Based on a velocity time graph, the average acceleration can be found by taking the start and end points, drawing a line between them, and then finding the slope of the resulting line. Instantaneous acceleration is the same except the line is the tangent to the curve at the point in question. This will not effect horizontal velocity, so this will usually remain constant. The direction of the forces should be shown, but it's usually not necessary to scale the lines for magnitude. The SI unit of mass is the kilogram.
Weight is a measure of force, specifically the force mass causes in the gravity of the earth.
The SI unit is the newton. Thus, an object traveling at constant speed has no net force acting on it. The resultant force on a free bond diagram can be found by vector addition. Usually most of the vectors will cancel out, making things simpler. B, B exerts an equal magnitude force on A in the opposite sense.