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, 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. The time period depends on the length of a pendulum. Applications of the pendulum should include simple fairground and playground rides.
The turning effect of a force is called the moment. 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. Applications should 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 cross-sectional areas on the effort and load side of a hydraulic system enables the system to be used as a force multiplier.
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.