AP Physics 1 Unit 2 Cheat Sheet: Dynamics
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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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