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How Forces Affect Motion 95 While learning about force earlier, did you notice that whenever any type of force acting on an object was described, its direction was also Grade 6 specified? For example, the phrases used were, force of friction acting on Curiosity an object in a direction opposite to the direction of its motion; like poles of Chapter 4 a magnet repelling each other or unlike poles attracting each other due to Grade 8 the magnetic force; like charges repelling or unlike charges attracting each Chapter 5 other due to the electrostatic force; the Earth attracting objects towards itself due to the gravitational force; buoyant force exerted by liquid in an upward direction on an object placed in it, and so on. Force is a physical quantity for which we need to specify direction along with its magnitude and unit, just like for the physical quantities — position, displacement, velocity and acceleration of an object which were introduced in Chapter 4. The SI unit of force is the newton (written with a small ‘n’) and its symbol is N. The magnitude of the force expresses its strength. Note If either the magnitude or direction, or the both, of a force applied on an object changes, the effect of the force also changes. 6.1.1 Measuring the magnitude of a force How can we measure the magnitude of a force? Do you remember using a spring balance earlier to measure the weight of objects (Fig. 6.2)? Do you Grade 8 Chapter 5 also remember that the weight of an object is the gravitational force with which the Earth pulls the object? A spring balance can be used to measure not just the weight of an object but the magnitude of the force in general. If you pull on the free end of the spring balance, it measures the force with which you pull on the spring inside the balance. Fig. 6.2: Measuring the weight of an object using a Threads of Curiosity spring balance In everyday life, the smallest forces we can directly feel are of the order of millinewtons (10 N), such as a light touch. Scientists, however, can measure –24 forces far smaller than this, down to yoctonewtons (10    N) in specialised Force by hand experiments (as of 2026). 6.2 Balanced and Unbalanced Forces Force of friction In real life, situations seldom exist where only one force acts on an object. Fig. 6.3: (a) Pushing a box Usually, there is more than one force acting on an object. For example, when kept on table or floor you are pushing a box placed on a surface then, apart from the force with which you are pushing it, the force of friction is also acting on the box in the Buoyant force direction opposite to that of the motion (Fig. 6.3a). Or, take the example of a ball floating on water (Fig. 6.3b). Two forces are acting upon it — gravitational force by the Earth acting downwards and Gravitational buoyant force applied by the liquid acting upwards. In such cases, what is force the effect of forces when more than one force is acting on an object at rest or in motion? Have you ever played a game of tug of war where two teams pull at Fig. 6.3: (b) A ball a rope in opposite directions? If both the teams pull the rope with equal floating on water