Fluids Study Guide and Practice Questions:
1. What is a fluid?
Any substance that flows (liquids and gasses)
2. Viscosity:
1. Measures how resistant to flowing a fluid is.
2. High viscosity = slow flowing
3. Low viscosity = fast flowing
4. Gasses are ultra low viscosity
3. Factors affecting Viscosity:
1. Temperature
2. Particle Size and Mass
3. Particle Complexity
4. Particle Properties like Polarity etc…
4. Mass vs Weight:
1. Mass measures the amount of matter in an object, uses “m” as a variable and is measured in grams (g) or kilograms (kg), does not change with location.
2. Weight measures the force due to gravity on an object, uses “Fg” as a variable, measured in Newtons (N), varies with location because gravity varies with location.
3. Calculations: Fg = m x g
m = Fg/g
g = Fg/m
Mass in this formula is measured in kg.
5. Density:
1. Measures the ratio of mass to volume of a substance or object; how tightly packed the matter is in an object or how much volume a substance takes up per unit of mass.
2. If two objects have the same mass, the denser of the two will take up less space (less volume).
3. If two objects have the same volume, the denser will have more mass.
4. Measured in g/ml or g/cm3
5. Calculations:
D = m/V
m = D x V
V = m/D
Mass in this formula is measured in grams (g)
Converting a value in grams to kilograms, divide by 1000
Converting a value in kilograms to grams, multiply by 1000
6. Buoyancy:
1. Buoyancy is a lifting force that objects experience while immersed in a fluid.
2. Needs gravity
3. Archimedes’ Principle:
“The buoyant force on an object is equal to the weight of the fluid it displaces.”
4. Three (+1) states of buoyancy:
* Positive: object displaces more than its own weight
The buoyant force (which is the weight of the displaced fluid) is greater than the weight of the object. The object ends up rising to the surface of the fluid.
* Negative: object displace less than its own weight
The buoyant force is weaker than the weight of the object.
The object sinks to the bottom
* Neutral: object displaces exactly its own weight
The buoyant force is equal to the weight of the object.
The object stays in place
* Special case: Floating object displaces exactly its own weight (like neutral buoyancy), but is only partially submerged. It only takes a portion of the object’s volume to displace enough fluid to match the weight of the object.
The fraction of the object that is submerged is equal to the ratio of the densities. EX: If the object has a density of 0.72 g/ml and the fluid has a density of 1.0 g/ml, then the object sinks 72% into the fluid since 0.72 is 72% of 1.0
5. Ultimately, if the density of an object is less than that of the fluid, it will float. If the density of the object is more than the density of the fluid, the object will sink.
7. Pressure:
1. Pressure is caused by a force acting on an area.
2. Pressure = Force/Area
3. P = F/A
4. Measured in N/m2 or Pascal (Pa)
8. Pascal’s Law:
Any force applied to an enclosed liquid is passed on without reduction everywhere, equally in that liquid.
Calculating forces within hydraulic systems:
F1 x A2/A1 = F2 The force on the output piston (F2) is equal to the input force (F1) times the ratio of the two areas of the pistons.
Practice Problems:
1. What is the weight of a 350kg box on Mars (g = 3.7 N/kg)?
2. What is the mass of an object that weighs 2000N on Pluto (g = 0.62 N/kg)
3. What is the density of a material if 400ml of it has a mass of 640g?
4. What is the volume of a 3000 g block of copper (D = 8.96 g / ml)?
5. What is the weight of a 2500ml block of gold (D = 19.3 g / ml) on Earth (g 9.8 N/kg)?
(Make sure to convert to kg before you calculate Fg)
6. Calculate the weight of the water (D = 1.0 g/ml) being displaced by a fully submerged 500ml block of Aluminum (D = 2.7 g/ml). Can this block float in water?