4. A mechanic exerts a constant horizontal force to push a 2.50 x 103 [kg] car from rest to a speed of v, doing 5.00 x 103 [J] of work in the process. During this time, the car moves 25.0 [m]. Neglecting friction between car and road, find: (a) the value of the constant horizontal force exerted on the car; (b) the final speed v; and (c) the time needed for the mechanic to move the car this distance. 5. (a) Calculate the average force needed to bring a 950-[kg] car to rest from a speed of 90.0 [km/h] in a distance of 120 [m] (a fairly typical distance for a non-panic stop). (b) Suppose instead that the car hits a concrete abutment at full speed and is brought to a stop in 2.00 [m]. Calculate the average force needed to bring the car to a stop in this case and compare it with the force found in part (a). 6. A 5.00105-[kg] subway train is brought to a stop from a speed of 0.500 [m/s] in 0.400 [m] by a large spring bumper at the end of its track. What is the force constant k of the spring