A body moves with uniform velocity when the resultant force on it is

Analyze the question We are asked what the resultant (net) force must be for a body moving with uniform velocity, meaning constant speed in a straight line.

Concept: Newton's First Law A body stays at rest or moves with constant velocity unless a net external force acts on it. Newton's Second Law gives the same result:

F = ma

Uniform velocity means the velocity does not change, so acceleration a = 0.

Solution

1. Uniform velocity means a = 0.
2. Substitute into F = ma: F = m × 0 = 0.
3. So the resultant force must be zero.

Checking the options

• A. Greater than the frictional force: The resultant would be positive, so the body would accelerate. Wrong.
• B. Less than the frictional force: The body would slow down (decelerate). Wrong.
• C. Equal to zero: No net force means no acceleration, so the velocity stays constant. Correct.
• D. Equal to unity: A resultant of 1 N is still a non-zero force, so the body would accelerate. Wrong.

Note that this does not mean no forces act on the body. For example, a car moving at constant speed has an engine thrust that exactly balances friction and air resistance. The forces are balanced, so the resultant is zero.

Answer: C

Exam tip: "Uniform velocity" and "constant speed in a straight line" both mean a = 0, which means resultant force = 0. If you see "uniform velocity" or "equilibrium," think "balanced forces."