Physics question thread

[Shrike]

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All users may post questions about MCAT and OAT physics here. We will answer the questions as soon as we reasonably can. If you would like to know what physics topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm), though be warned, there are subjects listed there that are rarely tested, or that appear in passages only and need not be learned.

Be sure to check the Physics FAQs and Topic Writeups thread if you have a general question; eventually, many of your answers will be located there. Also, a request: to keep this thread at least somewhat neat, when replying to someone else's post please refrain from quoting anything more than what's necessary for clarity.

Acceptable topics:

• general, MCAT-level physics
• particular MCAT-level physics problems, whether your own or from study material
• what you need to know about physics for the MCAT
• how best to approach to MCAT physics passages
• how best to study MCAT physics
• how best to tackle the MCAT physical sciences section

Unacceptable topics:

• actual MCAT questions or passages, or close paraphrasings thereof
• anything you know to be beyond the scope of the MCAT

Side note: anyone who knows how to post subscripts and superscripts in this system, please PM me the method. I also wouldn't mind knowing how to post some obvious symbols, such as Greek letters and an infinty sign. Should be a matter of changing fonts, but I don't know what's available; again, a PM would be appreciated.

If you really know your physics, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current official contributors to the this thread -- a team to which I hope to add more people:

Thread moderated by: Shrike. Shrike is a full-time instructor for The Princeton Review; he has taken the MCAT twice for no good reason, scoring 14 on the physical sciences section each time. He majored in mathematics, minored in physics, and spent several years accumulating unused school experience (in economics and law).

Also answering questions: Xanthines, a Kaplan instructor. He scored 13 on the PS section of the MCAT and 34 overall.

 

[vmp200]

An electron moves in a counterclockwise
circle in the plane of the page due to a uniform
external magnetic field. In what direction
does the external magnetic field point?
A. into the page
B. out of the page
C. counterclockwise
D. clockwise

Kaplan says its B, but wouldnt it be A since its an electron??

thanks
VP

 

[dermchick]

An object is initially at rest then moves in a straight line and accelerates uniformly at a rate of 2m/s^2. What is the average speed of the object in the third second of its motion?

The answer is 5m/s? Can someone explain?

 

[Nutmeg]

An object is initially at rest then moves in a straight line and accelerates uniformly at a rate of 2m/s^2. What is the average speed of the object in the third second of its motion?

The answer is 5m/s? Can someone explain?

aceleration is dv/dt, and velocity is the inverse, or int(a)dt.

V(t) = Vo + int(a)dt = 0 + t^2

The third second is from t = 2 to t = 3, so

average velocity = [int(V(t))]/t = [3^2 - 2^2]/1 = 9 - 4 = 5 m/s

 

[Nutmeg]

test

 

[Nutmeg]

An electron moves in a counterclockwise
circle in the plane of the page due to a uniform
external magnetic field. In what direction
does the external magnetic field point?
A. into the page
B. out of the page
C. counterclockwise
D. clockwise

Kaplan says its B, but wouldnt it be A since its an electron??

thanks
VP

dammit why can't I answer this damn thing?!?!?!?!?!?

 

[Nutmeg]

for the sixth effing time

A positive charge moving counterclockwise will, by the RHR, induce a field out of the page. So a negative charge would induce it in the opposite direction, or into the page. But the induced field is necessarily in the opposite direction of the extrenal field that forces an electron to go in circles, so the external field is necessarily out of the page.

And if my web browser crashes again, I'm going to smash my effing monitor into a million pieces.

 

[Nutmeg]

I could quote vmp200, I could answer the question, but every effing time I tried to do both in a single post, the damn thing crashed! I wrote the thing5 times and tried to post it 8 times!!!! WTLF!?!?!?!

Now if you'll excuse me, I have some broken glass and plastic monitor casing to clean up.

 

[GCT]

An electron moves in a counterclockwise
circle in the plane of the page due to a uniform
external magnetic field. In what direction
does the external magnetic field point?
A. into the page
B. out of the page
C. counterclockwise
D. clockwise

Kaplan says its B, but wouldnt it be A since its an electron??

thanks
VP

it's simple right had rule application. The force on the electron is centripetal as magnetic fields can't change the kinetic/translational energy of the electron, the palm of your hand represents the force, your thumb velocity, the rest of the four fingers pointing perpendicularly to the latter two, the magnetic field. Note the differences in application to a positively charged versus negatively charged. If a postively charged particle will follow a clockwise direction according to RHR, negatively charged particle will go counterclockwise.

 

[challie2385]

What exactly is terminal velocity, and how is it used on the MCAT? Is it anything like final velocity? Thanks for your reply!

 

[vmp200]

Thanks alot of the help nutmeg and GCT, and sorry about ur moniter nutmeg.

 

[GCT]

it's best that you look this up

due to the gravitational acceleration, velocity increases during free fall until it somewhat "equilibriates" with air resistance, the velocity will be constant furthermore.

 

[proteolytic]

Long time reader, First time poster.🙂 Is the normal path of light, the path that light would take if there was not a difference in the indices of refraction. For example, how would you find the normal when light travels through a prism from air?

 

[GCT]

not sure what you're asking here, the normal to the prism is simply perpendicular any of the planes of the prism

 

[Zuras]

Simple "displacement" question(I'm extremely physics rusty):

An object travels upwards for 6 km, north for 10 km, and east for 8 km. What is the displacement? The answer is supposedly 14, and they show that "sqrt(6^2+8^2+10^2) = net displacement."

I don't understand how they get this. They do try to explain, but it doesn't make any sense to me. I get 20, more like I "guessed" 20. I just added 10 to the hypotenuse of 8 and 6.

 

[GCT]

Draw a diagram to see what's going on. It has more to do with elementary math than physics (pythagorean theorem), or better yet try picturing the situation (aids may help e.g. pencil). You'll need to find the hypotenuse of 10 and 8, than the hypothenuse of "10 and 8" and 6.

 

[Zuras]

Draw a diagram to see what's going on. It has more to do with elementary math than physics (pythagorean theorem), or better yet try picturing the situation (aids may help e.g. pencil). You'll need to find the hypotenuse of 10 and 8, than the hypothenuse of "10 and 8" and 6.

Sorry, I don't recall 3-d displacement as elementary math. I'm not "seeing" why I am supposed to be solving it that way. I'm looking for an explanation why. I am getting the hypotenuse of 10 and 8, the north and up vectors. This gives me the north/upward facing vector. Now I have to get the east vector in as well, but that's what doesn't make any sense.

 

[Mister Pie]

Sorry, I don't recall 3-d displacement as elementary math. I'm not "seeing" why I am supposed to be solving it that way. I'm looking for an explanation why.

If you draw it out in 3D, you get a point in space. At some point in your math career, you probably learned that the distance from a point to the origin is the square root of x^2 + y^2 + z^2. Hope that helped.

 

[Zuras]

If you draw it out in 3D, you get a point in space. At some point in your math career, you probably learned that the distance from a point to the origin is the square root of x^2 + y^2 + z^2. Hope that helped.

Ok, I guess that does help. So, I either don't recall that formula/trick, or I just never learned it. It doesn't seem very intuitive. Thanks.

 

[GCT]

you just need to apply the pythagorean theorem. If you draw it out or visualize it you'll see, don't really need the formula. The verticle side of the displacement vector is 6, the horizontal side is actually the hypotenuse of z, x, represented by north and east.

 

[Zuras]

I don't understand centripetal acceleration/force.

I understand that:

An object at a constant speed about a circular path, the magnitude of the velocity is always the same, but it is constantly changing directions. This makes sense. What doesn't make sense, is that the net result of this is that it leads to continual acceleration.

 

[GCT]

I don't understand centripetal acceleration/force.

I understand that:

An object at a constant speed about a circular path, the magnitude of the velocity is always the same, but it is constantly changing directions. This makes sense. What doesn't make sense, is that the net result of this is that it leads to continual acceleration.

All of this can be best explained mathematically, but it may be a bit too advanced to discuss here. One way to think about it is to apply standard coordinates to the velocity vectors, observe the continual change in the magnitude of x and y velocity vectors.

 

[QofQuimica]

I don't understand centripetal acceleration/force.

I understand that:

An object at a constant speed about a circular path, the magnitude of the velocity is always the same, but it is constantly changing directions. This makes sense. What doesn't make sense, is that the net result of this is that it leads to continual acceleration.

This happens because velocity is a vector, and it is therefore dependent on direction. So even if you don't change the magnitude of the velocity (i.e., the speed), if you change the direction of the velocity, then your object has undergone an acceleration.

 

[hippocampus]

if you want a ball to travel far, would u want it to travel with the wind or against it? im confused because two passages say different things. one passage says u should fly against to increase lift (since velocity decreases at the bottom of the ball, and pressure increases) so it will increase time in flight, therefore increasing the distance. another passage says u should fly with the wind...? why??

also...
a s kg object is submerged in the unknown fluid has an apparent loss of mass of 0.5kg. what is the sp gravity of the object?

the answer is 20N... why? 🙁 i hate specific gravity. i know the equations, but i dont get it. also, what do they mean when they say buoyant force is equal the weight displaced? if you throw something into water, what does "displacing weight" mean? and what does that have to do with volume?

thanks

 

[vmp200]

a s kg ??????????????//
s kg? i have never heard of that
do you mean 5 kg??

 

[Zuras]

This happens because velocity is a vector, and it is therefore dependent on direction. So even if you don't change the magnitude of the velocity (i.e., the speed), if you change the direction of the velocity, then your object has undergone an acceleration.

I thought the same, but I cam across this question that I got wrong because I thought that as well. It shows a ball rolling down a curved/bowed incline plane. If what you say is the case, shouldn't a ball rolling down such a plane from the top of it to the bottom increase in both velocity and acceleration?

 

[QofQuimica]

I thought the same, but I cam across this question that I got wrong because I thought that as well. It shows a ball rolling down a curved/bowed incline plane. If what you say is the case, shouldn't a ball rolling down such a plane from the top of it to the bottom increase in both velocity and acceleration?

😕 That's not an example of centripetal acceleration!

 

[Zuras]

😕 That's not an example of centripetal acceleration!

But I was using the same logic that is used for concluding that circular motion is accelerating.

 

[QofQuimica]

But I was using the same logic that is used for concluding that circular motion is accelerating.

Ok, but an object rolling down an incline is not in circular motion, where it is traveling around in a circle. Yes, it is rotating as it rolls, but the overall motion is *linear*.

 

[parrothead_stud]

Consecutive resonances occur at wavelengths of 8 m and 4.8 m in an organ pipe closed at one end. What is the length of the organ pipe?


I know that L = n(wavelength) / 4 where n is an odd number

The answer is 6 m but I cannot see how you can figure it out, not knowing the answer.

 

[Mister Pie]

Consecutive resonances occur at wavelengths of 8 m and 4.8 m in an organ pipe closed at one end. What is the length of the organ pipe?


I know that L = n(wavelength) / 4 where n is an odd number

The answer is 6 m but I cannot see how you can figure it out, not knowing the answer.

Best bet would probably be to plug in the given answer choices and see which one works.

 

[hippocampus]

yes, i meant 5 kg.


when to use Av = Av or the bournallini equation???

Av=Av says that when the pipe is small, the velocity will be great. However, if the pipe is small, wouldn't there be great pressure, making the velocity slower (bournalini)..?

 

[frankrizzo18]

So, if a car is travelling at 5m/s and hits an identical parked car, and they form a perfectly inelastic collision, the two attached cars move at an angle 5 degrees east of north. At what angle was the moving car travelling prior to the inelastic collision? Assume constant friction. Would it be less than five degrees, five degrees, or more than five degrees? Beginning to feel like a brain teaser

 

[Nutmeg]

So, if a car is travelling at 5m/s and hits an identical parked car, and they form a perfectly inelastic collision, the two attached cars move at an angle 5 degrees east of north. At what angle was the moving car travelling prior to the inelastic collision? Assume constant friction. Would it be less than five degrees, five degrees, or more than five degrees? Beginning to feel like a brain teaser

5 degrees. If the first car was moving and the second was stopped, you're adding a zero vector to a non zero vector, resulting in no change in direction.

 

[Nutmeg]

yes, i meant 5 kg.


when to use Av = Av or the bournallini equation???

Av=Av says that when the pipe is small, the velocity will be great. However, if the pipe is small, wouldn't there be great pressure, making the velocity slower (bournalini)..?

No. Small pipes do not mean high pressure. If you open a pipe perpendicular to the direction of flow at a place with a small radius and at a place with a large radius, and measure the pressure at each, the pressure will be lower where the fluid is moving faster, ergo, at the place with the smaller radius.

Think about the airplane wing: faster moving air on top gives lower pressure, creating lift. And for constant flowrates in a pipe, the smaller the cross sectional area, the faster the fluid has to move.

 

[Dr Hibbert]

This is a homework problem from my physics professor. Any help would be greatly appreciated

Ignore all air friction in the following problems:

A baseball is thrown straight up from ground level at a speed of 128 ft/s
1. What is the acceleration at the highest point in ft/s^2:
2. How high will the ball go in ft?
3. How long will it take the ball to get back down to the ground (in sec)?
4. What will be the velocity in ft/s when the ball hits the ground again?

 

[DrBowtie]

This is a homework problem from my physics professor. Any help would be greatly appreciated

Ignore all air friction in the following problems:

A baseball is thrown straight up from ground level at a speed of 128 ft/s
1. What is the acceleration at the highest point in ft/s^2:
2. How high will the ball go in ft?
3. How long will it take the ball to get back down to the ground (in sec)?
4. What will be the velocity in ft/s when the ball hits the ground again?

1. -32ft/s with + being up and - being down.
2. Vf^2 = Vi^2 + 2ad
3. Vf= Vi + at
4. -128 ft/s

 

[Dr Hibbert]

1. -32ft/s with + being up and - being down.
2. Vf^2 = Vi^2 + 2ad
3. Vf= Vi + at
4. -128 ft/s

Thanks for the help Brett. Could you please elaborate a little further on your formulas for #2 and #3.

 

[QofQuimica]

Thanks for the help Brett. Could you please elaborate a little further on your formulas for #2 and #3.

All of the MCAT formulas have been posted already in the Physics FAQ thread.

 

[DrChandy]

"Dont rock the boat..."

nice avatar QofQuimica 👍

 

[Nutmeg]

"Dont rock the boat..."

nice avatar QofQuimica 👍

I was trying to figure that one out, but the rock wasn't clear. That, and the fact that I'm not clever like you. 😉

 

[QofQuimica]

"Dont rock the boat..."

nice avatar QofQuimica 👍

Thanks. I got tired of being a chemical spill. 😛

 

[Doc2007]

Question about momentum.

A women (50kg) is on the massless board with ten 5kg blocks which is on frictionless frozen lake which is 45m away from the rock. The rock is located to the right. She slides five blocks to the right at 6 m/s and after 2s she slides remaining five blocks to the left at 6 m/s. What is her location with respesct to the rock after 2s.

After analyzing the problem, my answer is 51m away from the rock and stationary. But EK did not have that answer choice and their answer choice is 49m from the rock and stationary. For some reason I dont know how they get to 49. Can someone help me?

 

[Nutmeg]

Question about momentum.

A women (50kg) is on the massless board with ten 5kg blocks which is on frictionless frozen lake which is 45m away from the rock. The rock is located to the right. She slides five blocks to the right at 6 m/s and after 2s she slides remaining five blocks to the left at 6 m/s. What is her location with respesct to the rock after 2s.

After analyzing the problem, my answer is 51m away from the rock and stationary. But EK did not have that answer choice and their answer choice is 49m from the rock and stationary. For some reason I dont know how they get to 49. Can someone help me?

I think you just forgot to consider the mass of the blocks she didn't throw.

Conservation of momentum: 25 kg of blocks * 6m/s, and in the other, the total wieght of the woman plus the five blocks is 75 kg. So she should be going 1/6th the speed of the blocks in the opposite direction, or 2 m/s. After two seconds, she goes 4m, meaning her distance is 49 meters.

If she then thows the other five blocks, she's giving them an additional 4 m/s speed, and the 25 kg mass means she's giving herself half of that speed relative to the blocks--meaning 2 m/s in the opposite direction, added to her initial speed. So she would, indeed, be stopped, with the final momentum of all parts being net zero.

 

[Doc2007]

That makes sense. Thank you so much for your help. I like this thread.

I think you just forgot to consider the mass of the blocks she didn't throw.

Conservation of momentum: 25 kg of blocks * 6m/s, and in the other, the total wieght of the woman plus the five blocks is 75 kg. So she should be going 1/6th the speed of the blocks in the opposite direction, or 2 m/s. After two seconds, she goes 4m, meaning her distance is 49 meters.

If she then thows the other five blocks, she's giving them an additional 4 m/s speed, and the 25 kg mass means she's giving herself half of that speed relative to the blocks--meaning 2 m/s in the opposite direction, added to her initial speed. So she would, indeed, be stopped, with the final momentum of all parts being net zero.

 

[theunraveler]

question about linear superposition

two in phase sorces of waves are seperated by a distance of 4m. these sources produce identical waves that have a wave length of 5 m. on the line between them, there are 2 places at which the same type of interference occurs

a) is it constructive or destructive interference? (my ans would be constructive)
b) where are the places located?

 

[Nutmeg]

question about linear superposition

two in phase sorces of waves are seperated by a distance of 4m. these sources produce identical waves that have a wave length of 5 m. on the line between them, there are 2 places at which the same type of interference occurs

a) is it constructive or destructive interference? (my ans would be constructive)
b) where are the places located?

There is only one place where constructive interference occurs on the line, and thiat's in the middle. But there are two places with destructive interference on the line in between Since the wavelength is 5 meters, and since the set up is symetric about the middle, there should be two places, seperated by half a wave length, where the two waves have equal and opposite values. So half the wave length is 2.5 m, and that means that your two similar points are .75 meters from each source, toward the middle.

 

[QofQuimica]

bump

 

[Caboose]

A study buddy of mine challenged me with this question:

A thin steel rod, 60cm in length has a mass of 500g. The rod rotates about an axis that is perpendicular to the rod and that passes through its center. What torque is required to bring the stationary rod to an anqular speed of 4 rev/s in 20s?

I don't think the MCAT covers centripital acceleration and I believe this to be one of those. I am not certain. At any rate, I couldn't do it and no one else seems to know. Here we stand.

Any Smarties still surfing around this forum?
Caboose.

 

[Iwy Em Hotep]

A study buddy of mine challenged me with this question:

A thin steel rod, 60cm in length has a mass of 500g. The rod rotates about an axis that is perpendicular to the rod and that passes through its center. What torque is required to bring the stationary rod to an anqular speed of 4 rev/s in 20s?

I don't think the MCAT covers centripital acceleration and I believe this to be one of those. I am not certain. At any rate, I couldn't do it and no one else seems to know. Here we stand.

Any Smarties still surfing around this forum?
Caboose.

It doesn't require centripetal acceleration, just an equation involving torque, angular speed, and moment of inertia.

 

[nekrogg]

I have a question. Something today discussed by my MCAT instructor really confused me. The statement was: if a person pushed a rock up an incline at a constant velocity, the total work performed on that rock was 0. He stated that this is due to gravity also exerting work on the rock. My question is, if you push a rock up an incline, arent you not only exerting work required to overcome gravity, but additional work to move the rock up the incline?