Fig. 3-1 The reaction from the ground balances the weight of the car.

Sometimes forces do not produce any noticeable effects, but we know that they are there. A car at rest on the ground does not fall under gravity to the centre of the Earth, because its weight (force of gravity) is balanced by the reaction force from the ground (Fig. 3-1). The two forces balance each other.

When two or more forces are acting on a body, they add together to produce a resultant effect. This is represented by a resultant force (or net force). If the resultant force is non-zero, the forces are said to be unbalanced and will produce a noticeable effect on the object.

Forces add up as vectors. Consider the ship pulled by two small towboats as shown in Fig. 3-2. We can use the so-called tip-to-tail method to find the resultant force.

Fig. 3-2 A ship is pulled by two boats (left). We can use the tip-to-tail method to find the resultant force (right).

	Toy towboats   Play [Low | High]    Download [Low(66kb) | High(118kb)]
	A toy ship is pulled by two toy towboats travelling in different direction. The ship moves in the direction of the resultant force.