How Forces Affect | Class 9 Science Notes

6.6 Newton's Third Law of Motion

This section explains Newton's third law of motion, which 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 law highlights that forces always come in pairs acting on different objects. The section includes activities such as pushing and pulling a table while sitting on a wheeled chair to observe the reaction forces causing the chair to move in the opposite direction. It explains how walking or running involves pushing the ground backward and the ground pushing the feet forward via friction, enabling movement. The section also includes an activity with two spring balances connected to verify that the forces they exert on each other are equal in magnitude and opposite in direction. Real-life examples include climbing a tree, rowing a canoe, and rocket propulsion. The rocket moves upward because the gases expelled downward exert an equal and opposite force on the rocket. The section clarifies that the action and reaction forces act on different objects and thus do not cancel each other. It also discusses that Newton's third law applies to both contact and non-contact forces such as magnetic, electrostatic, and gravitational forces. Examples illustrate why a heavy object like Earth does not noticeably move when attracting a small object like a fruit, due to its large mass. The section concludes with examples of recoil in guns and the difference in acceleration of bullet and gun due to their masses.

📊 Diagram: Fig. 6.22: Kicking a ball; Fig. 6.23: Applying force on a table in forward and reverse directions; Fig. 6.24: Pushing the ground backwards; Fig. 6.25: Walking; Fig. 6.26: Two spring balances connected together are pulled in opposite directions; Fig. 6.27: A person climbing a coconut tree; Fig. 6.28: Rowing a canoe; Fig. 6.29: Air rushing out of the balloon; Fig. 6.30: Rocket launching; Fig. 6.31, 6.32, 6.33: Magnetic, electrostatic, and gravitational forces acting as action-reaction pairs.

🧪 Activity: Activity 6.5: Push and pull a table while sitting on a wheeled chair to observe reaction forces; Activity 6.6: Connect two spring balances and pull to verify equal and opposite forces; Activity 6.7: Balloon and straw experiment to understand rocket propulsion.

🔗 Connection: Leads to application of Newton's laws to systems of objects and complex motions.