Answer:
The answer is the kinetic energy of sled B after it crash is 6439j.
The forces that make a passenger speed up, slow down, or
turn a curve are the same forces that have the same effect
on the driver and anybody else in the car.
-- Speeding up . . .
the back of the seat
friction between the car seat and the seat of your pants
-- Slowing down . . .
the seat belt
friction between the car seat and the seat of your pants
-- Turning away from a straight line . . .
the seat belt
friction between the car seat and the seat of your pants
the door, or whatever or whomever you're leaning against
The magnitude of the mass is 20, and the magnitude of the speed is 0.5. Neither one has a direction, since mass and speed are scalar quantities.