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Answer:
u = - 40 ft / s
Explanation:
The Galilean relation for the relative velocity is
v ’= v + u
where u is the speed between the two reference frames, v is the speed of the fixed system and v 'the speed of the mobile system.
In this case the truck has a speed with respect to the ground (fixed system) 0 m / s (it is stopped), the car has a speed with respect to the ground of v = 40 ft / s,
u = v'- v
u = 0 - 40
u = - 40 ft / s
the speed perceived by the car if the system is fixed on it is -40 ft / s