calling all woozies and other physics ppl

[Furtwangler]

I have a review packet for my physics final and my teacher is retarded and wont give us answers until tomorrow (the test) so for us that wanted to study, i have no way to know if im right. (or in some cases, where to even start because I've forgotten since the beginning of the year) so without FURTHER ADO, if someone can help me with these that'd be great.

1) A ball of Mass "m" is dropped down through the air in a gravitational field. The magnitude of the force of air restance is bv² where b is a positive constant. The magnitude of the accelration of the ball at any time is,
g - b
g - bv²/m
g + bv²/m
g/b
bv² + g


2) A 3kg block is supported on a 60degree incline by a string like shown http://img268.imageshack.us/img268/7145/99701855.png

if the coefficient of static friction is 1.0 the tension in the string is most nearly,
5.3 N
10.8 N
15 N
25.5 N
30 N


3)A block of 4kg is sliding down a frictionless ramp of 30degrees from the horizontal. The block starts at the top of the ramp and takes 4seconds to slide to the bottom.

How high is the top of the ramp?

The speed of the block when it reaches the bottom is most nearly?


Thank you tons if anyone decides to help meh

[Maguspk]

I'll help with 2:

Really simple, just free body diagram first and get your angles and axes setup. The orange dots represent equivalent angles, it's just a little geometry you should be able to do this on your own at this point.

http://www.bluegartr.com/forum/attac...1&d=1244610307

Now you want to break down the forces that aren't already falling onto your axes so that you can easily perform your calculations. For this problem you really only need to worry about the x-axis.

http://www.bluegartr.com/forum/attac...1&d=1244610312

Fgx and Fgy represent the x and y components of the gravitational force on the block, respectively. Ft represents the tension in the string. Fn represents the normal force and f represents the force of static friction opposing the otherwise -x direction of the block. If you have trouble deciding on the direction of the friction force, just imagine if the block was left alone without the applied forces, where would it go? Go opposite that direction.

The rest is easy, once you have a problem set up it's trivial. You should be expert at this by now but I'll go ahead and go through it.

The summation of the forces in the x direction:

f + Ft - Fgx = 0

f = (coefficient of friction) * Fn
Fn = -Fgy = -(-mgCos60)
Fn = 14.715

Fgx = -mgSin60

Ft = unknown

Solving for Ft:

Ft = Fgx - f
Ft = mgSin60 - mgCos60








If I had a tablet or some way to easily translate my work to your from my scratch paper in front of me I'd be more than happy to do 3 but I don't and I want to go to bed now. I don't like question one mostly because I have no idea what the units on air resistance are and I think it's a stupid question anyway.

[Furtwangler]

Thanks It seems like i never know what to do for any of the problems, but once i get started everything else kinda falls into place. I have the math background to be able to do all the algebra and calculus stuff involved in any of my problems, i just don't really remember how to set everything up every single time bc its always different than what i have in my notes, so that did help :D

[Rocl]

it's "further ado"

[Furtwangler]

thanks.

[Rocl]

its the only help i can provide, i sucked at physics

[Furtwangler]

[_chuchu]

42

[Jefe]

Use the work energy theorem for 3!

[Woozie]

I have a review packet for my physics final and my teacher is retarded and wont give us answers until tomorrow (the test) so for us that wanted to study, i have no way to know if im right. (or in some cases, where to even start because I've forgotten since the beginning of the year) so without FURTHER ADO, if someone can help me with these that'd be great.

1) A ball of Mass "m" is dropped down through the air in a gravitational field. The magnitude of the force of air restance is bv² where b is a positive constant. The magnitude of the accelration of the ball at any time is,
g - b
g - bv²/m
g + bv²/m
g/b
bv² + g

We want to find acceleration, and we already have force and mass. Is there a way to find the acceleration based on these two quantities?

How many forces are acting on the ball? What is the expression that represents these forces? One is given: air resistance is bv^2.

Remember, the total force is the sum of all the forces. We have mass, so find the total force, then find acceleration. Then compare your answer to mine

Spoiler: show

So as the ball falls, there are two forces acting on it. If we make down the negative direction, then we can call the force due to gravity Fg=-mg. (magnitude mg, negative because of it's direction). The force due to air resistance Fa=bv². The total force (Ft) is the sum of the two forces: Ft = Fg + Fa = mbv²-mg. Acceleration always equals force over mass, so the total acceleration would be bv²/m-g.

It's usually more intuitive to make down the negative direction, but since their answers all have g being the positive term, then obviously they took it to be the other way around. In this case, we'd expect the answer to be -1 times the answer I got: g-bv²/m

Another way to reconcile the discrepency is to note that the magnitude is basically the absolute value. g-bv²/m and bv²/m-g have the same absolute value, whereas the other choices don't. So we know the answer has to be g-bv²/m.

Or another way would be to walk through the steps I took, but with down as the positive direction, and you would still get g-bv²/m

[Woozie]

I have a review packet for my physics final and my teacher is retarded and wont give us answers until tomorrow (the test) so for us that wanted to study, i have no way to know if im right. (or in some cases, where to even start because I've forgotten since the beginning of the year) so without FURTHER ADO, if someone can help me with these that'd be great.

1) A ball of Mass "m" is dropped down through the air in a gravitational field. The magnitude of the force of air restance is bv² where b is a positive constant. The magnitude of the accelration of the ball at any time is,
g - b
g - bv²/m
g + bv²/m
g/b
bv² + g

We want to find acceleration, and we already have force and mass. Is there a way to find the acceleration based on these two quantities?

How many forces are acting on the ball? What is the expression that represents these forces? One is given: air resistance is bv^2.

Remember, the total force is the sum of all the forces. We have mass, so find the total force, then find acceleration. Then compare your answer to mine

Spoiler: show

So as the ball falls, there are two forces acting on it. If we make down the negative direction, then we can call the force due to gravity Fg=-mg. (magnitude mg, negative because of it's direction). The force due to air resistance Fa=bv². The total force (Ft) is the sum of the two forces: Ft = Fg + Fa = mbv²-mg. Acceleration always equals force over mass, so the total acceleration would be bv²/m-g.

It's usually more intuitive to make down the negative direction, but since their answers all have g being the positive term, then obviously they took it to be the other way around. In this case, we'd expect the answer to be -1 times the answer I got: g-bv²/m

Another way to reconcile the discrepency is to note that the magnitude is basically the absolute value. g-bv²/m and bv²/m-g have the same absolute value, whereas the other choices don't. So we know the answer has to be g-bv²/m.

Or another way would be to walk through the steps I took, but with down as the positive direction, and you would still get g-bv²/m

edit:

Spoiler: show

another way to solve this is through dimensional analysis

bv²/m is a force, so it has the units of force. The units of force are [mass]*[acceleration]=[mass]*[length]/[time]^2. So then b has to have units of [mass]^2/[time]

g - b Impossible. You cannot add/subtract to quantities with different units
g - bv²/m possible
g + bv²/m Impossible. Air resistance and gravity are working in two different directions so there should be a minus sign between the two terms
g/b Impossible. The expression "g/b" does not have units of acceleration
bv² + g Impossible. You cannot add/subtract to quantities with different units

By elimination, it has to be g - bv²/m

I'll post 2 and 3 in a moment.

[Woozie]

There's no need for me to explain problem 2, assuming you understood Maguspk's explanation (and his explanation is more detailed and more clear than mine would have been).

[Conrad]

Use the work energy theorem for 3!

Here's a jumbled sentence, see if you can solve the puzzle:

out the fuck Get.

[Woozie]

3)A block of 4kg is sliding down a frictionless ramp of 30degrees from the horizontal. The block starts at the top of the ramp and takes 4seconds to slide to the bottom.

How high is the top of the ramp?

The speed of the block when it reaches the bottom is most nearly?

Draw a force diagram like Maguspk did. Any time you ever see the word ramp anywhere ever, you always draw a force diagram.

Maguspk's diagram has two components: Fgx and Fgy. One of these will represent the force that's causing the block to slide. Which is it? If we know the force, can we find the acceleration? If you know the acceleration of something, and you know how much time has passed, how would you figure out the distance traveled? You've done this in problems where a ball is dropped from height x, this is kinda similar. Think about it, find an answer, and compare it to what I got.

Hint:

Spoiler: show

Again, we have a ramp, so use a force diagram like Maguspk did. If you understood his explanation, then you'll be able to figure out the "Fgx" and "Fgy" as he did. If we know the force that's causing the block to slide down the ramp, then we know the acceleration (since the mass was given). But since we know the acceleration, we know the average velocity over a time period. And since we know the time period, and the velocity, we know the distance it traveled. If we draw the ramp as a triangle as Maguspk did, then the distance the block traveled is the hypotenuse. If we know the hypotenuse and the angle (which was given 30 degrees), then we can figure out the height using basic trigonometry.

I'll start typing up the answer now.

Edit:

Spoiler: show

I'm still under the assumption that you understood Maguspk's method for problem 2. This is using the same method, but there's no friction involved. The force causing the block to slide is Fgx. We know this because it points in the direction that the block slides, whereas Fgy points in a perpendicular direction (meaning it doesn't contribute to the motion at all whatsover except for its effects on friction [and there is none]).

So Fgx=mgSinθ. If that's the force, then the acceleration is gSinθ. Since the acceleration is constant, we can work with the formula you've already learned: xf=x0+v0t+1/2*a*t^2.

v0=0
t=4
a=g*sinθ=9.8*sin(30degrees)=4.9

So

xf=x0 + 39.2

or xf-x0=39.2.

So if this is how far the block traveled, we can draw a ramp as in Maguspk's diagram, but with θ=30 degrees and the hypotenuse = 39.2. Use trigonometry: sin(30degrees)=height/hypotenuse => 1/2 = height/39.2. So height = 19.6

[Bagel]

Fuck yeah I got #1 right even though I haven't taken physics or math in 8 years

[Furtwangler]

thank you very kindly guys, did help when i looked back in the morning and I feel I did pretty good on my final