Question

Difficulty: HardNewton's Second Law (F=ma)

An arresting gear system is used to stop a 12,000 kg12,000 \text{ kg} aircraft landing on a flight deck. The aircraft touches down with a velocity of 144 km/h144 \text{ km/h} and is brought to a complete stop in 2.0 seconds2.0 \text{ seconds} by the arresting wire. Assuming a constant deceleration, what is the magnitude of the average net braking force applied to the aircraft?

  1. A
    600 N600 \text{ N}
  2. B
    864,000 N864,000 \text{ N}
  3. 240,000 N240,000 \text{ N}Answer
  4. D
    2,352,000 N2,352,000 \text{ N}

Answer

The correct magnitude of the net braking force is 240,000 N240,000 \text{ N}.
The correct answer accurately calculates the force by first converting the velocity from km/h to m/s (144 km/h×1000 m3600 s=40 m/s144 \text{ km/h} \times \frac{1000 \text{ m}}{3600 \text{ s}} = 40 \text{ m/s}). It then finds the magnitude of acceleration (a=Δvt=40 m/s2.0 s=20 m/s2a = \frac{\Delta v}{t} = \frac{40 \text{ m/s}}{2.0 \text{ s}} = 20 \text{ m/s}^2) and applies Newton's Second Law (F=m×a=12,000 kg×20 m/s2=240,000 NF = m \times a = 12,000 \text{ kg} \times 20 \text{ m/s}^2 = 240,000 \text{ N}).

Step-by-Step Solution

1
Convert the initial velocity from km/h to m/s.
144 km/h×(1000 m3600 s)=40 m/s144 \text{ km/h} \times \left(\frac{1000 \text{ m}}{3600 \text{ s}}\right) = 40 \text{ m/s}
Standard SI units (meters and seconds) are required to correctly calculate force in Newtons.
2
Calculate the magnitude of acceleration (deceleration) using the change in velocity over time.
a=40 m/s2.0 s=20 m/s2a = \frac{40 \text{ m/s}}{2.0 \text{ s}} = 20 \text{ m/s}^2
Acceleration is defined as the rate of change of velocity.
3
Apply Newton's Second Law to find the average net braking force.
F=12,000 kg×20 m/s2=240,000 NF = 12,000 \text{ kg} \times 20 \text{ m/s}^2 = 240,000 \text{ N}
Newton's Second Law states that force equals mass times acceleration (F=maF = ma).

Key Concept

Newton's Second Law (F=ma) and Kinematics
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