Lesson Plan

State Newton's second law of motion both verbally and mathematically (F_net = ma)

Explain that acceleration is directly proportional to net external force and inversely proportional to mass

Define the newton in terms of kilograms, meters, and seconds, and convert between newtons and pounds

Calculate the weight of an object using W = mg and explain how weight differs from mass

Define freefall and apply the sign convention to the acceleration due to gravity

Use Newton's second law to solve for acceleration, force, or mass, building the net external force from the individual forces

4.3

"Mathematically, the second law is most often written as F_net = ma or ΣF = ma, where F_net (or ∑F) is the net external force, m is the mass of the system, and a is the acceleration." "acceleration is directly proportional to the net external force ... It is also clear from the equation a = F_net/m that acceleration is inversely proportional to mass." "The SI unit of force is called the newton (abbreviated N) and is the force needed to accelerate a 1-kg system at the rate of 1 m/s². That is, because F_net = ma, we have 1 N = 1 kg × 1 m/s² = 1 kg·m/s²." "On Earth, g = 9.80 m/s², so the weight (disregarding for now the direction of the weight) of a 1.0-kg object on Earth is W = mg = (1.0 kg)(9.80 m/s²) = 9.8 N." "When the net external force on an object is its weight, we say that it is in freefall." "Suppose that the net external force (push minus friction) exerted on a lawn mower is 51 N parallel to the ground. The mass of the mower is 240 kg. ... a = 51 N / 240 kg = 0.21 m/s²." "T = [(2100 kg)(49 m/s²) + 650 N] / 4 = 2.6 × 10⁴ N."