NCERT Class 9 Science Exploration Chapter 06 How Forces Affect Motion PDF Download

Chapter 6: How Forces Affect Motion

In Chapter 4, you learnt to describe the motion of an object in terms of its position, velocity and acceleration. But you did not consider what causes motion. Is there an underlying cause for a change in position and velocity of an object? What is the nature of this cause? Do all motions require a cause? In this chapter, we will investigate what causes changes in the motion of objects. We will also discuss Newton's three laws of motion and learn how to apply them.

6.1 The Concept of Force

You may recall learning earlier that a force can make an object move from rest, change the speed and direction of motion of a moving object, and can even change the shape of an object. For example, a ball at rest starts moving when you apply a force, a force applied by a cricket bat on a cricket ball changes its direction, and a lemon can be squeezed by the force applied by your fingers (Fig. 6.1).

[Figure 6.1: Applying force on different objects, See in your textbook]

While learning about force earlier, did you notice that whenever any type of force acting on an object was described, its direction was also specified? For example, the phrases used were, force of friction acting on an object in a direction opposite to the direction of its motion; like poles of a magnet repelling each other or unlike poles attracting each other due to the magnetic force; like charges repelling or unlike charges attracting each 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 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.

[Figure 6.2: Measuring the weight of an object using a spring balance, See in your textbook]

Threads of Curiosity

In everyday life, the smallest forces we can directly feel are of the order of millinewtons (\( 10^{-3} \) N), such as a light touch. Scientists, however, can measure forces far smaller than this, down to yoctonewtons (\( 10^{-24} \) N) in specialised experiments (as of 2026).

Teacher's Note

When you use a spring balance to measure weight or force, remember that the scale reading directly gives you the magnitude of the force in newtons. If the scale shows 5 N, the force is 5 N. The direction is always along the line you are pulling, which is usually straight up or along the balance.

6.2 Balanced and Unbalanced Forces

In real life, situations seldom exist where only one force acts on an object. Usually, there is more than one force acting on an object. For example, when 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 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 buoyant force applied by the liquid acting upwards. In such cases, what is 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 a rope in opposite directions? If both the teams pull the rope with equal force, the rope does not move (Fig. 6.4a). Such two forces, which are equal in magnitude but opposite in direction are called balanced forces. However, if one team pulls harder, i.e., it applies a force of larger magnitude, the forces are no longer balanced and the rope moves in the direction of the larger force (Fig. 6.4b). The rope does not move if the forces applied on it are balanced but moves if the forces are unbalanced.

[Figure 6.3: (a) Pushing a box kept on table or floor, See in your textbook]

[Figure 6.3: (b) A ball floating on water, See in your textbook]

[Figure 6.4: Two forces applied in opposite direction of (a) equal magnitudes, (b) unequal magnitudes, See in your textbook]

If the forces applied on an object are not balanced, a non-zero net force acts on the object. When two forces are opposite in direction but unequal in magnitude (Fig. 6.4b), the magnitude of net force is equal to the difference between the magnitudes of two forces and the direction is along the force of the larger magnitude.

Now, think of a situation where two people are applying forces in the same direction on a stalled car to make it move (Fig. 6.5). In this case, the magnitude of the net force applied by them on the car is the sum of the magnitudes of two forces. The direction of the net force is in the same direction as the two individual forces.

[Figure 6.5: Two forces applied in the same direction, See in your textbook]

Example 6.1: Two forces of 10 N and 6 N are acting on a block lying on the table as shown in Fig. 6.6. What is the magnitude and the direction of the net force acting on the block in each case?

[Figure 6.6: Two forces acting on a block in three different manner, See in your textbook]

Answer:

1. Net force = 10 N + 6 N = 16 N, acting towards the right side.
2. Net force = 10 N - 6 N = 4 N, acting towards the right side.
3. Net force = 10 N - 6 N = 4 N, acting towards the left side.

Teacher's Note

When two forces act in the same direction, always add them. When they act in opposite directions, subtract the smaller from the larger. The net force always points in the direction of the larger force. Check yourself: if forces of 20 N and 12 N act opposite to each other, what is the net force? It should be 20 - 12 = 8 N in the direction of the 20 N force.

Ready to Go Beyond

There are situations in which forces do not act parallel or opposite to each other but act at an angle to each other. You will learn in higher grades how to calculate the net force in such cases. Also, there are situations where equal and opposite forces are applied to the two ends of an extended object which make the object rotate (Fig. 6.7). For example, applying equal and opposite forces to a handlebar or a tap makes it turn. You will also learn about this in higher grades.

[Figure 6.7: Equal and opposite forces on an extended object, See in your textbook]

Key Points

• Force is a physical quantity that requires both magnitude and direction to be fully described, and is measured in newtons (N).
• Balanced forces are equal in magnitude but opposite in direction, and they produce no net force; unbalanced forces produce a non-zero net force that causes motion.
• When forces act in the same direction, add their magnitudes; when they act in opposite directions, subtract the smaller from the larger to find the net force.

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