My question is Ch.2 #27 -- A car traveling 85 km/h strikes a tree. The front end of the car compresses and the driver comes to rest after traveling 0.80 m. What was the average acceleration of the driver during the collision? Express the answer in terms of g's where 1.00g = 9.80 m/s/s. The first thing I did was change 85 km/h to 23.6 m/s, then I decided that for the driver 'ds=.80m and vf=0m/s. I am not sure where to go from here. If we know 'ds and vf of the driver we need vo to solve for a. I am not sure how to interpret the 85 km/h(23.6 m/s) speed of the car.

You are correct that you need v0. The problem gives you v0:
You have ds and vf for the .80 m displacement. At the instant the car first contacts the tree it is moving at 85 km/hr, or 23.6 m/s. So v0 = 23.6 m/s.
Thus you have the three variables you need to calculate the acceleration.

You are correct that you need v0. The problem gives you v0: You have ds and vf for the .80 m displacement. At the instant the car first contacts the tree it is moving at 85 km/hr, or 23.6 m/s. So v0 = 23.6 m/s. Thus you have the three variables you need to calculate the acceleration.

You are correct that you need v0. The problem gives you v0:
You have ds and vf for the .80 m displacement. At the instant the car first contacts the tree it is moving at 85 km/hr, or 23.6 m/s. So v0 = 23.6 m/s.
Thus you have the three variables you need to calculate the acceleration.