A moving body of mass 25.0kg undergoes a uniform retardation of 20.0$ms^{-2}$. Calculate the magnitude of the retardating force

Analyze the question: A body of mass 25.0 kg is slowing down uniformly (retardation = deceleration) at 20.0 m/s². We need the magnitude of the force causing this.

Concept: Newton's Second Law states that force equals mass times acceleration:

F = ma

For retardation, we use the magnitude of the acceleration. The negative sign only shows the body is slowing down, so it does not change the size of the force.

Solution:

• Mass, m = 25.0 kg
• Retardation, a = 20.0 m/s²

F = m × a F = 25.0 × 20.0 F = 500 N

Checking the other options:

• A (1.25 N) comes from 25 ÷ 20, which is dividing instead of multiplying.
• B (8.00 N) has no sensible link to this calculation.
• C (45 N) comes from adding 25 + 20, which is wrong because force is a product, not a sum.

Answer: The retardating force is 500 N, which is option D.

Exam tip: When you see "retardation" or "deceleration," just use the magnitude in F = ma. Always check that your unit is Newtons (kg·m/s²). For quick recall: F = ma, so mass times acceleration gives force, never mass divided by acceleration.