TPR wtf moment ? (this is about inclined planes)

That is totally incorrect. The frictional constant (mu) depends on the two materials in contact. Unless the material is identical in both situations, you cannot assume that mu is the same.

unsung said:

(Edit: correction, I can't edit the title of the thread... but it's wrong. The question was actually about a block on a horizontal surface, connected via a pulley to another block over the edge of a table. Anyway the same question would apply there too.)

So, this bothers me a bit. In physics class today, we were told that for a block on a surface with friction, the frictional force depends only on the mass of the block.

Okay... I get that Ff = (mew)(N), where N is the normal force of the block. So, sure, Ff depends on mass.

But doesn't frictional constant mew (whether static or kinetic) depend on both the material of the surface and the material of the block?

There was a problem we did today in class (for those of you who've taken it, this is passage 1 out of the in-class compendium), where they essentially asked, if we replace the block on the surface with another block with identical *mass*, but with a different density, would frictional force change.

Well, I interpreted the question as- if the block is identical (i.e. same size/shape, same mass), yet still has a different density, it must be made out of a different material from the original block. If this block is made of a different material, then the frictional constant between it and the surface must be different.

Because mew as a constant, depends on the material of the block and the material of the surface...

...or so I thought.

But the *correct* answer (according to TPR) was that the frictional force would NOT change.

Please tell me I'm not delusional... is this really an assumption that the MCAT wants us to make? (i.e. we're not supposed to consider that the material of the block affects the frictional constant mew) Or, is this just something TPR wants us to assume?

I'd hate to adopt this assumption, then take the MCAT and find out their answer actually corresponds with reality...

Click to expand...

You have a valid point, but you made an assumption too. You assumed that the density was different because it was a different material. It could be less dense because it is a larger block of the same material that is hollow, thereby having the exact same mass as before but taking up more volume. Density changes with volume as well as material.

On the MCAT, you'll get a feel for what they want, most of the time. If the passage dealt with several different blocks of identical mass but different materials, then your logic is right on the mark. But if the passage shows that it's the same material shaped differently each time, then the µ is the same. Don't make any preconceived notions about what the MCAT wants, because each passage gas a different author and thus a different expectation level.

It's good to think, but doing well on this exam often comes back to keeping things simple.

BerkReviewTeach said:

You have a valid point, but you made an assumption too. You assumed that the density was different because it was a different material. It could be less dense because it is a larger block of the same material that is hollow, thereby having the exact same mass as before but taking up more volume. Density changes with volume as well as material.

On the MCAT, you'll get a feel for what they want, most of the time. If the passage dealt with several different blocks of identical mass but different materials, then your logic is right on the mark. But if the passage shows that it's the same material shaped differently each time, then the µ is the same. Don't make any preconceived notions about what the MCAT wants, because each passage gas a different author and thus a different expectation level.

It's good to think, but doing well on this exam often comes back to keeping things simple.

Click to expand...

Sorry,

but I think you're wrong on this one. You're making the assumption that the block is the same material. This is an ambiguous question. Actual MCAT questions will be clear as to what you are looking for.

bluemonkey said:

Sorry,

but I think you're wrong on this one. You're making the assumption that the block is the same material. This is an ambiguous question. Actual MCAT questions will be clear as to what you are looking for.

Click to expand...

That is my exact point, you have to make an assumption. If you read through my post, you will see that I say (1) if the passage addresses changing materials, then µ is different. But (2), if the passage addresses same material and different shape and possible hollowness, then µ is the same.

The question is ambiguous, which is why the answer is ambiguous.

unsung makes a good point that the material could be different. I'm adding that while that is true, it could also be a case of different dimensions, which would make the PR example correct. Both make valid points, but without more detail, no one is right.

As for the MCAT not having any ambiguous questions, if that were true, would there be huge threads after every MCAT of people complaining about a couple of questions? I'm sure most questions are well written, but on occassion, there will be something like this. You have to take the experiment into account.

unsung said:

Thanks, that's a good point. I think I get what they were asking now. I was just a bit wary since in lecture it was drilled into us that frictional force depends on *mass*, but we didn't go over how mew changes. Anyway, I think I get it now. Thanks.

Click to expand...

BR review scores again.

unsung said:

Well, I interpreted the question as- if the block is identical (i.e. same size/shape, same mass), yet still has a different density

Click to expand...

Just to note for future reference, if a block A has a density different from block B, but the same mass, they cannot be the same volume.

d=m/v
m1=m2 therefore d1v1=d2v2
v2=(d1/d2)v1 and since d1 != d2, v2!=v1

Otherwise, BerkReviewTeach did a good job, IMHO, illuminating things.

I wanted to throw in that I think your original assumption about the block changing materials is far-fetched. In problems where something is changed (mass, density, etc.) it is not pertinent to assume OTHER changes. The point of the problem is to test your understanding of the parameter that the problem altered. By adding your own changes to properties of the problem you are essentially changing the problem. They are trying to test a specific concept with the change that they made. Remember, these are artificial physics problems! Try and guess what they are trying to test.

I would say, (like BerkReivew) that unless something in the passage strongly leads you to believe that the material of the block is changing, you should never make an assumption like that. Use the information that they've given you.

To add, I think that its great that you are thinking about the problem in the detail that you are, but be wary about what the question is trying test rather than, what you think is *actually* right. Remember, try and get in the test-makers head to figure out what their intention was with the question.

In this question I realized that they were probably trying to test my understanding of how changing the density affects friction. Then you just take a quick look at the equation to see that density does not affect friction. The MCAT is basic concepts. Stick to that and you'll be right almost all of the time.

Threads like this make me realize why SDN is so dang valuable. What a great set of repsonses evolved out of this. It was well-thought, polite, and full of great ideas. Cudos to all!

So on that note, I figured it would be nice to develop some questions.

Consider the following experiment:

• Four different blocks of equal size and shape but different material and thus different densities are placed on the right side of a flat plank of slate on a horizontal table. Each plank is lifted from the right side such that the block lifts with the slate while the left edge of the slate surface remains in tact with the table. The research records the angle at which the block breaks free and begins to slide down the slate surface.

Table 1 represent the angles:

Block 1 37.9 degrees from horizontal
Block 2 49.2 degrees from horizontal
Block 3 26.1 degrees from horizontal
Block 4 31.0 degrees from horizontal


Q1
Which block has the greatest µstatic?
A) Block 1
B) Block 2
C) Block 3
D) Block 4

Q2
How can the differing angles be explained?
A) Each block has a different mass.
B) Each block has a different density.
C) Each block has a different µstatic.
D) Each block has a different shape.

Q3
During a subsequent experiment, the researcher quickly lowered the angle with respect to the horizontal surface until the block stopped slidding. In each case, to get the block to begin sliding again, the angle had to be returned the the threshold value listed in Table 1. How can this be explained?
A) The normal force varies when the block goes from stationary to moving.
B) Kinetic friction heats up the surface of the slate.
C) The µstatic for each material is greater than its µkinetic.
D) The µstatic for each material is less than its µkinetic.

Q4
If all four blocks were placed on a slab of slate that made a 50 degree angle with the horizontal, which one would reach the bottom of the slate fastest from rest, assuming µstatic to be a good predictor of µkinetic?
A) Block 1
B) Block 2
C) Block 3
D) Block 4

Good table/questions... would you consider them MCAT-like? If so, I can deal with that

BerkReviewTeach said:

Threads like this make me realize why SDN is so dang valuable. What a great set of repsonses evolved out of this. It was well-thought, polite, and full of great ideas. Cudos to all!

So on that note, I figured it would be nice to develop some questions.

Consider the following experiment:

• Four different blocks of equal size and shape but different material and thus different densities are placed on the right side of a flat plank of slate on a horizontal table. Each plank is lifted from the right side such that the block lifts with the slate while the left edge of the slate surface remains in tact with the table. The research records the angle at which the block breaks free and begins to slide down the slate surface.

Table 1 represent the angles:

Block 1 37.9 degrees from horizontal
Block 2 49.2 degrees from horizontal
Block 3 26.1 degrees from horizontal
Block 4 31.0 degrees from horizontal


Q1
Which block has the greatest µstatic?
A) Block 1
B) Block 2
C) Block 3
D) Block 4

Q2
How can the differing angles be explained?
A) Each block has a different mass.
B) Each block has a different density.
C) Each block has a different µstatic.
D) Each block has a different shape.

Q3
During a subsequent experiment, the researcher quickly lowered the angle with respect to the horizontal surface until the block stopped slidding. In each case, to get the block to begin sliding again, the angle had to be returned the the threshold value listed in Table 1. How can this be explained?
A) The normal force varies when the block goes from stationary to moving.
B) Kinetic friction heats up the surface of the slate.
C) The µstatic for each material is greater than its µkinetic.
D) The µstatic for each material is less than its µkinetic.

Q4
If all four blocks were placed on a slab of slate that made a 50 degree angle with the horizontal, which one would reach the bottom of the slate fastest from rest, assuming µstatic to be a good predictor of µkinetic?
A) Block 1
B) Block 2
C) Block 3
D) Block 4

Click to expand...

b,c, a, c

TheBoondocks said:

b,c, a, c

Click to expand...

I'd disagree on your third answer. The normal force is the same (given the same angle) whether the block is moving or stationary. However, since the mu(static) is greater than the mu(kinetic), a greater angle is required to start the block moving than the one that stops it moving.

I'd say c. I've been known to be wrong before, though. Any physics whizzes in the house?

JoeC said:

I'd disagree on your third answer. The normal force is the same (given the same angle) whether the block is moving or stationary. However, since the mu(static) is greater than the mu(kinetic), a greater angle is required to start the block moving than the one that stops it moving.

I'd say c. I've been known to be wrong before, though. Any physics whizzes in the house?

Click to expand...

i misread it, it's indeed c. i thought he brought it back all the way to the plank to stop it, but he lowered the angle and it stopped so this would be due to mu static being greater. even so, the normal force doesn't change from stationary to moving, so that couldn't be right. using poe i should have said c but i'm in physics 1 right now and got this confused with a different problem in class where we were concerned with normal changing.

1. B
2. C
3. C
4. C

I hope the MCAT gives me passages this straight-forward! Thanks for the questions.

biocmp said:

I hope the MCAT gives me passages this straight-forward! Thanks for the questions.

Click to expand...

That would be nice. From the sounds of things, you might get one or two this straight-forward out of seven.

BerkReviewTech,
When will your CBT's be made available? I am interested in all of the practice I can get.

biocmp said:

BerkReviewTech,
When will your CBT's be made available? I am interested in all of the practice I can get.

Click to expand...

Supposedly this Friday or Saturday, but my bosses are not always the best with deadlines. The exams are all set and the website is set, so I'm not sure what the hold up is. I'll keep checking the website and when it finally pops up as an option, I'll let you know.

On a different note, those questions from above seemed a bit easy. So...

Q5
If the block had a conducting loop around its perimeter that was parallel to the slate and there was a B field pointing directly up from the slate, what would be observed as the block slid down the slate?
A) A clockwise current (when viewed from the top of the block) would be induced into the loop; the block would take the same amount of time to slide down the slate as it did without the presence of a B field.
B) A counterclockwise current (when viewed from the top of the block) would be induced into the loop; the block would take the same amount of time to slide down the slate as it did without the presence of a B field.
C) A clockwise current (when viewed from the top of the block) would be induced into the loop; the block would take a greater amount of time to slide down the slate as it did without the presence of a B field.
D) A counterclockwise current (when viewed from the top of the block) would be induced into the loop; the block would take a greater amount of time to slide down the slate as it did without the presence of a B field.

Q6
A block sliding down a 50-degree slope strikes a stationary block of equal mass at the base of the slate and the two blocks slide together for a short distance across the table before coming to rest. Which of the following combination of blocks will likely slide the greatest distance after collision?
A) Block 1 sliding into a stationary block of Material 2
B) Block 2 sliding into a stationary block of Material 3
C) Block 3 sliding into a stationary block of Material 4
D) Block 4 sliding into a stationary block of Material 1

Q7
If the experiment were repeated with objects made from the same materials, but now with equal volume as before but a rounded base so that less of the object touched the slate, what would be expected?
A) The angle needed to break free would be reduced, and the speed at which the block reached the bottom of the slate would be greater.
B) The angle needed to break free would remain the same, but the speed at which the block reached the bottom of the slate would be greater.
C) The angle needed to break free would be reduced, but the speed at which the block reached the bottom of the slate would be the same.
D) The angle needed to break free would remain the same, and the speed at which the block reached the bottom of the slate would be the same.

I haven't finished physics II yet, but I wiki'd B fields, and here is what I came up with.

Q5. B, I think the right hand grip rule tells us which way the current would flow from above. Either field would have the same effect on the block, so I think I can rule out the other answers that say it would cause a change in time.

Q6. B. Block 2 has the greatest density, so I'm assuming it would carry the greatest momentum. Once it strikes block 3, which has the lowest static friction coefficient, they would continue the furthest. I think my reasoning is correct.

Q7. C. I'm guessing that the strength the static frictional force goes down with reduced area touching the table, so the blocks would break free quicker, but they will still be acting against the constant force of gravity, so the speeds will be constant. (this is tricky though, because they angle which they break free will be smaller, so the mgsin (theta) force will be smaller, taking longer for it to reach the bottom, but comparatively speaking, they will move at the same speeds.

Now those questions hurt my brain and made me second guess everything! great questions!

For seven, I'd say A, but there's really no way to tell, is there? I mean, if we don't know the actual numbers, we can't really work it out, but I would assume the lack of frictional force would overcome the greater gravitational potential energy of the blocks because of their higher mu-k's.

JoeC said:

For seven, I'd say A, but there's really no way to tell, is there? I mean, if we don't know the actual numbers, we can't really work it out, but I would assume the lack of frictional force would overcome the greater gravitational potential energy of the blocks because of their higher mu-k's.

Click to expand...

That's a trick question of sorts. If you think about sliding a box versus sliding a ball across the floor, if they have the same mass, then it takes the same amount of work to slide each over the same distance. The normal force and µkinetic are independent of the contact area. Assuming that the object and surface do not deform, the frictional force should remain the same, whether the object is flat or curved on bottom. The question gives us no reason to assume that either the object or the slate deforms from interacting with one another, so...

there should be no impact whatsoever.

On questions where they don't give you the numbers and details you think you need, it's often because those details are irrelevent, and therefore the answer is a simple one. Keep things simple!

biocmp said:

Q5. B, I think the right hand grip rule tells us which way the current would flow from above. Either field would have the same effect on the block, so I think I can rule out the other answers that say it would cause a change in time.

Q6. B. Block 2 has the greatest density, so I'm assuming it would carry the greatest momentum. Once it strikes block 3, which has the lowest static friction coefficient, they would continue the furthest. I think my reasoning is correct.

Click to expand...

For Q5, you are right to use the right-hand rule (coupled with Lenz/s law) to determine a counterclockwise current. The question lies in whether or nor that current will have an impact on the blocks motion. There are two lines of reasoning. The first is that electrical current is a form of energy, so if there is a current caused by the motion of the block, then some of the block's kinetic energy must be lost to generate the current. As such, the block must slide with a lower average speed, and therefore take longer to reach the bottom. The other line of reasoning come's from Faraday's perspective, where the induced current is the result the induced emf. The induced emf occurs to oppose the change in flux. One problem with my question is that we are not told if the loop is in the field the enrire time. Current will only be induced in the loop when there is a change in flux (when it enters the field, exits the field, or goes through a region of varying B-field density or orientation). As such, the second line of reasoning has some gaps in information.

But alas, because I'm taking a multiple choice exam, test-taking skills come into play. Every answer talks about an induced current, so there must be a change in the flux. This means that there must also be a loss of kinetic energy from the sliding block to generate the current. As such, the block must travel slower and thereby take longer.

For Q6, what makes you say that Block 2 is the densest?

It ends up that such a point was made irrlevent by the equal mass stipulation in the question. In every case, the mass of the system doubles when the blocks stick together. So, the velocity of the moving block is the most significant factor in determining its impact momentum. Because all of the blocks were released from a 50-degree angle for on slate, they are all released from rest with the same gravitational potential energy. As the blocks slide down, the one that reaches the bottom with the greatest kinetic energy (and thus greatest speed), is the one that loses the least energy to work done by friction. That should be block 3 according to the data in the table, as it has the smallest µs and therefore probably has the smallest µk. The question from here is how does having the greatest kinetic energy relate to having the greatest momentum.

There is an assumption made that the paired-objects sliding distance depends more on momentum at impact that their combined µk as they slide across the table, which may not be entirely correct. But it's an assumption that can be dismissed because of the answer choices. Afterall, the object combination with the greatest impact momentum will include the block with the lowest µk. It also happens that that a block made from Marterial 4 has the next lowest µk, so a block of Material 3 striking a block of Material 4 has the greatest impact speed and the lowest individual µk values.