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?
3. A*R*C*S: Net Force on a Driver.
You are driving in an everyday car when you decide to hit the accelerator to make your car begin to move with its maximum acceleration. Estimate the magnitude of the net force exerted on you, the driver. Remember to use the A*R*C*S steps from Figure 2.16.1.
Answer.
For a driver with a mass of 80 kg and a car with a maximum acceleration of 6 m/s\(^2\text{,}\) the net force on the driver would be 480 N.
4. Explanation - Three Cars.
Consider the following three situations:
- You are in a car when the speed limit changes from 35 mph to 45 mph.
- You are in a car on the freeway driving at a constant speed of 70 mph.
- 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.
