Physics Question + Answer

[AndyDude]

A lens is manufactured in such a way to allow the object and the image to be the same distance from the lens. If the lens is not flat, the only way this could be true is if the lens were:

 

A) a diverging lens with the object at the focal distance

a diverging lens with the object at twice the focal distance

C) a converging lens with the object at the focal distance

D) a converging lens with the object at twice the focal distance

 

 

Answer:_D_

 

 

If someone could explain how to solve this, it would be awesome. I read it in EK, and it kind of makes sense, but I wonder if someone has a strategy to solve this type of problem.

 

The answer would be D.

 

We are given that image distance (i) = object distance (o)

-> Also, converging lens f = + ; o = 2f

1/f = 1/2f + 1/i; 1/i = 1/f - 1/2f

Take the lowest common denominator, we get 2f. Now, 1/i = (2-1)/2f = 1/2f

 

i = o = 2f.

 

Intuitively, we know f<0 for a a diverging lens and will ALWAYS make a smaller, virtual upright image. So A and B are out . A converging lens will ONLY form real, inverted images when object is placed outside the focal length; at f no image forms, inside f virtual, upright image form. Same goes for mirrors but remember diverging mirror is convex while a diverging lens is concave.

 

Regarding strategies, I believe that although it can be extremely helpful in knowing the facts, it's much less confusing to just use plug and chuck method. But so long you know how rays would behave when placed at, within or beyond the focal point in a converging mirror or lens you'd be good to go. For diverging lenses/mirrors they can never form a real image.

[The Law]

Thanks AndyDude... I definitely have to review optics today. lol

[hate_physics]

Hello

 

I was wondering if anyone can help me with the following questions:

 

1. Assuming there are no limitations on the maximum allowable acceleration, what is the maximum angle that the pendulum can make with the vertical?

 

A. 90 degrees

B. slightly less than 90 degrees [correct]

C. slightly greater than 90 degrees

D. 180

 

 

2. Which of the following equations correctly defines the acceleration of the pendulum as a function of the angle.

 

A. a=MgCos theta

B. a=gCot theta

C. a=g tan theta [correct]

D. a = v^2/r

 

3. A passenger takes a reading each second of the total distance the car has traveled since starting. If the accelerometer reads a constant, positive value for 4 seconds, which of the following could be the values for total distance traveled over the four seconds?

 

A. 4m, 8 m, 12 m, 16 m

B. 1m, 4 m, 9 m, 16 m [correct]

C. 4m, 4m, 4m, 4m,

D. 10 m, 16m, 21m, 24m

 

 

can anyone be kind enough to help - I hate physics!

[hate_physics]

oh..one last one:

 

If the pendulum is suspended in mineral oil, what is the tension in the pendulum rod when the system is at rest? Assume the density of mineral oil is one-eighth the density of the pendulum weight.

 

A. 0

B. mg

C. (1/8) Mg

D. (7/8)Mg [correct]

[eng_dude786]

HI

 

I hope this is not coming in too late

I was wondering if anyone can help me with the following questions:

 

1. Assuming there are no limitations on the maximum allowable acceleration, what is the maximum angle that the pendulum can make with the vertical?

 

A. 90 degrees

B. slightly less than 90 degrees [correct]

C. slightly greater than 90 degrees

D. 180

 

it would be B because for the pendulum it is experiencing a force of gravity downward and the tension is pulling upward on the pendulum - assuming everything is in rest conditions. Therefore now if the pendulum were to swing all the way up to 90 degrees than the force of gravity would act downward and the tension force -due to string- experienced by the pendulum would be perpendicular to the force of gravity. Since you have one force of gravity pointing downward and nothing to oppose than it would accelerate downward - since that can not happen therefore the maximum allowable angle is less than 90 degrees. This question is more of a elimination type question where you remove the choices. It can't be A or Cand it can't be D -cuz D is just stupid.

 

2. Which of the following equations correctly defines the acceleration of the pendulum as a function of the angle.

 

A. a=MgCos theta

B. a=gCot theta

C. a=g tan theta [correct]

D. a = v^2/r

 

 

This one's a bit of derivation. I'll do this in the following post

 

 

3. A passenger takes a reading each second of the total distance the car has traveled since starting. If the accelerometer reads a constant, positive value for 4 seconds, which of the following could be the values for total distance traveled over the four seconds?

 

A. 4m, 8 m, 12 m, 16 m

B. 1m, 4 m, 9 m, 16 m [correct]

C. 4m, 4m, 4m, 4m,

D. 10 m, 16m, 21m, 24m

 

This one's also a bit tricky. Here's what you should look for it: Since the acceleration is constant, it means that the velocity is increasing or is a linear slope. Therefore the distance traveled can not be the same so choice C is wrong. Now lets look at the differences between the distance calculated at each second. Remember two things here: velocity is linearly increases and that the person is measuring total distance. This means that we look for the difference between the two numbers to be a positive difference and a difference value that is increasing. So for choice B, the difference between each consecutive value is increasing - which is what we want

can anyone be kind enough to help - I hate physics!

[eng_dude786]

oh..one last one:

 

If the pendulum is suspended in mineral oil, what is the tension in the pendulum rod when the system is at rest? Assume the density of mineral oil is one-eighth the density of the pendulum weight.

 

A. 0

B. mg

C. (1/8) Mg

D. (7/8)Mg [correct]

 

another derivation question:

 

first draw a FBD. At rest you have the following 3 forces working: tension, force of gravity and the buoyancy force due to the liquid.

 

Writing an equilibrium equation:

 

T+Fb=Fg

T=Fg-Fb

T=mg-Fb [equation 1]

 

BUT

Fb = density of fluild * volume of pendulum*g [equation 2]

also

density of fluid = (1/8)*density of pendulum

sub back into equation 1

 

T=(density of pendulum*volume of pendulum)*g - (Fb)

 

Sub equation 2 for Fb in the above equation.

 

T=(8/8)density.of.pendulum*volume*g - (1/8)density.of.pendulum*volume*g

T= choice D

[eng_dude786]

here's the answer I mentioned earlier:

2. Which of the following equations correctly defines the acceleration of the pendulum as a function of the angle.

 

A. a=MgCos theta

B. a=gCot theta

C. a=g tan theta [correct]

D. a = v^2/r

 

Again, draw a FBD and break down into vertical and horizontal components.

 

Vertically speaking:

Fg acts downward and to counteract the force of gravity, a component of the Tension force acts upward. Therefore component of tension acting upward would be Ty=TCos (theta) [where theta is the angle pendulum makes to the vertical]

 

So now you get the following equation:

Mg=TCos(theta) [equation 1]

 

Horizontally speaking:

horizontal component of tension would act with a magnitude of Tx=TSin theta

 

Let his now equal the net acceleration experienced by the pendulum and now you get:

 

Tsintheta = Ma [equation 2]

 

Sub equation 1 into equation 2

 

[Mg/Cos(theta) ] sine theta= Ma

 

M cancel out and you get

 

gSin theta/cos theta = a

 

sin theta/cos theta = tan theta

and the final result is:

 

a=gtan theta

 

I think it looks way complicated because I have typed it out but it not terribly bad on paper. Also I think I am over-analyzing this problem and a simple solution exists - probably by process of elimination.

[gamma]

For convention, down will be negative, up will be positive.

 

For the block to completely reverse direction, it must reduce it's velocity to 0m/s.

The problem requires it to reverse direction in 1sec, so that must mean the acceleration must be -2m/s^2.

 

Vf=Vi+at

0=2 + a(1)

-2=a

 

To give the block a -2m/s^2 acceleration, the net force on the block must be:

Fnet = ma

Fnet = (100)(-2)

Fnet = -200N (200N in the downwards direction)

 

The forces pulling the block in the down (negative direction) are mgsin30 and mgcos30*co.kinetic.fric and the tension of the rope pulls it upwards.

 

Fnet = T + mgsin30 + mgcos30*co.kinetic.fric

-200 = T + (100)(-10)(0.5) + (100)(-10)(0.87)(0.1)

-200 + 500 + 87 = T

T=387N (since it's positive, it's upwards).

 

sorry for reviving an old question but just wondering where the mgsintheta force is coming from. I know it's the force in the F_x direction but why is it pointing in the same direction as F_f and not opposite it? When it was moving at const v, wasn't it T + F_x - F_f = 0 ?

[The Law]

For resistors in parallel, if you add a capacitor in parallel to these resistors - will it experience the same voltage as the resistors do?

[eng_dude786]

you mean like this?

 

yea the voltage across all the elements would be the same

 

 

For resistors in parallel, if you add a capacitor in parallel to these resistors - will it experience the same voltage as the resistors do?

[cling]

Yeah, I guarantee that some of those pendulum questions you posted you are very unlikely to see on the real test. Especially the derivation one.

[The Law]

you mean like this?

 

yea the voltage across all the elements would be the same

 

Yep, precisely like that. Merci.

 

Ahh, the clock is ticking....

[The Law]

I`m looking at a circuit that has a 4V battery and 2 resistors in parallel to each other, each with a resistance of 2 ohms. The question asks, what will happen to the current through the remaining resistor if one of the resistors is removed.

 

The answer is that the current stays the same. The book says it`s because they are in parallel so the voltage across the remaining resistor does not change if resistors are added or removed. The thing I`m confused about is if you remove one of the resistors, then there will only be one resistor in the entire circuit. Doesn`t the voltage across resistors in a circuit always have to total to be the voltage of the battery source...

 

Is it because if you remove a resistor in a parallel circuit, the total current that the battery is releasing will go down and the remaining resistor will always keep the same voltage and current...

[avenir001]

I`m looking at a circuit that has a 4V battery and 2 resistors in parallel to each other, each with a resistance of 2 ohms. The question asks, what will happen to the current through the remaining resistor if one of the resistors is removed.

 

The answer is that the current stays the same. The book says it`s because they are in parallel so the voltage across the remaining resistor does not change if resistors are added or removed. The thing I`m confused about is if you remove one of the resistors, then there will only be one resistor in the entire circuit. Doesn`t the voltage across resistors in a circuit always have to total to be the voltage of the battery source...

 

Is it because if you remove a resistor in a parallel circuit, the total current that the battery is releasing will go down and the remaining resistor will always keep the same voltage and current...

 

oui, the 2 parallel resistors draw a total current of 4 amps whereas the sole resistor draws a total current of 2 amps.

[The Law]

Merci avenirio!

[Wallywarp]

Electric Circuits and Electricity:

 

Teach me about currents and voltages using analogies.

 

How are they the same, how are they different?

 

Analogy:

 

If I had 300 Spartans walking across the field, is that like current? Making the 300 Spartans run at full speed across the field, does that mean the current is the same, but the voltage would be increased?

[avenir001]

hmmm current is like the flow rate (ie amount of charge passing by per unit time) so i'd say if they're running faster, the current is higher. in high school we were taught the "current=water flow rate, voltage=water pressure" analogy.

[The Law]

Electric Circuits and Electricity:

 

Teach me about currents and voltages using analogies.

 

How are they the same, how are they different?

 

Analogy:

 

If I had 300 Spartans walking across the field, is that like current? Making the 300 Spartans run at full speed across the field, does that mean the current is the same, but the voltage would be increased?

 

Voltage is the thing that inspires electrons to flow. Resistance is the thing that hinders this flow of electrons.

 

I (current) = V over R

 

At a given resistance, the greater the voltage, the more current (flow of electrons you`ll get). The greater the resistance, however, the slower things happen.

 

So to correct your analogy, if you had 300 Spartans walking across the field without any enemies, it would only take them 15 minutes. Now, add a bunch of people from the Great Persian Empire, and it will take them 35 minutes (this is resistance). The more resistance you have, the longer it takes electrons (err, I mean, Spartans) to travel.

 

The voltage on the other hand, is the thing that motivates the electrons to move. In a circuit, this source is usually the battery. For the Spartans, this source was their hatred for the Persians (who could blame them, the Persians are pretty darn good looking ).

 

Now, if the current flows faster - it`s either due to 1) Increased voltage (bigger battery, or in our analogy, the need for revenge after someone you love is killed) or 2) less resistance (many resistors, vs fewer resistors OR No persians vs many persians).

 

Voltage and Resistance, howvever, are independent of each other. They both, however, factors that affect the current.

 

I = V over R

V = I R

-So just because you decrease V, that does not mean R will be affected. It does mean that if R does not change, that I will go down too.

 

Hope that helps.

[avenir001]

lool how dare u drag the great persian empire into ur analogy

[The Law]

lool how dare u drag the great persian empire into ur analogy

 

Forgive me for mentioning the name of your empire, oh Great Persian Princess.

[avenir001]

Forgive me for mentioning the name of your empire, oh Great Persian Princess.

 

that's ok Arabian Knight.

 

lol we derailed this thread..guess it's ok cuz u were the great creator.

 

*k back to serious business*

[The Law]

k, back to business for real... now EMCO!!!!!!!!!!!!!!!!!!!!!

 

To get back to business:

 

Voltage is the thing that inspires electrons to flow. Resistance is the thing that hinders this flow of electrons.

 

I (current) = V over R

 

At a given resistance, the greater the voltage, the more current (flow of electrons you`ll get). The greater the resistance, however, the slower things happen.

 

So to correct your analogy, if you had 300 Spartans walking across the field without any enemies, it would only take them 15 minutes. Now, add a bunch of people from the Great Persian Empire, and it will take them 35 minutes (this is resistance). The more resistance you have, the longer it takes electrons (err, I mean, Spartans) to travel.

 

The voltage on the other hand, is the thing that motivates the electrons to move. In a circuit, this source is usually the battery. For the Spartans, this source was their hatred for the Persians (who could blame them, the Persians are pretty darn good looking ).

 

Now, if the current flows faster - it`s either due to 1) Increased voltage (bigger battery, or in our analogy, the need for revenge after someone you love is killed) or 2) less resistance (many resistors, vs fewer resistors OR No persians vs many persians).

 

Voltage and Resistance, howvever, are independent of each other. They both, however, factors that affect the current.

 

I = V over R

V = I R

-So just because you decrease V, that does not mean R will be affected. It does mean that if R does not change, that I will go down too.

 

Hope that helps.

[avenir001]

LOL u just had to quote urself to bring that onto this page...for fear of someone possibly missing ur great great analogy.

[The Law]

LOL u just had to quote urself to bring that onto this page...for fear of someone possibly missing ur great great analogy.

 

omg, stop derailing the thread! i was quoting to bring it back to where it was before it got derailed! sheesh! lol... as for my analogy, people will see it regardless of what page it`s on.

[The Law]

A woman of height h can see her entire body in a flat mirror. What is the minimum length of the mirror?

 

A) 1/4 h

1/2 h

C) h

D) any mirror will work depending upon how far the woman stands from the mirror