Why is Physics So Hard? Advice to Improve

[Angry Citizen]

That's correct. In order to affect a decreasing change in velocity, the acceleration would have to be in the opposite direction of movement.

 

[Ryker]

Use your definitions. Kinetic friction is dependent on four things: The mass of the object, the magnitude of local gravity conditions (little g), the coefficient of kinetic friction, and any extra force in the y direction. We know little g is 9.8 meters per second squared (assuming this question wasn't one of those alien planet questions). Do we know the masses of the objects? If not, are they all the same mass? Can we assume that all the coefficients of kinetic friction are the same?

From the problem definition, we know that one of the boxes has extra force in the y direction, while one of the boxes does not. The extra force in the y direction, depending on whether it worked with gravity (pushed down on the box) or against gravity (pulled up on the box), would affect the normal force acting on the box. Use the fact that force is a vector quantity to separate it into its x and y components. For the horizontal force, this is unnecessary because all the force is in the x direction. For the funny one at an angle, you need to separate it into its components using vector algebra.

So, if you have three boxes of equal mass (m1=m2=m3), all of the same material and on the same surface (μk1=μk2=μk3, or in other words all the coefficients of kinetic friction are the same), and they're all in the same gravity field (Earth's), then the only possible difference can come from the direction in which the equal force is applied. Thus, the box with the greatest kinetic friction is going to be the box whose force vector is directed downwards (with gravity, thereby increasing the acceleration, thus increasing the normal force, thus increasing the kinetic friction), and the one with the least kinetic friction is going to be the one whose force vector is directed upward (against gravity, thereby counteracting some of the acceleration, thus reducing the normal force, thus reducing the kinetic friction).

Hope this helps.

Hold on, if the force is applied straight downward, then there will be no kinetic friction, since only the normal force will be counteracting the force. You can't have kinetic friction if the force is parallel to the movement (well, there is no movement) and doesn't try and move the box in the direction parallel to the surface.

 

[Angry Citizen]

Hold on, if the force is applied straight downward, then there will be no kinetic friction, since only the normal force will be counteracting the force. You can't have kinetic friction if the force is parallel to the movement (well, there is no movement) and doesn't try and move the box in the direction parallel to the surface.

You're right, of course, but I felt that was pretty obvious given that the problem was about kinetic friction. If the vector is pointed downward while still retaining an x component, and given that the force exceeds the maximum static friction, then my statement remains true.

 

[NeedsHelp1212]

got a 94:) Highest grade in my period :)

 

[Chunkysalsa]

Congratulations, you seem like you've been working hard this year and deserve it.

Do you guys do any E&M stuff, I don't remember doing anything relating to that or optics in High School (but I barely remember high school physics anyway, snoozed to a C).

 

[Brown Arrow]

Step 3 is absolutely terrible. Since when did Physics become an exercise in memorizing formulas?

i agree. i learned it the hard way :/

never sit there and just memorize formulas or constant... when you do question it automatically gets built into your memory... this is how i got to know my stufff in time for exam...

 

[NeedsHelp1212]

Congratulations, you seem like you've been working hard this year and deserve it.

Do you guys do any E&M stuff, I don't remember doing anything relating to that or optics in High School (but I barely remember high school physics anyway, snoozed to a C).

As of this point no, but i remember looking ahead in the book and i definitely saw stuff about optics

 

[NeedsHelp1212]

What is up everyone! I finished with an A+ in the 3rd marking period and just got 1 more marking period to go. But i actually decided to take ap physics next year as a senior. This is crazy, at the beginning of the year I HATED PHYSICS and was horrible at it, but now i actually like physics and am good at it. Thanks for the motivation everyone. Next big decision; what should i major in college. Either something with engineering or business. Big difference but I like both and have taken classes that prepare for both so far in high school (accounting)

 

[Disinterred]

Physics is easy, Math Is Hard.

This is hilarious since any physics problem requires

1) Mathematical Knowledge
2) Physical Intuition

Any math problem requires

1) Mathematical Knowledge

What is harder?

The answer is both can be made incredibly difficult, but physics will always involve that one extra ingredient which can makes problems that much more difficult: physical intuition.

Furthermore, in physics we cannot make up axioms and prove stuff based on them, we are limited by the true test of all truth: experiment. Which also makes physics that much harder.

 

[Shackleford]

Operationally, they are hard because, in many cases, to solve problems, it requires quite a bit of creativity.

My cal 2 professor used the term "mathematical trickery" on more than one occasion when deriving things. I've noticed in CM, QM, and to a lesser extent surprisingly in Thermal, that they pull stuff out of their ***. It is also the same for mathematics.

"Where the heck did that come from?"

I think I like mathematics more because of its axiomatic approach. You use definitions to create more definitions and identify new relationships, etc.

 

[Chunkysalsa]

This is hilarious since any physics problem requires

1) Mathematical Knowledge
2) Physical Intuition

Any math problem requires

1) Mathematical Knowledge

What is harder?

The answer is both can be made incredibly difficult, but physics will always involve that one extra ingredient which can makes problems that much more difficult: physical intuition.

Furthermore, in physics we cannot make up axioms and prove stuff based on them, we are limited by the true test of all truth: experiment. Which also makes physics that much harder.

Is there no such that as Mathematical Intuition. I'm pretty sure it took a lot of creativity and intuition to devise all of this knowledge you call upon to do your physics.

 

[lisab]

What is up everyone! I finished with an A+ in the 3rd marking period and just got 1 more marking period to go. But i actually decided to take ap physics next year as a senior. This is crazy, at the beginning of the year I HATED PHYSICS and was horrible at it, but now i actually like physics and am good at it. Thanks for the motivation everyone. Next big decision; what should i major in college. Either something with engineering or business. Big difference but I like both and have taken classes that prepare for both so far in high school (accounting)

Wow, excellent job, congrats!

You can always double major - not a bad combination, those two.

Keep in mind, having an engineering degree doesn't prevent you from going into business, but having just a business degree pretty much excludes you from getting an engineering job.

 

[flyingpig]

As Walter Lewin would say "you must have had a bad teacher"

 

[NeedsHelp1212]

last day of school today!

I'm not sure what my final grade is until report cards are mailed out but I am hoping i somehow got to an A+ for the year. The final exam was surprisingly easy and i feel i aced it.

The bad news is AP physics will most likely not be available in my school next year. Under 10 people elected to take the class so there most likely not going to offer it. This really sucks since I wanted to challenge myself further. Instead I am going to take AP Bio, since i want an AP science for colleges to see (and i just hate chemistry even though i get good grades in that class).

So yeah, thanks for the advice guys. It's weird looking back at this thread and seeing what people wrote back in October when i felt so horrible about my grades and where i am now. Hopefully, somehow they offer ap physics next year.

**Btw fylingpig: My teacher is not in any way bad, on the other hand I feel he is one of the best teachers I have ever had. He teaches the material clearly and will always be available for help.