What are the best steps for understanding engineering dynamics?
Dynamics is often difficult for engineering students (or even engineers). But there is a very important key to understanding engineering dynamics. The key is to focus on a systematic approach to solving dynamics problems. This blog post will explain how to use the systematic approach.
Understanding Engineering Dynamics Categories
You may have noticed that there can be many different ways to solve any given dynamics problem. I have taught dynamics several times in my academic career, and I have noticed that students have a hard time finding the best method to use for a given problem.
Any dynamics problem will fall into one of four categories:
- Particle kinematics
- Particle kinetics
- Rigid body kinematics
- Rigid body kinetics
The key for fully understanding engineering dynamics and solving dynamics problems is to identify the proper category and understand the solution procedure for that category.
Particles and Rigid Bodies
The first step to understanding engineering dynamics problems is to know the difference between particles and rigid bodies. It is common to hear the word particle and automatically assume something small. However, that is not necessarily the case. In dynamics problems it is not uncommon to have something like a car or train be a particle. The defining difference between particles and rigid bodies is rotation. If the object is not rotating about its own centroid it can be treated as a particle because the object’s rotational properties do not matter. If the object is rotating about its own centroid, the rotational properties are important and it is considered a rigid body.
Kinematics and Kinetics
One very important area in understanding dynamics is the distinction between kinematics and kinetics. The determining factor here is force. Kinematics problems concentrate on motion without any regard to the force required for that motion. In other words, kinematics focuses on the geometry of motion. Kinetics problems involve forces.
The Systematic Approach to Understanding Engineering Dynamics
So, now we can put some ideas together and define the systematic approach. When you read a dynamics problem, the very first step is to ask yourself ‘is this problem dealing with particles or rigid bodies?’. Remember, if the object is not rotating about its own center of mass it is a particle. Also, rigid body problems will give additional information about the object such as moment of inertia.
Once you determine if the problem relates to particles or rigid bodies, the next step is to ask ‘is this a kinematics or kinetics problem?’. Force is the key here.
Let’s look at some examples:
So, the first thing we need to determine is if this is a particle or rigid body problem. Because the disk is rotating about its own center of mass it is a rigid body. The next step is to determine if the problem is in the category of kinematics or kinetics. The problem only asks about the motion (position and velocity for this problem) and does not ask or include information about forces. Therefore, the problem is a kinematics problem. Now we know the problem is a rigid body kinematics problem.
The problem involves a piston connecting rod that is rotating about its own center of mass (it is in general plane motion). The problem also gives information about the center of mass location and the radius of gyration (which will give moment of inertia). Therefore, the problem is a rigid body problem. The question specifically asks for force, which is a kinetics problem. So, the problem is a rigid body kinetics problem.
Understanding engineering dynamics first requires the ability to determine the type of problem to be solved. This blog was all about understanding the four categories. You should be able to read a dynamics problem and identify the proper category. Future blogs will go through solution methods for each category.
Get more tips like this in the study guide