Physics Explained Through a Video Game/Forces and Free Body Diagrams

Topic 2.2 - Forces and Free Body Diagrams

Unlike most other sections of this course, this topic is reserved for reading and as a reference. As such, there are fewer practice materials for this topic. Consider the later topics in Unit 2 to broaden and apply the skills discussed below.

Dynamics and Forces

Two players bouncing on a set of colorful springs in the map "Mushroom Forest" by Nexooo. Physical interactions, such as those pictured, involve the presence of numerous forces that affect the motion of the players.

So far during much of this course, we have focused on kinematics, a way of describing the motion of objects, such as through their position, velocity, and acceleration. However, we also need to discuss what causes for objects to move in the first place. In this unit, we will be largely focusing on dynamics. This content area focuses on linking together the existence of forces and how they impact the motion of objects.

A force can be described as a push or a pull that is acting on an object. It is a vector quantity, meaning that it has both a magnitude and direction. We can use the symbol ${\vec {F}}$ to denote the presence of a force. Also, forces have their units in Newtons ($N$). As will be explored closely in the next topic, a Newton is equivalent to ${\text{kg}}\cdot {\frac {\text{m}}{{\text{s}}^{2}}}$. In other words, a force's magnitude is related to the mass of the object (described in $kg$ multiplied by the acceleration of that object (described in ${\frac {\text{m}}{{\text{s}}^{2}}}$). This can give us an intuition that forces—through their ability to push or pull—influence objects in accelerating towards a particular direction.

Characterizing Forces

A video of a rhinoceros walking in the map "Safari" by O_dot.

There are a wide variety of forces that exist, ranging from frictional forces, gravitational forces, tension forces, and many more. To characterize these forces, we distinguish between forces that are external and internal. An external force is a force that is originating from outside of an object or system and is applying a push or pull on it. For instance, assume that we're considering a rhinoceros as a system of interest. The gravitational force exerted by the Earth that pulls the rhinoceros down towards the ground is an example of an external force on the creature.

In contrast, there also exists numerous forces inside of the rhinoceros as well, such as skeletal and muscular forces that enable it to be able to eat, walk, and breathe. These can be characterized as internal forces.

Often, we take a particular interest towards the external forces that are acting on a system. This is because if we combine add all of the external forces vectors together**, we can derive the net external force, denoted by ${\vec {F}}_{\text{net}}$. This vector is important as the direction angle of ${\vec {F}}_{\text{net}}$ determines the direction in which an object is accelerating. With this property and others that will be discussed later in this unit, we can garner information concerning the kinematic behavior of an object directly through considering the forces that are acting on it.

** This requires the use of vector addition. Although this concept will later discussed in Topic 2.3 - Newton's Second Law, consider reading the introduction of vector addition by FHSST Physics, another Wikibooks project.

The Difference Between Mass and Weight

[To be added.]

Free Body Diagrams

[To be added.]

References