Topic 5 - Forces, Density and Pressure Flashcards
Forces
actions upon an object that results in an interaction between objects.
Common type of forces. ( 6 )
- Gravitational forces
- Electrival forces
- Magnetic forces
- Frictional forces
- Drag forces / viscous drag
- Upthrust force / Buoyant force
Gravitational Force
Region where mass experience a force in gravitational field.
- acts on all masses
- weakest force
- field lines are directed towards centre of m
Electrical Force
attractive and repulsive interactions that acts on all charged particles
Magnetic Forces
acts on magnetic materials / moving charges
Frictional Forces
forces which opposes motion that are in contact with each other
Drag force / Viscous drag
forces that oppose / prevent motion
- found in fluids
- directly proportional with speed
- also known as air resistance
Upthrust / Buoyant Force
vertically acting upwards force on an object partially or fully immersed in fluid due to difference in pressure of fluid
• bottom pressure - top pressure
Terminal velocity
Maximum constant velocity when an object is in motion in a fluid / resisting medium and net force is zero.
How different masses varies the terminal velocity?
- higher mass - higher terminal velocity
- higher mass requires a larger air resistance to balance out the weight of the object
- higher velocity is needed to gain a larger air resistance
- hence results in a higher terminal velocity
How energy is conserved in free fall at terminal velocity?
- Object experiences loss in GPE
- No gain in kinetic energy (constant velocity)
- Energy is used to do work against air resistance
- Heat increases - thermal vibration also increases
- Energy converted to internal energy, increase in internal energy
Centre of gravity
(def.) a point where the whole weight / gravity is considered to act, regardless of its orientation.
• uniform shape
- at the geometrical centre
• non-uniform shape
- towards higher mass
Pivot point
- stationary point
* if centre of g > pivot point, object will topples.
Moment of a force
(def.) Turning effect of a force
Product of force and the perpendicular distance of pivot to the line action of force
- M = F x d
- clockwise & anti clockwise rotation
System in equilibrium
TRANSLATIONAL EQUILIBRIUM
• Sum of forces in any direction = 0
ROTATIONAL EQUILIBRIUM
• Sum of moments about any point = 0
Principle of Moments
(def.) for an object to be in equilibrium, the sum of moments about any point must be zero
enables us to :
• check whether object will rotate / remains balanced
• calculate unknown force or distance if the object is balanced
Common point
Intersection points of all forces so that the sum of moments at this point is zero
Couples
(def.) a pair of equal and oppositely directed paralled forces whose line of action does not coincide.
Torque
(def. ) turning effect or moment of a couple
(def. ) product of one of the forces with the perpendicular distance between two forces
Torque = F (d1 + d2)
Difference in moment and torque
Moment
• turning effect of ONE force
• affected by the perpendicular distance of force from PIVOT
Torque
• turning effect of a COUPLE
• affected by the perpendicular distance BETWEEN TWO FORCES
Density
(def.) mass per unit volume
• constant for :
- solid
- liquid that is not compressed
• not constant if
- there’s change in T / V
Pressure
(scalar quantity)
(def.) Force acting normally per unit surface area
Pressure in liquid
- increases the with depth below its surface
- equal in all directions at the same depth
- proportional to the density
- depends on height of liquid only
P = pgh
Pressure in gas
due to collision between particles and the wall of container
• change in momentum (N2L)
• impulses
• impulsive force is exerted on the wall (N3L)
• gas pressure produced
Type of meters
- Barometer
- Manometer
Barometer : determines Patm
Manometer : determines Pgas
also known as U-tube
