TLDR

• Dynamics explains why objects move using forces.

• Net force determines acceleration, not motion itself.

• Free body diagrams are required for correct reasoning.

• Newton’s Laws connect force, mass, and acceleration.

• Most AP errors come from force misidentification, not math.

Why This Unit Matters

Unit 2 is where physics becomes causal. In Unit 1, you described motion. In Unit 2, you explain what causes that motion.

On the AP Physics 1 exam, dynamics is everywhere. Forces appear directly in this unit and indirectly in energy, momentum, and rotation. The College Board heavily emphasizes reasoning with forces, especially through free response questions.

If you master forces and Newton’s Laws here, later units feel far more intuitive.

1. Forces and Interactions

A force is a push or pull that results from an interaction between objects.

Key ideas:

• Forces are vectors

• Forces come from identifiable sources

• An object can experience multiple forces at once

Common forces in AP Physics 1:

• Weight

• Normal force

• Tension

• Friction

• Applied force

Forces do not describe motion. They describe interactions.

2. Newton’s First Law

Newton’s First Law describes inertia.

An object:

• At rest stays at rest

• In motion stays in motion at constant velocity

Unless a net force acts on it.

Key takeaway:

• Zero net force means no acceleration

• Objects can move with constant velocity even when forces are present, as long as they cancel

This law explains equilibrium situations tested on the exam.

3. Newton’s Second Law

Newton’s Second Law connects force and acceleration.

Core relationship:
ΣF = ma

Important implications:

• Acceleration points in the direction of the net force

• Larger mass results in smaller acceleration for the same force

• Net force, not individual forces, determines motion

This is the most tested equation in AP Physics 1.

4. Newton’s Third Law

Newton’s Third Law explains force pairs.

For every force, there is:

• An equal force

• In the opposite direction

• Acting on a different object

These force pairs:

• Never cancel each other

• Must act on different objects

This concept is frequently misunderstood and tested conceptually.

5. Free Body Diagrams

Free body diagrams are the backbone of dynamics problems.

Rules:

• Draw only forces acting on the object

• Represent forces as arrows starting from the object

• Label each force clearly

• Do not include forces the object exerts on others

AP graders expect a correct free body diagram before calculations.

Tutor Tip:
If you are stuck, pause and redraw the free body diagram. Many dynamics problems become obvious once forces are correctly identified.

6. Weight and Normal Force

Weight is the gravitational force on an object.

W = mg

Important clarifications:

• Weight always acts downward

• Normal force is perpendicular to the surface

• Normal force does not always equal weight

On inclines or accelerating systems, normal force changes and must be calculated using force components.

7. Friction

Friction opposes relative motion between surfaces.

Two types:

• Static friction resists motion before sliding

• Kinetic friction acts during sliding

Key relationships:

• fₛ ≤ μₛN

• fₖ = μₖN

Direction of friction depends on relative motion, not applied force direction.

8. Systems and Multiple Objects

Many AP problems involve connected objects.

Strategies:

• Treat connected objects as a system when possible

• Apply Newton’s Second Law to each object when needed

• Be careful with internal forces, especially tension

Action-reaction forces appear between objects but are never placed on the same free body diagram.

Common Pitfalls

• Forgetting to draw a free body diagram

• Including action-reaction forces on the same object

• Assuming normal force always equals weight

• Treating friction as always μN

• Ignoring force directions

• Confusing mass with weight

• Using motion direction instead of net force direction

Ready to Master Forces?

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