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P3.2 Using Physics To Make Things Work
Many things, from simple toys to complex fairground rides, are constructed from
basic machines such as the lever. A knowledge of the physics involved in balancing
and turning can help us to make these appliances work.
P3.2.1 Centre of Mass
The centre of mass of an object is that point at which the mass of the object may be thought to be
concentrated. ) If freely suspended (hanged from above i.e. experiment with Mr Collins), an object will
come to rest with its centre of mass directly below the point of suspension. The centre of mass of a
symmetrical object is along the axis of symmetry. For a simple pendulum:
T = 1/ f
T is periodic time in seconds, s
f is frequency in hertz, Hz
The time period depends on the length of a pendulum.
the turning effect of a force is called the moment.
The size of the moment is given by the equation:
M= F x d
M=moment of the force in newtonmetres, Nm
F =force in newtons, N
d=perpendicular distance from the line of action of the force to the pivot in metres, m
If an object is not turning, the total clockwise moment must be exactly balanced by the total
anticlockwise moment about any pivot.
The calculation of the size of a force, or its distance from pivot, acting on an object
that is balanced.
If the line of action of the weight of an object lies outside the base of the object
there will be a resultant moment and the body will tend to topple. An exemplification
of this would include vehicles and simple balancing toys.
Liquids are virtually incompressible, and the pressure in a liquid is transmitted equally in all directions.
The use of different crosssectional areas on the effort and load side of a hydraulic system enables the
system to be used as a force multiplier. The pressure in different parts of a hydraulic system is given by:
P = F/A
P is the pressure in pascals, Pa
F is the force in newtons, N
A is the crosssectional area in metres squared, m2
P3.2.4 Circular Motion
when an object moves in a circle it continuously accelerates towards the centre of the circle.
This acceleration changes the direction of motion of the body, not its speed. The resultant force causing
this acceleration is called the centripetal force and is always directed towards the centre of the circle.
The centripetal force needed to make an object perform circular motion increases as:
the mass of the object increases
the speed of the object increases
the radius of the circle decreases.