forces and moments Flashcards
(24 cards)
what is the definition of a field of force?
a field of force refers to a region of space within which a body experiences a force without being in physical contact with any other body
what is always zero in this topic?
resultant force = 0
net moment = 0
what are the 4 most common forces drawn in FBDs?
1) W
2) N
3) f
4) T
how does contact force arise?
in general, contact force has 2 perpendicular components: friction and the normal reaction
what is frictional force?
frictional force is a type of resistive force that arises when 2 surfaces move relative to each other or are in attempted motion
when is viscous force present and what is an example of it?
it is present when an object moves in a fluid
air resistance is a common example
explain the origin of air resistance
air resistance is the force exerted on a moving body due to the collisions of the body and the air molecules in its path
what do friction and viscous force have in common?
they are both dissipative in nature (work done by them always leads to the production of thermal energy)
what are the 3 factors affecting viscous force?
1) speed of object
- viscous force increases with speed of moving object (need not be proportional)
2) viscosity of fluid
- higher viscosity = greater viscous force encountered
3) shape and size of object
- more aerodynamic/streamline = less viscous force encountered
what is tension and what is compression?
tension is the ‘pulling’ force exerted by a spring, rope etc on another object
compression is the ‘pushing’ force exerted by a spring, column, rod etc
relate direction of force exerted by spring to direction of extension/compression of the spring
direction of force exerted by spring is opposite to direction of extension/compression of the spring
what is upthrust caused by?
upthrust is caused by the difference in pressure between the upper and lower surfaces of the object
what are the 2 factors affecting upthrust?
1) density of fluid
- denser medium = larger upthrust
2) volume of fluid displaced / volume of object submerged
- larger volume of fluid displaced = larger upthrust
define Hooke’s Law
if the limit of proportionality is not exceeded, the extension is directly proportional to the force/load applied
what is the formula for Hooke’s Law?
F = kx, where
F is force applied
k is force constant (spring constant for springs)
x is the extension/compression of the material
define moment of a force
moment of a force is defined as the product of the force and the perpendicular distance of its line of action from the pivot of rotation (VECTOR)
when is the moment of a force always 0?
when the line of action of the force passes through the pivot. this means that the perpendicular distance of its line of action from the pivot is 0
define couple
a couple is a pair of equal and opposite forces whose lines of action do not coincide. hence it tends to produce rotation only.
define moment of a couple
moment of a couple is defined as the product of one of the forces of the couple and the perpendicular distance between the lines of action of the forces
define centre of gravity
the centre of gravity of an object is the point through which the entire weight of a body may be considered to act
it is also the point where the summation of moments due to the gravitational forces of all the particles that make up the body add up to zero
state the principle of moments
principle of moments states that for a body to be in rotational equilibrium, the sum of anticlockwise moments about ANY point must be equal to the sum of clockwise moments about the same point
if a body is acted upon by 3 forces only and remains in equilibrium, then
1) the lines of action of the forces must pass through a common point
2) when a vector diagram of the 3 forces is drawn, they will form a closed triangle, with resultant force = 0
relate F = kx to springs in series
same force exerted on each spring
total extension is sum of each individual extension
1/k total = 1/kA + 1/kB
relate F = kx to springs in parallel
force is shared between all the springs
each spring has the same extension
k total = kA + kB + …
