Alex7592

Can someone provide explanations to the CB Physics practice test questions?

Is there a document of all of them somewhere?
If not, I can provide the questions

damnit

Provide the question

Alex7592

*Picture of stupid cannon with no information*

15. Assume that every projectile fired by the toy cannon shown above experiences a constant net force F along the entire length of the barrel. If a projectile of mass m leaves the barrel of the cannon with a speed v, at what speed will a projectile of mass 2m leave the barrel?

Answer: v/sqrt(2)

darks0ulz

The energy at that point is the same (same force etc).

Let v1 = when mass is m
Let v2 = when mass is 2m.

E = 1/2*m*(v1)^2
E = 1/2*(2m)*(v2)^2

Energy is the same so..
1/2*m*(v1)^2 = 1/2*(2m)*(v2)^2
Masses cancel out.

1/2*(v1)^2 = (v2)^2
sq root both sides.
v2 = v1/sqrt(2)

darks0ulz

Or you can just use some sort of common sense..

Velocity has to decrease because mass increased...so it has to be either v/2 or v/sq2..

And enough time spent with physics should indicate that mass isn't unit proportional (don't know if it's the right term) to speed.

Alex7592

70. A child on a swing can greatly increase the amplitude of the swing's motion by "pumping" at the natural frequency of he swing. This is an example of what of the following?

a. conservation of momentum
b. newton's first law of motion
c. newton's third law of motion
d. resonance
e. interference

darks0ulz

Resonance.

Increased amplitude infers a constructive interference. By definition, the same natural frequency would be resonance.

Alex7592

Thanks!

Can you explain diffraction?

darks0ulz

It's the whole concept of how light (a wave) bends when it runs into an obstacle.

Think of the double slit experiment. You have two openings, the light bends into either of the openings. Now you have two "arches" of light.

Since they are at different wavelengths relative to each other (like both arches aren't in identical positions) you'll have different points of constructive and deconstructive which will cause different slits of maximum and minimum brightness.

Alex7592

Thanks yet again!

How about polarized sheets?
explain?

darks0ulz

It's regarding that question in the BB no?

When I read it, I interpreted it as both sheets cause the polarized light to cancel out. Imagine sheet X forcing polarized light to only be on the X axis. Sheet Y forces polarized light to be only on the Y axis. So once it goes through both sheets, no light is emitted as everything just cancels out.

Rotating it 180 and 360 will do nothing as it's just the same thing. Adding more sheets will do nothing too as light is already canceled out.

So you'll have to rotate it 90 degrees.

Good luck on the test tmmw.

Alex7592

Thanks man!