Inertia Physics Example 2

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Example 2

medium
A passenger in a car is not wearing a seatbelt. The car, traveling at 15 m/s15 \text{ m/s}, suddenly stops in 0.3 s0.3 \text{ s}. What happens to the passenger (mass 70 kg70 \text{ kg}) and what force would be needed to decelerate them at the same rate?

Solution

  1. 1
    Due to inertia, the passenger's body continues moving forward at 15 m/s15 \text{ m/s} even though the car has stopped.
  2. 2
    The deceleration of the car is: a=ΔvΔt=0−150.3=−50 m/s2a = \frac{\Delta v}{\Delta t} = \frac{0 - 15}{0.3} = -50 \text{ m/s}^2
  3. 3
    The force needed to decelerate the passenger at the same rate: F=ma=70×50=3500 NF = ma = 70 \times 50 = 3500 \text{ N}

Answer

F=3500 NF = 3500 \text{ N}
Inertia causes an unrestrained passenger to continue forward when a car stops suddenly. A seatbelt provides the restraining force to decelerate the passenger along with the car, preventing injury.

About Inertia

The intrinsic tendency of an object to resist any change in its state of motion, whether at rest or moving.

Learn more about Inertia →

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