Manometer Homework Help: Solving Problems and Understanding Concepts

[Frisky90]

Homework Statement

Can someone help with this problem?

http://img854.imageshack.us/i/img6405.jpg/

Homework Equations

The Attempt at a Solution

http://img151.imageshack.us/f/img6404v.jpg/

I got an answer which is very close to the one from the textbook. I can't find the problem.
I'm talking about a).

It'd be nice if someone explain how do we assign the signs of the r*g*h terms. As far as I know, its plus when we go down, and minus when we go up.

Thx :)

 

[anathema]

Hi there,

Sure, I can help with this problem. Let's start with the equations we can use to solve it:

1. Potential energy equation: PE = mgh, where m is the mass, g is the acceleration due to gravity, and h is the height.

2. Conservation of energy equation: KEi + PEi = KEf + PEf, where KE is the kinetic energy, PE is the potential energy, and the subscripts i and f represent the initial and final states, respectively.

Now, for the attempt at a solution, it looks like you've correctly used the first equation to find the potential energy at the top of the incline (PEi) and the potential energy at the bottom of the incline (PEf). However, the second equation is missing the kinetic energy terms. Since the object starts from rest at the top of the incline, the initial kinetic energy (KEi) is equal to 0. Therefore, the equation should be:

PEi = KEf + PEf

Now, let's assign the signs for the r*g*h terms. As you mentioned, when an object goes down, the sign for the r*g*h term is positive, and when it goes up, the sign is negative. In this problem, the object is initially at the top of the incline, and it goes down to the bottom. Therefore, the sign for the PEi term should be positive, and the sign for the PEf term should be negative. This is because the object is losing potential energy as it goes down the incline.

I hope this helps! Let me know if you have any other questions.