A morsel of food with a mass of 4.2g is injected into the esophagus with an initial speed of 2.5cm/s. On the way down to the stomach, the walls of the esophagus exert an upward resistive force of 0.0027 N on the morsel. Ifthe esophagus is 20cm long, with what speed does the morsel of food enter the stomach? I solve it by : Wf = 1/2 m (v^2 - u^2) and I got the wrong answer. Can anyone tell me what`s wrong with it? I feel very blur with enerygy concept, especially relate with non conservative force. Can anyone summarize the important concept about energy for me?