A2 Mechanics Flashcards
what is a moment?
the turning effect of the force on a rigid body
when do moments occur?
around a point
when object has size & force is not acting through the centre of mass
what is the formula for moment with units?
moment (Nm) = force (N) x perpendicular distance (m)
moment = |F| x dsinθ
d = distance from point
define equilibrium
when the resultant force = 0
& when the resultant moment = 0
describe how to solve equilibrium problems
- resolve forces vertically
- resolve forces horizontally
- resolve moments around a point you choose that will eliminate a variable
what is the difference b/w a uniform & non-uniform mass?
uniform mass - the centre of mass is at the middle of the object
non-uniform mass - the centre of mass is not necessarily at the centre of the object
describe the ‘point of tilting’
one support is the pivot, one support has 0 reaction force or 0 tension
Ra = 0N
see Wilson OneNote
describe resolving forces
components of the force in the direction of motion - perpendicular & parallel using
y = Fsinθ
x = Fcosθ
–> always choose triangle that creates these equations
F = (Fcosθi + Fsinθj) N
see Wilson OneNote
F = ma
F - resultant force
if F = 0, a = 0
when to use F = ma?
if equilibrium in a certain direction, equate forces in opposing directions
if no equilibrium in certain direction, use F=ma
describe resolving forces along a plane
perpendicular & parallel to the plane
Fr =
max. friction at limiting equilibrium
μR
μ = roughness of plane, if μ = 0, plane is smooth
R = reaction force
if pulling force < Fmax?
object won’t move & Fr will = pulling force
define static particle
at equilibrium
no moments
a = 0
F=ma=0
describe how to solve problems in statics
draw diagram
resolve the forces in horizontal & vertical directions or, if the particle is on an inclined plane, in perpendicular & parallel directions
set the sum of components in each direction = to 0
solve the resulting equilibrium to find the unknown forces
modelling with static particles
weight
tension - force of string in both directions
pulleys
wires
planes
strings
beads - tension on each side is the same (see OneNote)
pegs
+ skills from static particles
define light, inextensible string
light - no mass
inextensible - no stretch
practice modelling with static particles & tension, friction
define lamina
like a piece of paper - 2d surface with area & thickness = 0
mass is distributed across the 2d surface
what point should you resolve forces about in statis rigid bodies?
a point that eliminates a force
draw diagram for inclined rod & peg with reaction & friction forces
see Wilson OneNote
draw diagram for ladder & wall with reaction & friction forces
see Wilson OneNote
draw diagram for suspended rod & wall with reaction, tension & friction forces
see Wilson OneNote
particle = no moments
what equations do dynamics questions involve?
SUVAT
F = ma
resolving forces
Fmax = μR
kinematics SUVAT vectors note!
when asked to find position vector & given initial position vector, do
r = r0 + ut + 1/2at^2
bc r = ut+1/2at^2 is displacement from the original point
