Chapter 6 - Forces in equilibrium Flashcards
What is a vector?
Any physical quantity that has a direction as well as a magnitude.
What is a scalar?
Any physical quantity that is not directional.
What is the moment of a force about any point?
The turning effect of a force.
The force x the perpendicular distance from the line of action of the force to the point.
M = F x d
What is the principle of moments?
If a body acted on by forces is in equilibrium,
The sum of the clockwise moments about any fixed point = The sum of the anticlockwise moments about that point.
F1d1 = F2d2
No overall turning effect.
What is the centre of mass of a body?
The point through which a single force on the body has no turning effect.
It is the point in an object through which all forces on the body appear to act.
What is a uniform object?
An object that has its centre of mass at its centre.
What is a couple?
A pair of equal and opposite forces acting on a body, but not along the same line of action.
The moment of a couple (torque)= Fd,
where d = distance between line of action of each force-
Moment of couple defined as…
…force × perpendicular distance between the lines of action of the forces.
What are the conditions for equilibrium?
The resultant force must be zero. If there are three forces, they must form a closed triangle.
The principle of moments must apply (i.e., the moments of the forces about the same point must balance out).
(Overall M = 0).
Resolving a vector means to…
…split it up into two components.
What is the normal contact force (R)?
The force exerted by a surface on an object, acting perpendicularly to the surface.
What is tension?
The pulling force exerted by a string, cable, rope, chain or by each end of a rod on an object.
What is upthrust (buoyancy)?
An upward force exerted by a fluid that opposes the weight of a immersed object.
What is the triangle of forces?
For three forces to give zero resultant, their vector sum F1 +F2 + F3 = 0 and form a closed triangle.
An object will topple over if…
…the centre of mass falls outside its base area. This is because a resultant moment occurs which provides a turning force.
The lower the centre of mass and the wider the base area, the more stable the object is.
