#4). The concepts are: A). acceleration is always in the direction of the force, and B). friction always acts in the direction opposite to motion. (that's B)
#5). The concepts are: A). the NET force is the sum of all the individual forces acting (on this car, it's 1600N forward). and B). Force = (mass) x (acceleration). So Acceleration = (force) / (mass). For this car, that's (1600N forward) / (800 kg) .
#6). The concept is: As long as you don't exceed the "proportionality limit" of a spring, its extension is proportional to the load on it. That means that the change in extension is always proportional to the change in the load. So now, look at the table: As long as the load is 10N or less, the spring stretched 3cm longer for every 2N more of load. But if the load is somewhere between 10N and 12N, that relationship disappears. Something changes between 10N and 12N of load. The spring's "elastic limit" is somewhere in that slot.
