Each of these is a real situation with a velocity-time graph already measured. Start with the puck if this is new.
The top graph is given. In the bottom panel, draw what you think the net force was doing at every moment. The dashed line is zero — you can draw above it or below it. Draw all the way across.
Compare your sketch to the answer. Name one section you got right and one you did not.
Look at both graphs together. What feature of the velocity graph tells you the size of the net force? Write the rule in your own words.