Chapter 8- Modelling In mechanics Flashcards
Particle
Dimensions of the object are negligible
Particle model assumptions
Mass of the object is concentrated at one point
Rotational forces and air resistance can be ignored
Rod
All dimensions but one are negligible, like a pole or a beam
Rod model assumptions
Mass is concentrated along a line
No thickness
Rigid (does not bend or buckle)
Lamina
Object with area but negligible thickness, like a sheet of paper
Lamina model assumptions
Mass is distributed across a flat surface
Uniform body
Mass is distributed evenly
Uniform body model assumptions
Mass of the object is concentrated at a single point at the geometrical centre of the body- the centre of mass
Light object
Mass of the object is small compared to other masses, like a string or a pulley
Light object model assumptions
Treat object as having zero mass
Tension is the same at both ends of a light string
Inextensible string
A string that does not stretch under load
Inextensible string model assumptions
Acceleration is the same in objects connected by a taut (tight) inextensible string
Smooth surface model assumptions
Assume that there is no friction between the surface and any object on it
Rough surface
When it is not smooth
Rough surface model assumptions
Objects in contact with the surface experience a frictional force if they are moving or are acted by a force
Wire
Rigid thin length of metal
Wire
Treated as one-dimensional
Smooth and light pulley
All pulleys you consider will be smooth and light
Smooth and light pulley model assumptions
Pulley has no mass
Tension is the same on either side of the pulley
Bead
Particle with a hole in it for threading on a wire or string.
Bead model assumptions
Moves freely along a wire or string
Tension is the same on either side of the bead
Peg
A support from which a body can be suspended or rested.
Peg model assumptions
Dimensionless and fixed
Can be rough or smooth as specified in the question
Air resistance
Resistance experienced as an object moves through the air.
Air resistance model assumptions
Usually modelled as being negligible
Gravity
Force of attraction between all objects. Acceleration due to gravity is denoted as g.
Gravity model assumptions
All objects with mass are attracted towards Earth
Earth’s gravity is uniform and acts vertically downwards
g is constant and is taken as 9.8m/s^2
Normal reaction
This the force that acts perpendicular to a surface when an object is in contact with the surface.
Friction
Force which opposes the motion between two rough surfaces
Tension
The force acting on the object which is being pulled by a string is known as the tension (in the string).
Thrust or compression
When an object is pushed by a light rod, the force acting on it is known as to the thrust r compression in the rod
Buoyancy
Upward force on a body that allows it to float or rise when submerged in liquid
Air resistance (force diagram)
Opposes motion.
Displacement definition
Position relative to where you started from
Unit: metres
Velocity definition
Rate of change of displacement
Unit: ms^-1
Acceleration definition
Rate of change of velocity
Units: ms^-2
Force/weight definition
Described by magnitude, direction and point of application
Units: Newton (N)
Distance definition
Measure of length
Units: metres
Speed definition
Measure of how quickly a body moves
Units: m/s
Time definition
Measure of ongoing events taking place
Units: seconds
Mass definition
Measure of the quantity of matter contained in an object
Unit: kg
