As I watched my dad pick fruits, papayas in particular, from our backyard, I was amazed at how many physics concepts the simple act of picking fruits demonstrates. The scene reminded me of Newton's laws, forces, potential and kinetic energy, and work.
As the papaya falls down from the tree to the ground, Newton's third law states that forces come in pairs and in that these forces are equal and opposite to each other. As the papaya accelerates towards the ground, the Earth exerts a downward force of mg while the apple exerts an upward force F on the earth. These forces are exerted in opposite directions and are equal to each other.
In addition, as my dad caught the papaya before it reached the ground, he demonstrated a familiar work concept. As the papaya falls down and in catching the fruit, my dad's hands applied a force on the papaya, opposing its downward motion and slowing it down. This results in negative work because the force is in the opposite direction of the papaya's displacement. The negative work done on the fruit slows it down and then stops it down, reducing the papaya's kinetic energy to zero. According to the work-kinetic energy theorem, work equals the change in energy and in this case, the negative work results in a reduction in the papaya's energy. On the other hand, the situation can also be seen in another perspective. The papaya also exerts a force on my dad's hands, causing his hands to move downward. In this case, the fruit does positive work on my dad's hands becuase the force it applied is in the direction of its displacement.
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