Summary
Highlights
Newton's third law states that for every action, there is an equal and opposite reaction (F1 on 2 = -F2 on 1). The video demonstrates this using two spring scales connected by a loop. When one scale pulls with a force of 3 Newtons, the other scale simultaneously reads 3 Newtons, illustrating that the forces are always equal and opposite, even as they fluctuate. The only exception is when forces produce rapid acceleration, preventing an instantaneous equilibrium reading.
A common misunderstanding involves confusing Newton's third law with Newton's second law. The video uses an example of an orange resting on a stool. Many mistakenly believe that the downward force of gravity (mg) and the upward normal force are an action-reaction pair due to Newton's third law. However, this is incorrect. These two forces act on the same object (the orange) and are equal only when the orange is in equilibrium (zero acceleration), according to Newton's second law (F_net = ma = 0). If the stool is removed or disturbed, the forces will not be equal, and the orange will accelerate.
Newton's third law applies to forces acting between two different objects. For example, the force the orange exerts on the stool and the force the stool exerts on the orange constitute a Newton's third law pair. Similarly, the initial spring scale experiment shows that the force exerted by one scale on the other is always equal and opposite to the force the second scale exerts on the first, regardless of motion. The video concludes by emphasizing that Newton's third law is always at play and cannot be 'beaten'; if you exert a force on an object, that object simultaneously exerts an equal and opposite force back on you.