A 4.0 kg mass and a 2.0 kg mass are next to each other on a horizontal frictionless surface. Somebody pushes them from the left with a 12.0 N force. What is the acceleration of the masses

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You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero. 

There is not a picture so i will answer this as best as i can usually when the roller coaster has the greatest potential energy is when it is sitting at the crest of the hill.