NCERT Solutions for Class 9th Science Chapter 6 .6 Newton's Third Law of Motion — Activity 6.6: Let us verify

Book page 1086 Updated on2026-09-08

Q1.

Imagine that you are pulling the free end of the other spring balance with your other hand. Predict what will be the readings of their scales if the spring balances are stationary.

Answer

Prediction: both scales will read exactly the same number of newtons.

Balance A pulls Balance B with force F_AB
Balance B pulls Balance A with force F_BA
Newton's third law: |F_AB| = |F_BA|
Each spring balance shows the magnitude of the force pulling it
→ Reading A = Reading B

Why the prediction is safe: the two balances are hooked together, so whatever force one experiences comes entirely from the other. Newton's third law says such a pair is always equal in magnitude and opposite in direction, whether you pull gently or hard. And because the balances are stationary, the net force on each is zero, so nothing else can be adding to the reading.

Q2.

Now, carry out step 3. Repeat it multiple times by varying the magnitude of the force applied by you. Is your observation same as your prediction?

Answer

Yes. Whatever force you pull with — small, medium or large — the two scales read the same value every single time.

Trial	Reading of balance A	Reading of balance B	Conclusion
Gentle pull	e.g. 2 N	2 N	Equal
Medium pull	e.g. 5 N	5 N	Equal
Hard pull	e.g. 9 N	9 N	Equal

What this proves: the forces the two balances apply on each other, in opposite directions, are equal in magnitude — and they stay equal no matter how the magnitude is changed. Repeating with different forces is what turns a single lucky reading into evidence for a general law: Newton's third law of motion.

Tip: both balances must be horizontal and reading zero to start with, otherwise their own weights add a systematic error to the comparison.

Was this helpful? Report an error