How Forces Affect Motion | Class 9 Science Notes

Newton's Third Law of Motion

Newton's third law of motion states that whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object. This means forces always come in pairs, called action and reaction forces. These forces are equal in magnitude, opposite in direction, and act on different objects, so they do not cancel each other out. For example, when you push a table, your hand applies a force on the table, and the table applies an equal and opposite force on your hand. This mutual interaction explains many everyday phenomena, such as why a person sitting on a wheeled chair moves backward when pushing a heavy table forward. Similarly, when walking or running, your feet push the ground backward, and the ground pushes your feet forward with frictional force, propelling you ahead. The third law also explains the motion of boats or canoes, where pushing water backward with a paddle results in a forward force on the paddle and boat. Experimental verification using spring balances shows that forces measured on connected springs pulling against each other are equal in magnitude. Newton's third law applies to all forces, including contact forces like friction and tension, and non-contact forces like gravity, magnetism, and electrostatics. For instance, the Earth and a falling fruit exert equal and opposite gravitational forces on each other, but due to Earth's large mass, its acceleration is negligible. The law is fundamental in understanding interactions and is essential in analyzing systems ranging from simple pushes to rocket propulsion, where expelling gas downward produces an upward thrust. Recognizing action-reaction pairs helps in solving problems involving forces and motion.

📊 Diagram: Figure 6.22 shows a girl kicking a ball illustrating action and reaction forces. Figures 6.23a and 6.23b depict a girl pushing a table forward and backward, showing forces exerted by the girl and table on each other. Figure 6.24 shows a person pushing the ground backward with feet, and Figure 6.25 shows frictional force pushing the person forward while walking. Figure 6.26 shows two spring balances connected and pulled in opposite directions to verify equal forces. Figure 6.27 shows a person climbing a coconut tree using frictional forces. Figure 6.28 shows a canoe moving forward as the paddle pushes water backward. Figure 6.29 shows a balloon moving opposite to the direction of air expelled, illustrating rocket motion. Figure 6.30 shows a rocket launching.

🧪 Activity: Activity 6.5 explores pushing and pulling a table while sitting on a wheeled chair to observe reaction forces. Activity 6.6 uses two spring balances connected to verify equal and opposite forces. Activity 6.7 demonstrates rocket motion using a balloon and straw on a thread.

🔗 Connection: This section concludes the study of Newton's laws by explaining force interactions, leading to the application of these laws to systems of objects.