When the linear momentum of a body is constant, the net force acting on it

Step 1: Analyze the question We are told the linear momentum (p = mv) of a body does not change. We need to decide what that means for the net force.

Step 2: The concept Newton's Second Law, in terms of momentum, says:

F_net = Δp / Δt (the rate of change of momentum)

Step 3: Solution

• Momentum is constant, so Δp = 0.
• Therefore F_net = 0 / Δt = 0.

This agrees with Newton's First Law. If the net force is zero, the body stays at rest or moves with constant velocity, so its momentum does not change.

Step 4: Checking the options

• A. is zero: Correct, because there is no change in momentum.
• B. increases: A growing force would change the momentum.
• C. decreases: A force that is non-zero, even if decreasing, would still change the momentum.
• D. remains constant: A constant force that is non-zero would cause constant acceleration, so the momentum would keep changing. This is a common trap. The force would be constant only if it were constantly zero, and option A states that more precisely.

The correct answer is A. is zero.

Exam tip: Remember "constant momentum means zero net force." Whenever a question says "constant velocity" or "constant momentum," think equilibrium and net force = 0. Watch out for option D, which is tempting but wrong.