Chapter 4: Forces, Density and Pressure Flashcards
(28 cards)
Definition of Centre of gravity
The point at which the weight of the object may be considered to act.
Where is the centre of gravity for symmetrical objects with uniform density?
At the point of symmetry
What do most stable objects have?
Wide base and low centres of gravity.
What is the difference between centre of gravity and centre of mass?
In a uniform gravitational field, the centre of gravity is identical to the centre of mass. The centre of mass does not deemed on the gravitational field. Since weight is the product of mass and acceleration due to gravity (W=mg), the centre of gravity does depend on the gravitational field.
When an object is in space:
Its centre of gravity will be closer to the object with larger gravitational field.
It’s centre of mass will be closer to the to it’s geometric centre.
What is a moment?
A moment is the turning effect of a force. Moments occur when forces cause objects to rotate about a pivot.
Moment formula
Moment = Fd
F = force in newtons
d = perpendicular distance from the pivot in metres
State what is meant by a couple.
A couple is a pair of forces that acts to produce rotation only.
State 3 conditions if a couple
Equal in magnitude
Opposite in direction
Perpendicular to the distance between them
What a torque?
The moment of a couple is known as a torque.
State the principle of moments
For a system to be in equilibrium, the sum of the clockwise moments about a point must equal to the sum of the anti-clockwise moments (about the same point).
State the conditions for equilibrium.
A system is in equilibrium when all the forces are balanced:
There is no resultant force
There is no resultant torque
Density definition
Mass per unit volume
Density formula
p = m/V
ρ (rho) = density in kilograms per metre cubed (kg m^-3)
m = mass in kilograms (kg)
V = volume in metres cubed (m^3)
Volume of Speere formula
V = 4/3pi r^3
Volume of cube formula
d^3
Volume of cyclinder formula
pi r^2 l
Pressure definition
Force per unit area
Pressure formula
p = F/A
p = pressure in pascals (Pa)
1Pa = 1Nm^-2
F = force in newtons (N)
A = area in metres squared (m^2)
Note the area referred to is the ‘cross-sectional’ area of the 3D object. This is the area of the base that the force is applied on. E.g: For a cylinder this will be a circle.
What type of quantity is pressure?
Scalar
What is hydrostatic pressure?
Hydrostatic pressure is the pressure at any given point within a fluid, that is exerted by the weight of the fluid above that point.
Pressure in fluids formula/ equation of hydrostatic pressure and it’s derivation
p = ρ * g * h
w = mg = ρ * A * h * g
ρ = m/V = m/Ah
p = F/A = W/A = ρ * g * h
What is atmospheric pressure?
Atmosphere if pressure also known as barometric pressure is 101325 Pa
How r find the total pressure acting on an object submerged in fluid?
Total pressure = hydrostatic pressure + atmospheric pressure
What is a U-tube manometer?
A manometer is an instrument used to measure pressure and density of 2 liquids.
