Lesson Plan

State Newton's third law of motion both verbally and mathematically

Identify the two forces in an action-reaction pair and explain why they do not cancel

Use the third law to determine which forces are external to a chosen system of interest

Describe the normal force and explain why N = mg on a horizontal surface follows from the second law, not a third-law pairing

Describe tension and calculate the tension in a rope supporting a hanging mass (T = mg)

Explain thrust as a third-law reaction and combine the third and second laws to solve for acceleration

4.4

"Newton's third law of motion states that whenever a first object exerts a force on a second object, the first object experiences a force equal in magnitude but opposite in direction to the force that it exerts." "You might think that two forces of equal magnitude but that act in opposite directions would cancel, but they do not because they act on different systems." "The normal force is the outward force that a surface applies to an object perpendicular to the surface, and it prevents the object from penetrating it. The normal force is a third law reaction force to the object's force into the surface." "If an object on a flat surface is not accelerating, the net external force is zero and, as a consequence of Newton's second (not third!) law, the normal force has the same magnitude as the weight of the system but acts in the opposite direction. In equation form, we write that N = mg. ... This does not violate the third law since gravity and the normal force are not third law pairs." "Tension is a pull that acts parallel to the connector, and that acts in opposite directions at the two ends of the connector." "T = mg = (5.00 kg)(9.80 m/s²) = 49.0 N." "Rockets move forward by expelling gas backward at a high velocity. This means that the rocket exerts a large force backward on the gas in the rocket combustion chamber, and the gas, in turn, exerts a large force forward on the rocket in response. This reaction force is called thrust." "F_net = F_floor − f = 150 N − 24.0 N = 126 N. ... a = 126 N / 84 kg = 1.5 m/s²."