Highway safety engineers build soft barriers such as the one shown in Figure 3-21, so that cars hitting them will slow down at a safe rate. A person wearing a safety belt can withstand an acceleration of -3.0 102 m/s2. How thick should barriers be to safely stop a car that hits a barrier at 121 km/h?

Answer 1

almost 2 feet in width

Answer 2

first 121 km/h (1000m/1km)(1h/3600s) = 33.6 m/s

v_f² = v_i² + 2a(&Delta x)

&Delta x is what we want to solve for, it tells us the distance which we will compress the barrier so it needs to be atleast that thick

a is the max acceleration given which I don't really understand from what you've written (-3.0 102?)

v_f is 0 because the car will stop

v_i is 33.6 m/s

solving for &Delta x we get

[v_f² - v_i²] / 2a = &Delta x