Forces in Equilibrium Flashcards
Define a scalar
A scalar is any physical quantity that is not directional
Give examples of vector quantities
Displacement Velocity Acceleration Momentum Force Pressure
When drawing polygons to add vector quantities, what represents the resultant vector?
The arrow that closes the polygon of vectors
What can be said about the object if the polygon of vectors acting upon it is closed?
There is no resultant vector
Define a moment
The force applied to an object, making it rotate, multiplied by the perpendicular distance to the line of action of the force from pivot
M = F x d
State the principle of moments
If an object is NOT rotating, the sum of al the moments about any point must be equal to ZERO
Define an object’s centre of mass
The point from which all of the weight of an object appears to act from.
State the 2 conditions and object must have for maximum stability
A large base area
A low centre of mass
Define a point object
An object that is small enough that all the forces acting on it can be considered to act at one point.
Define coplanar forces
Forces that all act in the same plane
Give the 2 conditions required for the static equilibrium of coplanar forces
1) The force vectors acting upon the point object must form a closed polygon (therefore the resolved components of all the force vectors in any two perpendicular directions must be zero).
2) The resultant moment about any point in the plane of the forces must be zero.
State the principle of concurrency
The lines of the forces actions must all pass through one point
Name the condition required for the static equilibrium f three coplanar forces
The forces acting on the object must be concurrent
N.b. if the forces are concurrent, condition 1 is enough to ensure static equilibrium
Define a vector
A vector is any physical quantity that has a direction as well as a magnitude
Define a couple
A couple is a pair of equal and opposite forces acting on a body, but not along the same line
Define stable equilibrium
If a body in stable equilibrium is displaced then released, it returns to its equilibrium position
(e.g. a coat hanger or a hanging basket)
Define unstable equilibrium
If a body in unstable equilibrium is displaced, the displacement will be increased by the centre of mass of the body
(e.g. a plank on an oil drum)
Describe when a tilted object will topple
A titled object will topple if it is tilted too far. If the objects line of action of its weight (acting from the centre of mass) passes beyond the pivot, the object will topple over.
What is the furthest point an object can be tilted without it toppling over?
The position where the line of action of the objects weight passes through the pivot
Explain why a taller uniform object will topple over at smaller angled slopes than smaller uniform objects
Because the lower the centre of mass of an object, the more stable it is
This is because an object with a higher centre of mass doesn’t need to be titled as far for its line of action of its weight to pass through the point it would pivot around
For a point object acted on by a three forces, describe the condition required for the object to be in equilibrium
The vector sum of all 3 forces must = 0
F₁ + F₂ + F₃ = 0
Give the 2 conditions required for a body to be in equilibrium
1) The resultant force must be 0. If there are only 3 forces, they must form a closed triangle
2) The principle of moments must apply (i.e. the moments of the forces about the same point must balance out)
