Cambridge Secondary (7-9). Stage 9. Science. Learner's Book - Unit 3: Forces and energy - Question 74

Activity 3.3.1: Conservation of energy. Use two identical plastic bottles with lids, water, string, scissors, ruler and two strong supports about 70–80 cm apart. Fill both bottles completely with water, with no air, to give equal masses. Tie about 10 cm of string to each neck. Tie a supporting string securely between the supports, high enough that the bottles do not touch the surface. Hang the bottles about 40–50 cm apart at equal heights, as shown. Pull the bottom of one bottle towards you so it swings at 90° to the supporting string; observe both bottles.



Explain this observation using the law of conservation of energy.