Draw the forces on one chosen object
You will be able to: Identify force-producing interactions and draw only forces on the selected object.
Which arrows belong on a free-body diagram?
A book rests on a desk. Earth pulls it downward and the desk pushes it upward. The book’s push on the desk belongs to the desk’s diagram, not the book’s.
A useful starting point: Center of mass of a nonuniform rod →
Words and symbols before equations
- Force
- A vector interaction, measured in newtons; 1 N = 1 kg·m/s².
- Free-body diagram
- A dot for the chosen object with labeled arrows for forces exerted on it.
- Normal force N
- Surface contact force perpendicular to the surface.
- Weight mg
- Gravitational force on mass m near Earth, with g = 10 m/s² here.
What this picture assumes
A 2 kg block stays on a horizontal table; opposing horizontal force is 4 N. Vertical acceleration is zero. Only actual forces appear on the dot diagram; the net force is calculated separately.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- ΣFₓ = 2 N, ΣFᵧ = 0. Acceleration 1 m/s²; normal force stays 20 N.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Name an agent for every force: Earth on book, desk on book, rope on block. Motion is not an agent and does not supply an extra forward force. Contact forces arise from microscopic interactions between materials.
Draw the actual force vectors from the object’s dot. Resolve them into components separately when writing equations; drawing weight and both weight components as three forces counts the same interaction twice. Keep distinct same-direction forces side by side so neither disappears.
Choose axes that make the equations clear. For an incline, a downhill axis and a perpendicular axis simplify motion, while weight still points vertically down. A force’s component depends on the chosen axis; its physical direction does not change when the axes rotate.
A worked example, step by step
A 2 kg block is at rest on a frictionless horizontal table while two horizontal strings pull with 6 N each in opposite directions. Describe its diagram and net force.
- Select the block alone.
- Draw weight 20 N downward and normal force 20 N upward.
- Draw two separate 6 N tensions, one left and one right.
- Each component sum is zero; the net force is zero. There is no additional “motion force.”
An acceleration arrow is not a force. Do not add ma as another force to the force diagram.
Does a moving block always need a force in its direction of motion?
Compare with an explanation
No. A force is needed to change velocity, not to maintain constant velocity in an inertial frame.
Predict. Change one thing. Explain.
Keep the 2 kg block on the table. Change the horizontal pull while holding the opposing force fixed. Predict which vertical forces stay unchanged and how the horizontal sum changes.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
ΣFₓ = 2 N, ΣFᵧ = 0. Acceleration 1 m/s²; normal force stays 20 N.
A 2 kg block stays on a horizontal table; opposing horizontal force is 4 N. Vertical acceleration is zero. Only actual forces appear on the dot diagram; the net force is calculated separately.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant force, system boundary, acceleration or calculus relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA 3 kg block on a horizontal frictionless surface is pulled right by 9 N. Draw and label all forces, state the vertical sum and calculate the horizontal acceleration.
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Compare with the answer and four-point rubric
- 1 point: Weight is 30 N downward, normal is 30 N upward.
- 1 point: Applied force is 9 N right; no friction force in the stated model.
- 1 point: Vertical sum is zero.
- 1 point: Horizontal acceleration is 9/3 = 3 m/s² right.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1How do you decide whether an arrow belongs?
Identify an external agent exerting that force on the selected object.
RECALL 2What does “normal” mean?
Perpendicular to the contact surface.
RECALL 3Should components appear as extra forces?
No. Use them separately in component equations.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Draw the forces on one chosen object
- List interactions before drawing arrows.
- Draw actual forces; resolve components separately.
- ΣFₓ and ΣFᵧ are algebraic sums of the chosen components.
Remember: An acceleration arrow is not a force. Do not add ma as another force to the force diagram.
Conditions: A 2 kg block stays on a horizontal table; opposing horizontal force is 4 N. Vertical acceleration is zero. Only actual forces appear on the dot diagram; the net force is calculated separately.
Refresh Kid · AP Physics C: Mechanics Unit 2 (official Unit 2) · Objectives 2.2.A; 2.2.B · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.2, objectives 2.2.A; 2.2.B. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026 alongside the Fall 2026 clarifications. This is Mechanics Unit 2: Force and Translational Dynamics. The unit covers Topics 2.1–2.10. Calculus is introduced where it is needed for continuous mass and velocity-dependent forces. Shell theorem is applied without requiring a proof; spring combinations are purely series or purely parallel. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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