turning effects of forces Flashcards
(12 cards)
what is a moment?
the turning effect of a force
what is a lever?
a lever is a rigid bar resting on a pivot, used to move a load when a force is applied
a longer rigid bar has a longer perpendicular distance from the edge of the lid, creating a greater turning effect, load is moved more easily
the greater the force applied, the greater the turning effect, load is moved more easily
what is the definition of moment?
the moment if a force about a pivot is the product of the force and perpendicular distance from the pivot to the line of action of the force applied
in order to create a moment, the line of action of the force must not pass through the pivot
a clockwise rotation about a pivot = clockwise moment
an anticlockwise rotation about the pivot = an anticlockwise moment
what is the formula of moment?
M = Fd
what is the SI unit of moment?
newton metre (N m)
what is the principle of moments?
DEFINITION=
the principle of moments states that when a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot
context = load has mass and hence weight, so it exerts a turning effect as well (that is opposite to that of the applied force)
when the lever is in equilibrium (balanced/does not rotate) = clockwise and anti clockwise moment are equal
what are the conditions for equilibrium?
for an object to be stationary or in equilibrium = must not have any rotational motion
1. resultant moments on the body = zero
2. resultant forces acting on the body = zero
what is centre of gravity?
the centre of gravity of an object is an imaginary point where the entire weight of the object seems to act
For the body to be in balance, the sum of clockwise moments (due to individual particles) must be equal to the sum of anticlockwise moments. This is equivalent to having a force acting through a point below the pivot, since the net turning effect is zero.
the sum of all the individual particles’ weight = a resultant force acting through this point below/above the pivot = weight of the body.
what is stability?
DEFINITION = the stability of an object is a measure of its ability to return to its original position
when an object is displaced:
topple / return to its original position
what are factors affecting stability?
- position of the centre of gravity
- area of the base of the object
how does cg affect the stability of an object?
(a) return to its original position
If the block is tilted slightly, it can return to its original position.
The vertical line through its centre of gravity still falls within its base. The block’s weight creates an anticlockwise moment about the pivot that helps the block return to its original position.
(b) balanced
If the block is tilted a little more, it will reach a point where its centre of gravity is in line with the pivot and the block would balance on its edge.
Since the centre of gravity (line of action of weight) is directly below the pivot, no moment is generated in any given direction. Hence, the toy will not topple.
(c) fall over
If the block is tilted even further, the block will topple.
The vertical line through its centre of gravity falls outside its base. The centre of gravity of the block will generate a clockwise moment and cause the block to topple.
how does the area of the base of an object affect its stability?
the lower the centre of gravity of an object, the more stable it is
greater height = when slowly tilted through the same angle, it will topple first = it has a higher centre of gravity = hence, vertical line through the centre of gravity of block A falls outside its base at a smaller tilt angle
