Exercise 6.3
Two blocks P and Q on a smooth horizontal surface are shown in acting in opposite directions on block P, while block Q is moving with a constant velocity. Which of the following statement is correct? (i) P experiences a net force and Q does not experience a net force. (ii) P does not experience a net force and Q experiences a net force. (iii) Both P and Q experience a net force. (iv) Neither P nor Q experiences a net force.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(i)
Correct statement: (i) P experiences a net force and Q does not experience a net force.
On P the two forces are opposite and unequal, so the net force is the difference of their magnitudes: \(\displaystyle 5\ \text{N} - 4\ \text{N} = 1\ \text{N} \), directed along the $\displaystyle 5$ N force.
A non-zero net force means P is accelerating.
Q moves with a constant velocity, so its acceleration is zero and by Newton's first law the net force on Q is \(\displaystyle 0\ \text{N} \).
Hence P has a net force, Q does not — option (i).