4. Forces, density and pressure Flashcards
Define center of gravity
The point where the entire weight of an object appears to act
Define moment
The moment is the turning effect of force.
N m is the SI unit.
Calculating perpendicular force
and non-perpendicular force.
- perpendicular force- moment= force* perpendicular distance from the pivot.
- Non-perpendicular force= f*d cos(theta)
what is the principal of moments.
For a system to be in equilibrium, the sum of clockwise moments about a point must be equal to the sum of the anticlockwise moments (about the same point).
also;
clockwise moments = anticlockwise moments
Define a couple and torque
couple: a pair of equal and opposite forces that act on an object in different points producing rotation only.
torque: the turning effect of a couple is known as torque.
What are the conditions for a couple
two forces must be:
1. equal in magnitude
2. parallel, but in opposite direction
3. separated by a distance d
How do you calculate torque-
torque(N m) = one of the forces (N) * perpendicular distance between the forces (m)
What are the conditions for a system to be in equilibrium-
- there is no resultant force
- there is no resultant torque
What are the conditions of forces being in equilibrium-
- at rest
- moving at constant velocity
How are coplanar forces represented in equilibrium-
in closed vector triangles.
Define density
Density is mass per unit volume
density= mass/ volume
units= g/cm3 and kg/m3
Define pressure
Pressure if force per unit area
pressure= force/ area
UNITS- N/cm2 , N/m2 and pascals where;
1 P= 1N / m2
what is u-tube manometer:
U-tube manometer is an instrument used to measure and density of two liquids.
Define hydrostatic pressure-
Hydrostatic pressure is the pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.
What is the equation for hydrostatic pressure-
change in pressure= density x acceleration due to gravity x change in height
Δp=density x g x Δh
How to calculate the total pressure-
total pressure= hydrostatic pressure + atmospheric pressure
atmospheric / barometric pressure= 101325 pa
Deine upthrust-
Upthrust is a force which pushes upwards when an object is submerged in a fluid- i.e- gases or liquids
What causes upthrust-
Upthrust is caused by the difference in hydrostatic pressure at the top and the bottom of an object immersed in the fluid.
The force of upthrust is significantly larger in liquids than in gases, this is because liquids are much denser than gases.
NOTE- upthrust is influenced by pressure and not by the density of the object.
What does Archimedes’ principle state: elaborate
Archimedes’ principle states that an object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid displaced by the object.
The object sinks until the weight of the fluid displaced is equal to its own weight.
Therefore the object floats when the magnitude of the upthrust equals the weight of the object.
How to calculate the magnitude of the upthrust?
force=density x acceleration due to gravity x volume displaced
