Definition 3.4.1. Net force.
The net force on an object or system is equal to the (vector) sum of all forces acting on it:
\begin{equation*}
\vec{F}_{net} = \sum_i \vec{F}_i = \vec{F}_1 + \vec{F}_2 + \vec{F}_3 + \dots\text{.}
\end{equation*}
Section 3.4 The Law of Motion (Newton’s Second Law)
Principle 3.4.2. The Law of Motion.
The net force on an object or system is equivalent to the product of object’s mass and its acceleration:
\begin{equation*}
\vec{F}_{net} = m\vec{a}\text{.}
\end{equation*}
Exercises Activities
1. Summarize What You Learned - Law of Motion.
Above, you learned a relationship between net force and acceleration. Write a 2-3 sentence description of what this symbolic equation says in your own words.
2. Sensemaking - Units of Force.
Using what you learned above, how would you write the units of a force (newtons) in terms of other units you are familiar with?
Answer.
\(\mathrm{\frac{kg m}{s^2}}\)
3. Explanation - Three Cars.
Consider the following three situations:
- You are in a car when the speed limit changes from \(35 \mathrm{~mph}\) to \(45 \mathrm{~mph}\text{.}\)
- You are in a car on the freeway driving at a constant speed of \(70 \mathrm{~mph}\text{.}\)
- You are in a car waiting for a red light to turn green.
Rank the three situations by the magnitude of the net force on the car from largest to smallest. Explain your reasoning.
