# Specific Gravity, weight and acceleration

• jan2905
In summary, we discussed an object with a mass of 30kg and specific gravity of 3.6 being placed in a fluid with a specific gravity of 1.2. After neglecting viscosity, we calculated the object's acceleration to be 2/3 times the acceleration due to gravity and its weight to be 196.1 N. We also discussed the concept of buoyancy force and how it affects the apparent weight of an object submerged in a fluid.
jan2905
An object with mass 30kg and specific gravity 3.6 is placed in a fluid whose specific gravity is 1.2. Neglecting viscosity, what is the objects acceleration and weight?

I guessed on this one. Not sure how to make things come together.

I said that a=2/3(g) and F=200N

Refer to Archimedes' Principle then show your approach if you do not get the desired result.

is specific gravity (rho)?

jan2905: No. Density is mass per unit volume, and is denoted by the symbol rho. Density (rho) has SI units of kg/m^3. Specific gravity is the density of a substance divided by the density of water, and is therefore a dimensionless ratio. Specific gravity is sometimes denoted by the symbol SG, or maybe G (?), but never rho. Thus, specific gravity SG = rho/rhow, where rhow = density of water.

Regarding your acceleration answer, excellent work! That is correct. From the definition of specific gravity, and from the definition of density, you can solve for volume of the object. Your answer for the apparent weight of the submerged object is currently incorrect. However, remember Newton's second law? Try it again.

how can that be? F=mg=30kg(2/3)(9.81)=196...

That is correct; F = m*a = m*(2/3)g = (30 kg)(2/3)(9.807 m/s^2) = 196.1 N.

How can that be? The water pressure on the bottom of the object is pushing upward harder than the water pressure on the top of the object is pushing downward. This creates a net upward force, called buoyancy force, which reduces the magnitude of the downward acceleration of the object.

you said it wasn't 200N... because it's 196N? ... sorry I rounded.

## 1. What is specific gravity?

Specific gravity is a unitless measurement that compares the density of a substance to the density of water at a specific temperature. It is typically used to determine the relative density of liquids and solids.

## 2. How is specific gravity calculated?

Specific gravity is calculated by dividing the density of a substance by the density of water. This can be expressed mathematically as: Specific Gravity = Density of Substance / Density of Water.

## 3. How does specific gravity relate to weight?

Specific gravity is related to weight through the concept of density. The higher the specific gravity of a substance, the more dense it is and therefore the more it will weigh compared to an equal volume of water.

## 4. What is the difference between weight and mass?

Weight is a measure of the force of gravity acting on an object, while mass is a measure of the amount of matter in an object. Weight can vary depending on the strength of gravity, but mass remains constant.

## 5. How does acceleration affect weight?

Acceleration has a direct effect on weight. When an object is subjected to acceleration, its weight will increase due to the force of acceleration acting on it. This is known as the force of gravity.

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