Couple of Phys Questions

[FlipDoc2Be]

Hey guys I just need some help in solving these 2 Phys questions. Thanks a lot!

What is the specific gravity for an object that weighs 50N in air and weights 20N in water?

A. 5
B. 2.5
C. 1.67
D. 0.4

For a violin string that produces consecutive harmonic frequencies of 196 Hz, 392 Hz, 588 Hz, what is the frequency of the third harmonic?

A. 294 Hz
B. 490 Hz
C. 588 Hz
D. 784 Hz

The answers are C & C. Thanks.

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[RogueUnicorn]

The first question is answer as follows -

The object weights 20N, meaning the buoyant force is 30N. Knowing how buoyancy is generated, you must know then that the volume of water equal to the volume of this object is 30N. So, what we have here is an object with a weight of 50N, and a body of water with the exact same volume as this object with a weight of 30N. Specific gravity is a ratio of densities, but recall that density is merely mass (for which we can substitute weight) over volume. Here we can simply take the ratio of their weight since the volumes are the same. The ratio of the weights of the object/water is 50/30, or 1.67.

The second question is simply a plug on chug for a standing wave.

 

[BerkReviewTeach]

Hey guys I just need some help in solving these 2 Phys questions. Thanks a lot!

What is the specific gravity for an object that weighs 50N in air and weights 20N in water?

A. 5
B. 2.5
C. 1.67
D. 0.4

For a violin string that produces consecutive harmonic frequencies of 196 Hz, 392 Hz, 588 Hz, what is the frequency of the third harmonic?

A. 294 Hz
B. 490 Hz
C. 588 Hz
D. 784 Hz

The answers are C & C. Thanks.

Rather than posting so many questions from the BR inclass handouts and wait around for an answer, why don't you go to their office or email one of the instructors to get the detailed answer explanations? The explanations are really helpful, especially for the inclass questions.

 

[rinsterman12]

How can you place weight into the density equation? Could you explain that a little more please

 

[BerkReviewTeach]

How can you place weight into the density equation? Could you explain that a little more please

What Bleargh has done in a brilliant and to-the-point fashion is simply set up a ratio between forces in such a way that they include density:

W = mg = rhoobjectVobjectg

Fbuoyant = rhomediumVdisplacedg = rhomediumVobjectg

The reason Vdisplaced = Vobject is because the object is fully submerged (the only way it could have a weight in the fluid).

By setting W/B = rhoobjectVobjectg/rhomediumVobjectg we get:

50/30 = rhoobjectVobjectg/rhomediumVobjectg

50/30 = rhoobject/rhomedium = specific gravity