Chapter 9 – Forces in Action

Forces can start motion, stop motion, speed up or slow down motion, change the direction of motion, change the shape of an object, or have no visible effect at all.

A force is a push or a pull. Weight is the property of an object caused by the force of gravity pulling it towards earth’s centre. Forces may be contact forces, where an object strikes or rubs against another, or the forces may be invisible – no touching is necessarily involved.

Permanent magnets retain their magnetism and are made of a combination of iron, nickel, and cobalt substances. Magnetite is a natural magnetic substance. Magnets have two poles – a north pole and a south pole. When a bar magnet is suspended its south pole points to the South Pole of the earth’s magnetic field. When bar magnets are placed near each other, it is found that like poles repel whereas unlike poles attract.

Electromagnets are temporary magnets made from a coil of wire wrapped around an iron core. They only act as magnets when electric current passes through the wire.

When swimming, heavy objects seem lighter because the water exerts an upward force on the object. This force is called buoyancy. Liquids seem to produce a "skin" type of effect at their surfaces. This force is called surface tension and permits some objects or animals to float on the surface even when the density is greater than that of the fluid.

The force that causes an object to slow down or stop, when moving on a surface or through a fluid, is called friction. Without this force there would be no grip when you run, a car’s wheels would spin, and anything you grasped would feel slippery. Oil is used to reduce friction in car engines, because friction produces heat and causes parts to wear. It is an example of a lubricant. To reduce fluid friction vehicles are steamlined. This friction causes falling objects to reach a terminal speed. Besides fluid friction, there is sliding friction and rolling friction. Rolling friction is less than sliding friction, and, inside wheels, ball bearings are used to reduce friction with the axles.

Weight is measured in kilograms, whereas mass is measured in Newtons. Mass relates to the amount of material in an object whereas weight also depends on gravity.