Static Friction??

[G1SG2]

---

Members don't see this ad.

A 10-kg block is at rest on an incline plane (theta=10 degrees). The coefficient of static friction is 0.75 and the acceleration due to gravity is 10 m/s^2. The force of static friction is approximately 17 N. Which of the following would NOT cause the force of static friction to increase?

A) Increasing the mass of the block to 20 kg.
B) Increasing theta to 30 degrees.
C) increasing the acceleration due to gravity to 12 m/s^2
D) Increasing the coefficient of static friction to 0.80

Which would you pick, and why? I just want to see some of you would work this out before I state the answer, as I got this question wrong and don't know what I'm missing! Thanks.

 

[Fort]

Increasing the angle would ultimately decrease the normal force on the block. This should make sense intuitively because if the block laying flat, gravity is maximized in the y-direction. But if you put the block on an incline, the force acting perpendicular to the block will decrease because there are now two components. Increasing theta will cause the gravity component perpendicular to the block to decrease; thus, decreasing the normal force. Decreasing the normal force will decrease the frictional force; the best answer is B.

 

[G1SG2]

Increasing the angle would ultimately decrease the normal force on the block. This should make sense intuitively because if the block laying flat, gravity is maximized in the y-direction. But if you put the block on an incline, the force acting perpendicular to the block will decrease because there are now two components. Increasing theta will cause the gravity component perpendicular to the block to decrease; thus, decreasing the normal force. Decreasing the normal force will decrease the frictional force; the best answer is B.

YES, that's exactly what I thought too, but the answer is D 🙁 Apparently, increasing theta will decrease the maximum value of static friction (from 74 N to 65 N), but it also increases the force applied (from 17 N to 50 N). The block continues to remain at rest, but the force of static friction is now 50 N instead of 17 N?

 

[Fort]

YES, that's exactly what I thought too, but the answer is D 🙁 Apparently, increasing theta will decrease the maximum value of static friction (from 74 N to 65 N), but it also increases the force applied (from 17 N to 50 N). The block continues to remain at rest, but the force of static friction is now 50 N instead of 17 N?

It's a tricky question, and a bit poorly worded. The maximum static friction to hold the block up would decrease from 74 N to 65 N, so that is true. But the force required actually increases from 17 N to 50 N. As long as the the maximum static friction exceeds the parallel component of the block on the incline, then the block will remain at rest. So, yeah I guess, increasing theta to 30 degrees will increase the frictional force to keep the block at rest, but the maximum frictional force will decrease; that's what I immediately thought when I read the question.

For the answer, increasing the coefficient would increase the maximum frictional force, but it doesn't actually change the frictional force being applied. The frictional force applied is mgsin(theta), since none of these variables change then the frictional force doesn't change.

Like I said, I think they could've worded it better; but it just goes to show that drawing diagrams is better than doing things in your head. I guess I would know the parallel and perpendicular components of gravity on an inclined plane.

Parallel = mgsin(theta)

Perpendicular = mgcos(theta)

I know you can derive these from a diagram and what not, but it may be faster to just know them off the top of your head like Coulomb's law and such. Oh, and my apologies; I wasn't thinking too hard about this problem.

 

[ThirdEye]

Ugh... I don't like this question at all.

 

[matth87]

Force Friction = mgcos(theta) (coefficient static friction)

Increasing the angle will make cos(theta) smaller. Thus making Ff smaller.

Answer is B.

 

[Doodl3s]

Force Friction = mgcos(theta) (coefficient static friction)

Increasing the angle will make cos(theta) smaller. Thus making Ff smaller.

Answer is B.

Sorry, matth, you'd be WRONG on the MCAT 😱

Fort has the answer explained perfectly. Changing the coefficient only effects the maximum static friction!!

A good trick here for everyone is to remember when dealing with kinetic friction, its always the max, HOWEVER, WHEN DEALING WITH STATIC FRICTION. You only use enough to balance its own weight. You almost NEVER use the maximum for static UNLESS you're trying to overcome it.

 

[loveoforganic]

tricky question

 

[sv3]

static friction only exists to trick people on tests...i truly believe that. Just remember that and you'll always think twice. Otherwise, a static friction question is just too damn easy.

You've gotto set it equal or compare it to an opposing force usually. That can tell you a few things, mainly if the object is at rest it'll tell you the force of static friction, or it can also tell you if there's enough force to overcome the force of static friction and therefore you have a moving object.

pookiez...nice post......helpful indeed

 

[matth87]

ouch good question lol

 

[sv3]

i have yet to do the AAMCs but is this typical of an official MCAT type question? It seemed logic would lead you to one answer, but you had to go a couple of steps further to actually get the right one. I'm hoping they are not all like this. It would be ok if we had tons of time, not so much as it is.

 

[G1SG2]

i have yet to do the AAMCs but is this typical of an official MCAT type question? It seemed logic would lead you to one answer, but you had to go a couple of steps further to actually get the right one. I'm hoping they are not all like this. It would be ok if we had tons of time, not so much as it is.

This is a Kaplan question.

How would we know what the maximum static friction is without knowing what cos 10 is? I mean, how would we know that, the maximum static friction wasn't exceeded when theta increased? Or do we assume that it wasn't exceeded since there was no mention of the block sliding down the plane or something? I hate static friction.

 

[loveoforganic]

You can calculate whether or not it would slide by calculating force gravity is pulling it down the slope with and comparing it to the static force.

 

[G1SG2]

You can calculate whether or not it would slide by calculating force gravity is pulling it down the slope with and comparing it to the static force.

But how would I know if it will slide if I don't know what the maximum force of static friction would be?

 

[loveoforganic]

You can calculate the maximum static force too. mgu cos theta

 

[G1SG2]

You can calculate the maximum static force too. mgu cos theta

Yeah but what if I don't know what cos theta is (for example, in our problem theta=10 degrees-do I have to know what cos 10 degrees is)? 😕 I mean, I know it's close to cos 0, so do I take it to be around 1, and thus assign a high value to static friction?

 

[loveoforganic]

That's what I'd do. If rounding doesn't work... Oh well!

 

[EyEnStein 07]

Yeah but what if I don't know what cos theta is (for example, in our problem theta=10 degrees-do I have to know what cos 10 degrees is)? 😕 I mean, I know it's close to cos 0, so do I take it to be around 1, and thus assign a high value to static friction?

You dont have to memorize it but i dont think you have enough time to do all the calculations. You can just pick a number like..maybe....".99" or ".98" you know its going to be very close to 1 because cos 30deg. = about .866

I remmber learning something about ranges of static friction and kinetic friction. I believe coeff. static friction is typically around .5 - .7

 

[G1SG2]

That's what I'd do. If rounding doesn't work... Oh well!

You dont have to memorize it but i dont think you have enough time to do all the calculations. You can just pick a number like..maybe....".99" or ".98" you know its going to be very close to 1 because cos 30deg. = about .866

I remmber learning something about ranges of static friction and kinetic friction. I believe coeff. static friction is typically around .5 - .7

Yeah, that's what I thought, rounding would probably be the way to go.

Thanks everyone.