NCERT Solutions for Class 9th Science Chapter 6 .6 Newton's Third Law of Motion — Activity 6.7: Let us understand
Book page 1096 Updated on2026-09-08
Q1.
Remove the thread tied to the neck of the balloon and observe in which direction the straw and the balloon move.
Answer
The balloon and straw shoot along the thread in the direction opposite to the direction in which the air rushes out of the neck.
Stretched balloon pushes air molecules out through the neck, with force F
Escaping air pushes the balloon material the other way, with force F
The two forces act on different objects — air and balloon — so they do not cancel
Net force on balloon + straw is forwards → it accelerates along the thread
Why it happens: as the stretched rubber shrinks it squeezes the air inside and expels it backwards. By Newton's third law the escaping air pushes forward on the balloon with an equal force. The straw threaded on the taut line simply keeps the motion in a straight line so you can see the effect clearly.
The balloon rocket: air is expelled one way, and the balloon is pushed the other way by an equal force.
Did you know? A rocket works in exactly this way. Its engine expels hot gas downwards; the gas pushes the rocket upwards with an equal force, and because that upward force is larger than the rocket's weight, the net force is upwards and the rocket lifts off.