Two physics questions I just don't get....

[sv3]

Hi, just looking for some insight in two problems I am having - I'm using TPR and just got railed on a couple of passages.

1) A runner was running in a straight line along the ground. He was accelerating to the right as he ran for the first little bit. What is the force of the ground on the runner? The answer choices were pictures of vectors. I chose the one that went straight up as I thought that the ground exerts a normal force, and normal forces are can only be perpendicular to the surface. The right answer was a vector pointing diagnally up and to the right. In a way i somewhat understand that, but can't get my head around the fact that normal forces have to be perpendicular - did the ground exert some other force? and if so...what?

2) And nevermind about this one.....finally came to me!

thanks in advance
steve

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[Bernoull]

Hi, just looking for some insight in two problems I am having - I'm using TPR and just got railed on a couple of passages.

1) A runner was running in a straight line along the ground. He was accelerating to the right as he ran for the first little bit. What is the force of the ground on the runner? The answer choices were pictures of vectors. I chose the one that went straight up as I thought that the ground exerts a normal force, and normal forces are can only be perpendicular to the surface. The right answer was a vector pointing diagnally up and to the right. In a way i somewhat understand that, but can't get my head around the fact that normal forces have to be perpendicular - did the ground exert some other force? and if so...what?

2) And nevermind about this one.....finally came to me!




thanks in advance
steve

I think this is Newton's 3rd Law at work, ur right about normal force being equal and opposite to runner's weight, however you need to consider that the runner has to push the ground back and the ground pushes him forward thus propelling him. The resultant vector applied by the runner will be diagonal SW direction and the ground equal and opposite force will be NE, I drew out a diagram took a pic and tried to upload but the dimensions exceed the limit n i can't quite figure out how to resize it with photoshop

 

[sv3]

the force from the ground is a vector NE - perfectally correct and now I understand this is newton's 3rd law........the only thing i am trying to tie up is what is the normal force then? Is it the vertical component of this NE vector? the normal force always has to be perpendicular (90 degrees) to the surface right? Or am I wrong there? (cause that would be a big conceptual mistake on my understanding of the normal force). I get the newtons third law thing, but don't know if there is a second force (the normal force) or if the normal force is somehow part of this NE vector. Sorry if it sounds confusing........likely cause im still cloudy!

thanks very much !
steve

I think this is Newton's 3rd Law at work, ur right about normal force being equal and opposite to runner's weight, however you need to consider that the runner has to push the ground back and the ground pushes him forward thus propelling him. The resultant vector applied by the runner will be diagonal SW direction and the ground equal and opposite force will be NE, I drew out a diagram took a pic and tried to upload but the dimensions exceed the limit n i can't quite figure out how to resize it with photoshop

 

[Bernoull]

the force from the ground is a vector NE - perfectally correct and now I understand this is newton's 3rd law........the only thing i am trying to tie up is what is the normal force then? Is it the vertical component of this NE vector? the normal force always has to be perpendicular (90 degrees) to the surface right? Or am I wrong there? (cause that would be a big conceptual mistake on my understanding of the normal force). I get the newtons third law thing, but don't know if there is a second force (the normal force) or if the normal force is somehow part of this NE vector. Sorry if it sounds confusing........likely cause im still cloudy!

thanks very much !
steve

thanks for da tip with paint!! Yes normal force is the vertical component of the resultant force and the horizontal force is what the earth exerts on the runner to propel him forward, you take sa vector sum of the two and voila!! see the attachments and let me know if it's not clear..

 

[sv3]

Thanks for your pics as they really did help (cool stick figure...) and totally make sense. The two things are, I thought the runner is applying force down and back, so the force directly down would be greater than mg since it would be the runner's weight plus the force the runner exerts down? (or perhaps you can't exert more force than your weight?) and then of course you also have the runner's force that pushes back in order to propel the runner forward.

the second and more important thing is: Do the forces you drew cancel out? The SW and NE look like they cancel (i could be wrong in interpretting your diagram), but the runner is accelerating forward, so the net force has to be forward right?........... The answer explanation in TPR didn't help me understand but here it is: The runner is accelerating to the right, so the net force has to be to the right. Since the force of gravity is straight down, the ground force has to be up and to the right. That's what they write but doesn't help me much - it sounds like they deduced the answer but im looking for the why.......one question.....21 head bangs against my desk...brutal

thanks again, really appreciating the help!
steve

 

[Bernoull]

Thanks for your pics as they really did help (cool stick figure...) and totally make sense. The two things are, I thought the runner is applying force down and back, so the force directly down would be greater than mg since it would be the runner's weight plus the force the runner exerts down? (or perhaps you can't exert more force than your weight?) and then of course you also have the runner's force that pushes back in order to propel the runner forward.

the second and more important thing is: Do the forces you drew cancel out? The SW and NE look like they cancel (i could be wrong in interpretting your diagram), but the runner is accelerating forward, so the net force has to be forward right?........... The answer explanation in TPR didn't help me understand but here it is: The runner is accelerating to the right, so the net force has to be to the right. Since the force of gravity is straight down, the ground force has to be up and to the right. That's what they write but doesn't help me much - it sounds like they deduced the answer but im looking for the why.......one question.....21 head bangs against my desk...brutal

thanks again, really appreciating the help!
steve

Good questions, first whether the forces cancel out: Vertical components of both NE & SW forces cancel since the runner is neither floating up or sinking through the ground (sounds trivial right...) NEWTON 3 LAW FORCES NEVER CANCEL OUT!!! THEY ACT OF DIFFERENT BODIES!!. Therefore both the Earth and RUnner accelerate, given the massiveness of dsa earth it's acceleration is imperceptible a=F/m whereas for the runner he has a net horizontal force hence acceleration.


You make another good point here:
"The two things are, I thought the runner is applying force down and back, so the force directly down would be greater than mg since it would be the runner's weight plus the force the runner exerts down? (or perhaps you can't exert more force than your weight?) and then of course you also have the runner's force that pushes back in order to propel the runner forward."

I think you are overanalyzing when you think about whether the runner applies an additional downward force beside mg. I doubt that the mcat implies that. I try to simplify problems so I'll focus on the region where the foot contacts the ground and analyze the forces acting. If u really want to get technical from a biomechanical perpective, ur mucles across ur hip apply a force to lift ur leg from the ground and then ur quad straigthens out ur leg, im not convinced there's a net downward force, ur body has enough trouble dissipating the forces on ur knees and other joints everytime ur foot hits da ground, so it doesn't make sense to increase these forces if anything human gait attempts to lessen them.

 

[sv3]

God...I am kicking my own butt over here b/c when I studied Newton's 3rd law i told myself "don't forget they are on different bodies, opposite the concept of normal force"......and there I go forgetting it!! Your explanation was great and much better than the one TPR gave me, thanks.

Right on overanalyzing...I am getting better at not doing this by trying to remind myself these aren't supposed to be long questions although some can be "tricky".

I also second batteries question...its something I've wondered myself - that is identifying the concept before answering the question - I just don't know if there's enough time for that......or for someone like me....I could likely talk myself into switching back and fourth between concepts and get frustrated. Typically I just read, look at answer choices for format, and then go at it. But lately I have been having the same thoughts Batteries does. Whats your take Bernoull ?

cheers bud
steve

 

[Bernoull]

I like your explanations I bet you're gonna rock the MCAT.

Question since I haven't started *hardcore* studying for it yet...do you feel with most problems it's ideal to first identify the concept they are trying to test (i.e. every action has an equal and opp rxn) and if you can't figure that out you're thinking about the problem wrong?

I plan to spend almost as much time strategizing for this as I do taking practice tests...LOL

Another good question. I think you both are referring to whether to actively, consciously think about concepts being tested by MCAT questions as we attempt to answer them. Besides guessing, we always apply scientific (sometimes mathematical) concepts to answer ALL question, most of the time this is done subconsciously though.

To hepl answer this, I'll pose a question. For the problem above, what is the direction of the net frictional force and it's type. I'm referring to the runner's shoe/ground interface... I want ONLY you two to answer this.

 

[sv3]

Another good question. I think you both are referring to whether to actively, consciously think about concepts being tested by MCAT questions as we attempt to answer them. Besides guessing, we always apply scientific (sometimes mathematical) concepts to answer ALL question, most of the time this is done subconsciously though.

To hepl answer this, I'll pose a question. For the problem above, what is the direction of the net frictional force and it's type. I'm referring to the runner's shoe/ground interface... I want ONLY you two to answer this.

I think it's static friction b/c the runner's shoe and the ground are not in continous contact (i.e the runner isn't doing the moonwalk) - when you take off your foot shouldn't slide backward either hence another reason for static. I think the direction of friction strangely enough is in the direction of the net force - to the right going forward. The runner pushes back, and friction opposes this, helping the runner move forward. It seems strange as TPR said friction always opposes velocity so TPR would say friction goes backwards (I just dont get this explanation b/c when a car is moving, friction i thought is actually it the same direction since the tires are rolling back).

damn....wait.....the above is for the take off part, but when the runner lands, then friction is going backwards, opposing the runners forward motion. But this time I'm not sure which type of friction it is - i'll go with static again for similar reasons stated above

and going back to thinking about concepts, I am not sure what concept your testing here........ugh

thanks
sv3

 

[Bernoull]

I think it's static friction b/c the runner's shoe and the ground are not in continous contact (i.e the runner isn't doing the moonwalk) - when you take off your foot shouldn't slide backward either hence another reason for static. I think the direction of friction strangely enough is in the direction of the net force - to the right going forward. The runner pushes back, and friction opposes this, helping the runner move forward. Not sure how I put this together in my head though. It seems strange as TPR said friction always opposes velocity so TPR would say friction goes backwards.

damn....wait.....the above is for the take off part, but when the runner lands, then friction is going backwards, opposing the runners forward motion. But this time I'm not sure which type of friction it is - i'll go with static again for similar reasons stated above

and going back to thinking about concepts, I am not sure what concept your testing here........ugh

thanks
sv3

Excellent answer!! You definitely have friction down right in ur first paragraph.

"damn....wait.....the above is for the take off part, but when the runner lands, then friction is going backwards, opposing the runners forward motion. But this time I'm not sure which type of friction it is - i'll go with static again for similar reasons stated above"

Here I think ur overanalyzing again!!. Think about when u run do u speed up at takeoff and slow down at landing, NO. Not if u wanna go anywhere fast. When u land u knees are slightly bent and ur center of mass accelerates forward and ur knee moves out in front of ur feet. Da feet then push back on the ground for another take-off. I took a human gait course a while back, i may be rusty on some things but anyhow, this overanalysis is an overkill.

Keep it simple for physics, identify a system and analyze the forces acting on that system. Since we're interested on friction the shoe/pavement interface will work.

Friction can be a confusing concept bcos some books/professors suggests that friction opposes an object's motion which is not necessarily true. Friction opposes relative motion b/t contiguous surfaces and when the contiguous surfaces are not sliding/"moonwalking" wrt each other, that's static friction otherwise its kinetic. In this problem both horizontal forces (F applied by runner and F applied by ground) are infact frictional (may sound strange). For this question, knowledge of and correct application of the concepts is critical, bcos it seems counterintuitive for friction and displacement to have the same direction, but if u recalled the true definition of friction and assessed the problem to see in what direction da runner's shoe is likely to slide over da ground, then friction's direction will opposite to oppose this sliding. Car tire works da same too!!

Back to ur question, generally I'll break questions in 2 types, one I'll think about concepts b4 and another I won't. Straightforward questions that u've practiced so much that u can do them in ur sleep (Kinematics comes to mind) i dont' think about concept b4 since it's da concept being tested is obvious/explicit but I'll quickly mentally verify whether my answer is reasonable. The second Q type is the one u PAUSE at in the beginning, if u need to pause and think, it's probably bcos the question is too obscure and ur digesting it or ur have competing approaches/concepts u want to apply. Here I'LL DEFINITELY ask myself what basic science concept is being tested? The answer to this is SAVE ME TIME bcos this essentially clarifies and simplifies the question and stops u from running a fool's errand. It's like rephrasing a VR question stem to simplify it, it can only help. MCAT questions can be obscure, esoteric and foggy. I believe points should be award for simply deciphering what the question wants, besides this is half the battle!!

Another benefit of thinking about concepts is some passage-based question might as well be stand-alones bcos the answer has absolutely nothing to do with the passage, now if u reflexively started scouring the passage for a non-existent answer n then rereading the question etc.. u can easily lose a couple of mins. Determining what basic concept was being tested would have saved u precious time and energy. Basically, this is a tool and like any tool, u need to use it when appropriate and not blindly. So IMO using this tool for specific question types will be more efficient and u'll make less mistakes.

For instance:
Shape is important for binding of the relevant peptide to the receptor protein on cell surface, Which AA will best bind to a sterically hindered receptor site?

a. Phenylaline
b. Glycine
c. Histidine
d. Isoleucine

If u look at da question and realize that it's asking you for da smallest AA (in MCAT-speak of course), u will answer it CORRECTLY and QUICKLY, it saves u time and prevents overanalyzing!!

Good luck

 

[sv3]

I wasn't sure it was static (again me and my brain also thought that there could be slight slippage of the shoe vs the ground hence some kinetic friction too) but I went with the bigger idea. I am trying to be a little more narrow/more focused on the physics questions and I think knowing the concepts that are being tested will help me and I'll implement your advice......cheers.

That was a great explanation all around, thanks for your time Bernoull.

Steve