Struggling with Physics please help!

[Dochopeful13]

A 2kg box slides down a ramp a distance of two meters before it reaches the ground. The ramp has an angle of 30o. The coefficient of kinetic friction for the ramp is 0.1.

What is the work done by the normal force on the box?

I was under the impression for a ramp problem I would use w cos sin to solve for normal force? However, the book uses f cos zeta and gets the answer zero. Is this correct?

The other problem is asking
A fluid is flowing through a pipe from left to right. If you increase the vertical height of the left end, what will happen to the pressure and velocity of the fluid at the left end?

Bernoulli’s equation:

P1+12ρv21+ρgh1=P2+12ρv22+ρgh2


The answer is there is
Correct answer:

The relative changes to pressure and velocity cannot be determined from the given information

However, I was under the impression that height decreases pressure. In addition according to the book a decrease in pressure accompanies an increased in velocity. Is this relationship correct? If so why is it not correct in this problem?

Thank you!

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[5words]

For the first one, the Normal Force is always perpendicular to the direction of motion except in a "banked curve", thus does no work because the angle between the ground and the F vector is 90 and cos of 90 is 0

Second one, height only decreases Hydrostatic pressure, that is when a fluid is stationary and not flowing. So this is an example of Hydrodynamics, so different rules apply. but your premise is still true. Still, in order to answer the question we need to know the velocity. If V was constant, assuming that the fluid was ideal, then an increase in height would have decreased the pressure. As in P1 + pgh1 = P2 + pgh2 (because this is greater) --- > P2 = P1 + pgh1 - pgh2 . However, in the example above, we don't know the velocity at the right or left end of the pipe, nor the pressure tbh.

Also the Bernoulli principle is indeed correct ( increase in P = decrease in v)

 

[Dochopeful13]

For the first one, the Normal Force is always perpendicular to the direction of motion except in a "banked curve", thus does no work because the angle between the ground and the F vector is 90 and cos of 90 is 0

Second one, height only decreases Hydrostatic pressure, that is when a fluid is stationary and not flowing. So this is an example of Hydrodynamics, so different rules apply. but your premise is still true. Still, in order to answer the question we need to know the velocity. If V was constant, assuming that the fluid was ideal, then an increase in height would have decreased the pressure. As in P1 + pgh1 = P2 + pgh2 (because this is greater) --- > P2 = P1 + pgh1 - pgh2 . However, in the example above, we don't know the velocity at the right or left end of the pipe, nor the pressure tbh.

Also the Bernoulli principle is indeed correct ( increase in P = decrease in v)

Thank you very much. I'm still a bit confused about problem one. Can you please explain why this video is incorrect? He uses the angle of the ramp for normal force.

 

[sullen-burger]

What is the work done by the normal force on the box?

The video above shows you how to find the normal force.

Your question asks what work does the normal force do. The normal force doesn't do any work because it's vector arrow points straight up, at a 90 degree right angle to the surface the box sits on.

Work = force * distance * cos(angle)

And since Work is defined as force times distance in the direction of motion, the normal force is "working" perpendicular to the direction of motion (box sliding down). So there's no work done when force is at a right angle to motion.

Don't think of the angle inside the triangle as your angle for the work equation. Your work equation angle is 90 and cos(90) is zero. Regardless of the normal force, you'll be multiplying it by zero.

Your box is sliding down a ramp all the while the normal force is pointing up. It's not doing any work.


Just think like when you pick up a box and carry it as you walk forward, are you doing work on the box?

 

[jdawgydawg]

Hi, since we are talking about physics, please help me understand this one.
So I understand that at the peak of a projectile motion, vertical velocity=0 and there is only horizontal velocity.
I thought if the sound source is moving, whether that be a vertical or horizontal velocity, then the detected frequency will decrease because of doppler effect.
But, the answer says that it is 2.00m because at the peak, the projectile has stopped moving and there is no Doppler shift.
I don't understand how the projectile stopped moving if there is still a horizontal velocity... and thus there should be a Doppler shift?!
HELP ME!

 

[5words]

View attachment 218146
Hi, since we are talking about physics, please help me understand this one.
So I understand that at the peak of a projectile motion, vertical velocity=0 and there is only horizontal velocity.
I thought if the sound source is moving, whether that be a vertical or horizontal velocity, then the detected frequency will decrease because of doppler effect.
But, the answer says that it is 2.00m because at the peak, the projectile has stopped moving and there is no Doppler shift.
I don't understand how the projectile stopped moving if there is still a horizontal velocity... and thus there should be a Doppler shift?!
HELP ME!

Well, the Doppler effect equation is fd = fo (v+-vd)/(v+-vs) vd being V of the detector and Vs being V of the source. Now, you agree with me that because the recording device or detector is on the ground vd = 0. As the speaker travel upwards, its changes however, like you've correctly mentioned above at the top of the projectile Vs (speed of the speaker) = 0. And also, there is no horizontal velocity, this isn't a projectile motion, you have highlighted the keywords here "straight upwards". So, the motion is synonymous to a free-fall. Thus, because there is no horizontal velocity because the object was propelled straight upwards, and that at h vs = vd , we have no Doppler motion.

 

[jdawgydawg]

Well, the Doppler effect equation is fd = fo (v+-vd)/(v+-vs) vd being V of the detector and Vs being V of the source. Now, you agree with me that because the recording device or detector is on the ground vd = 0. As the speaker travel upwards, its changes however, like you've correctly mentioned above at the top of the projectile Vs (speed of the speaker) = 0. And also, there is no horizontal velocity, this isn't a projectile motion, you have highlighted the keywords here "straight upwards". So, the motion is synonymous to a free-fall. Thus, because there is no horizontal velocity because the object was propelled straight upwards, and that at h vs = vd , we have no Doppler motion.

Wow that cleared up everything. I feel so dumb lol I even highlighted it and still called this a projectile motion...
Thank you so much!

 

[jdawgydawg]

Can someone please explain to me why this is 45 degrees?

 

[5words]

View attachment 218163
Can someone please explain to me why this is 45 degrees?

This is Snell's Law, Angle of incidence = Angle of reflection.

The question tells you that the light is reflected at a right angle.

So if the angle between the incident beam and the reflected beam is 90 degrees, then the angle of incidence must be 45 degrees, because it is measured from a vector normal to the mirror.

Using the diagram, the beam splitter is set at a position to create "equal distances" or "equal angles". I then figured that they must both be 45 to create 90 degrees total. I probably got really lucky after reading that other response.

If you go back to your physics book, it will say that for reflected light, theangle of incidence equals the angle of reflection. The passage says that the reflected light reflects "at a right angle", which is 90 degrees. Thus the angle of incidence (and the angle of reflection) are each 45 degrees, which sum up to the 90. The question asks what the angle of incidence is.

Reflection (physics) - Wikipedia