Unit 2 Overview: Dynamics

Dynamics is the study of the forces 💪, or the interactions of an object with another object, that cause objects and systems to move. The basic understanding of a force as a push or pull helps solidify that it is a vector quantity and has both magnitude and direction 🔁. 

Similar to that of Unit 1, translation is key in Unit 2. In Unit 1, you learned how to analyze the motion of an object.  Unit 2 takes this idea further and teaches you not just how but why translational motion occurs.  

The first major concept that you will learn about in this unit is the idea of defining a system ⚙️ as a portion of the universe that you choose to study.  You will be able to identify internal and external forces to the system.  The aim of the unit is to show the same object–force interactions through different graphs 📈, diagrams, and mathematical relationships. During Unit 2, you will also learn a necessary skill throughout the remaining units of AP Physics 1: how to derive new expressions from fundamental equations to form predictions in unfamiliar scenarios. 

The backbone of this unit is a variety of different types of forces.  These forces are typically classified into two categories: contact forces and non-contact forces.  Contact forces are exactly what they sound like–forces that occur when two objects are directly touching each other 🙏.  Non-contact forces are forces that occur at a distance 🙌. 

The most common forces that you will study in this unit are weight, normal force, tension, friction, and spring force.  Weight is the force exerted on an object by gravity.  It is the only non-contact force you will encounter in this unit, and you calculate it by multiplying the mass of the object by the acceleration due to gravity.  Normal force is the force of a surface pushing against the object’s weight. Tension is the pulling force transmitted by a string, cable, or similar object 🪢. You will find that there are a lot of hanging signs and ropes when you have situations involving tension!  Friction is the force between two surfaces that resists motion.  Rougher surfaces (like sandpaper) have lots of friction, and smoother surfaces (like ice 🧊) have less friction.  Finally, spring force is precisely what it sounds like–the force exerted by springs!  We treat this differently from tension because springs and other elastic items act differently than a rope that is not as stretchy.  We use Hooke’s Law to relate the stretchiness of the spring, how far it stretches, and the spring force.

After learning about the different forces, you will start to add them together using force vectors and Free-Body Diagrams. This tool will allow you to write net force equations and calculate the net force acting on a system. This is probably the hardest part of the unit 😓, but doing practice problems will help you see patterns in the different types of questions.  Once you have that, you can relate it to the mass and acceleration of an object, culminating in Newton’s 2nd Law.  In this section, you will also learn how to determine if a system is in equilibrium (if the net force is zero) or accelerating (net force is not zero).  

There are two special 😎 cases that you will practice in this unit.  The first one is called an Atwood’s Machine.  One of these setups usually involves a pulley, a string, and a system of masses.  To work through these problems, you should be able to decide what your system is and be able to shift between the entire machine as one system and each mass separately.  

The second special case that you will practice is when you need to calculate the Apparent Weight of an object.  The apparent weight of an object will be different from the actual weight of an object when the force of gravity is not balanced by an equal normal force ⚖️.  This case typically arises when an object is accelerating vertically, such as in an elevator. 

The exam weight of this unit is 12-18%, and it tends to span over approximately 19-22 45 minute class periods.

Big Idea #1: Systems - Objects and systems have properties such as mass and charge. Systems may have internal structures.

Bid Idea #2: Fields - Fields existing in space can be used to explain interactions.

Big Idea #3: Force Interactions - The interactions of an object with other objects can be described by forces.

Big Idea #4: Change - Interactions between systems can result in changes in those systems.