Forces in Equilibrium

a moment

• A turning force

• force x perpendicular distance from the line of action of the force to a fixed (stationary) point (pivot)

• f x d

scalar

a physical quantity with magnitude only

weight vs mass

mass is a quantity of matter of an object. the greater an object’s mass, the greater its resistance to a change in velocity (inertia).

weight is a force experienced by an object with mass due to a gravitational field

Describe the conditions at which a body is in equilibrium

state of an object when at rest (stationary) or in uniform motion. no resultant external forces

• the sum of the forces in the horizontal plane must be 0

• the  sum of forces in the vertical plane must be 0

• sum of clockwise moments must be equal to sum of anticlockwise moments (net moment must be 0)

vector

a physical quantity with magnitude and direction

object not in equilibrium

the object will be experiencing a resultant external force and be accelerating

newton’t first law

an object at stays at rest (stationary) or in motion at a constant velocity (uniform motion) unless acted upon by a resultant external force

newton’s second law

the resultant force on an object is proportional to the rate of change of momentum

newton’s third law

every force has an equal and opposite reaction force

balanced force diagrams

in each plane, the sum of all the forces in that plane must equal 0

resultant vectors

moving vectors so that the head of one connnects to the tail of another will show the direction of the resultant vector 

if they form a closed loop, the forces are balance and the system is in equilibrium

adding vectors not in the same plane

resolve forces into horizontally and vertical (2 perpendicular) components using trigonometry. the vectors in the two planes can be added

the principle of moments

for an object in equilibrium, the sum of the clockwise moments about any fixed point is equal to the sum of the anticlockwise moments about that point

centre of mass

single point that you can consider it’s whole mass to be concentrated. and its whole weight to act through

(may fall outside of the object)

for a uniform regular solid, this is considered to be at the centre of the object 

a point through which the line of action of a single force on the body has no turning effect

a couple

a pair of equal in magnitude forces which act parallel to each other but in opposite directions and not along the same line of action. 

they do not produce a resultant linear force but do cause a turning effect

the size of the turning effect depends on the size of the forces and the distance between them