Using Physics to Make Things Work (P3.2)
Pendulums
Centre of Mass
Circular Motion
Hydraulics
Moments in Balance
Moments
Stability
Turning effect of a force is called a moment
M = F x d
M is the moment of the force in Newton metres, Nm
F is the force in Newtons, N
d is the perpendicular distance from the line of action of the force to the pivot in metres, m
To increase the moment
Force must increase
Or pivot must increase
Levers make jobs easier as it acts as a force multiplier so the effort applied can be smaller than the load
Mass is concentrated at one single point, the centre of mass
Any object freely suspended will come to rest with its centre of mass directly below the point of suspension. The object is then in equilibrium
Finding the centre of mass of a thin irregular sheet of material
Hang the sheet with the pin at another point and repeat it
Mark the position of the plumbline against the sheet
When it comes to rest, hang a plumbline from the same pin
Suspend the sheet from a pin held in a clamp stand
The centre of mass is where the lines marked the position of the plumbline cross
Plumbline: String with a small weight on the end
The centre of mass depends on the shape of the object
For a symmetrical object the centre of mass is along the axis of symmetry
If an object is in equilibrium it is balanced
The moments about any point will find the total clockwise moment and total anticlockwise moment are equal
Everyday principle of moments include seesaws and balance scales
The line of action of the weight of an object acts through its centre of mass
The wider the base of an object and the lower its centre of mass, the further it has to tilt before the line of action of the weight moves outside the base
The stability of an object is increased by making its base wider and its centre of mass lower
Pressure is given by the equation P = F/A
F is the force in Newtons, N
P is the pressure in pascals, Pa (N/m²)
A is the cross-sectional area at right angles to the direction of the force in metres squared, m²
Liquids are virtually incompressible and pressure in a liquid is transmitted equally in all directions
The force exerted on one part of a liquid will be transmitted to other points in the liquid, like in hydraulic systems
The force exerted by a hydraulic pressure system depends on
The area of the cylinder which this force acts on
The area of the cylinder that exerts this force
The force exerted on the system
When an object moves in a circle it is continuously changing direction therefore continously changing velocity as it is changing direction
This is called the centripetal acceleration
An object only accelerates when a resultant force acts on it. This force is called the centripetal force and always acts towards the centre of the circle
If the centripetal force stops acting, the object will continue to move in a straight line in tangent to 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
A pendulum moves to and fro along the same line, this is oscillating motion
A simple pendulum consists of a mass, called a bob, suspended on the end of a string
When the bob is displaced to one side and let go, the pendulum oscillates back and forth, through the equilibrium position
The amplitude of the oscillation is the distance from the equilibrium position to the highest point on either side
The time period of the oscillation is the time taken to complete one cycle, which is
The time taken from the highest position on one side to the highest position on the other side and back to the start position
The time taken between successive passes in the same direction through the equilibrium position
The time period depends only on the length of the pendulum and increases as its length increases
The frequency of oscillations is the number of complete cycles of oscillation per second
Time period and frequency are given by T = 1/f
T is the time period in seconds, s
f is the frequency in hertz, Hz