand is equal in magnitude and opposite in direction to the force of the hand on the string. Therefore, (c) and (d) form a "force pair" and they cannot be answers.

This leaves (e).

I believe this is the logic your instructor had in writing this problem, though I agree it is worded rather clumsily.

 

[2ndyrengneer]

Thanks! That is exactly what i was looking for. It makes sense now. If the question was worded only slightly differently I could have gotten the answer. It makes more sense the way you put it with the force pair. And now I see what the prof was going for but he did NOT make that clear in the question. Oh well. I'll go argue with him on monday!

 

[diazona]

Well... I hate to break it to you but I don't think this is a hard question, as long as you know what a reaction force is. The reaction to the force exerted by A on B is the force exerted by B on A.

The question asks about the force of gravity on the block, and hopefully you know that the force of gravity is exerted by the Earth. So the reaction to the force exerted by the Earth on the block is the force exerted by the block on the Earth, which is choice (e).

Sure, maybe the question could be worded a little better, but I don't think it's so confusing that a student wouldn't be expected to figure out the right answer.

 

[D H]

I concur with diazona with one exception: The question wasn't even all that badly worded. The only possible answer is (e).