Unit 2: Force and Translational Dynamics

What this unit is about

In Unit 1 you learned to describe motion: where something is, how fast it goes, how its speed changes. Unit 2 answers the next question: why does it move that way?

The answer is forces. A push, a pull, the weight of an object, the floor pushing back, a rope, friction, a spring. Newton's three laws tell you how forces change motion. Once you can draw the forces on an object and add them up, you can predict its acceleration. Then everything from Unit 1 tells you the rest of the motion.

Exam weight: Unit 2 is about 18% to 23% of the AP Physics 1 multiple choice section (College Board Course and Exam Description, fall 2024). It is one of the two biggest units, and its ideas (free body diagrams, net force, Newton's laws) show up in almost every later unit.

The topics

Work through them in order. Each page has a story, the idea in plain words, worked numbers, an animation you control, examples, AP style practice with hidden answers and the common mistakes.

  1. 2.1 Systems and Center of Mass
    Choose what counts as "the system". Internal forces cannot move its center of mass; only outside forces can.
  2. 2.2 Forces and Free-Body Diagrams
    A force is a push or pull between two objects. Draw every force on one object as an arrow from a dot.
  3. 2.3 Newton's Third Law
    Forces come in pairs: if A pushes B, B pushes A back just as hard, in the opposite direction.
  4. 2.4 Newton's First Law
    With zero net force, velocity does not change: at rest stays at rest, moving keeps moving in a straight line.
  5. 2.5 Newton's Second Law
    Net force = mass × acceleration. Elevators, Atwood machines and fan carts.
  6. 2.6 Gravitational Force
    Every two masses pull on each other. Near Earth the weight of a mass m is mg.
  7. 2.7 Kinetic and Static Friction
    Friction opposes sliding. Static friction holds things still up to a limit; kinetic friction acts while sliding.
  8. 2.8 Spring Forces
    A spring pushes or pulls back in proportion to how far you stretch or squeeze it: F = kx.
  9. 2.9 Circular Motion
    Moving in a circle at steady speed still needs a net force, pointing to the center of the circle.

Play and practice

How to study this unit

1. For every problem, first pick the object (the system). 2. Draw its free body diagram. 3. Pick a positive direction. 4. Add the forces to get the net force. 5. Use Fnet = ma. 6. Use Unit 1 kinematics if you need positions or times. This six step recipe solves most Unit 2 questions.

Want a second explanation? See the free resources page.