AP Physics 1: Algebra-Based, AP exams, 18–23%

Unit 2: Force and Translational Dynamics

Unit 2 is one of the two most heavily weighted units. It covers systems and center of mass, free-body diagrams, all three of Newton's laws, and the specific forces the course uses: gravitational, friction, spring, and normal. Circular motion and gravitation are treated as applications of the same laws.

Position: 2 of 8 units
Published weighting: 18–23% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 1: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Systems and center of mass: Choosing a system boundary, treating an extended object as a point mass, and locating the center of mass.
Forces and free-body diagrams: Identifying every interaction acting on a chosen system and drawing the diagram to scale.
Newton's three laws: Inertia, the proportionality between net force and acceleration, and force pairs acting on different objects.
Gravitational force: Weight near a surface, the universal law of gravitation, and the field produced by a massive body.
Friction and spring forces: Static versus kinetic friction, the normal force dependence, and the restoring force of an ideal spring.
Circular motion: Centripetal acceleration, the net inward force requirement, and applications including orbits and banked turns.

Study tips

How to practice Force and Dynamics.

Section-specific guidance that follows from how this material is usually tested.

Draw a labeled free-body diagram for every dynamics problem before writing an equation, without exception.
Practice identifying the third-law partner for each force and stating which object it acts on.
Give this unit and Unit 3 the largest share of practice time; together they carry the heaviest published weighting.

Where this fits

Unit 2: Force and Translational Dynamics is 2 of 8 units in the AP Physics 1: Algebra-Based guide, and it carries a published weighting of 18–23% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 1: Algebra-Based guide Official exam information

Sample Force and Dynamics questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
A 4.0 kg block rests on a frictionless horizontal surface and is pushed by a single constant horizontal force of 12 N. What is the magnitude of the block's acceleration?
1. A0.33 m/s^2
2. B3.0 m/s^2
3. C8.0 m/s^2
4. D48 m/s^2
Show answer
Answer: B. 3.0 m/s^2
Newton's second law gives a = F/m = 12 N / 4.0 kg = 3.0 m/s^2. The 48 m/s^2 choice multiplies force by mass instead of dividing, and 0.33 m/s^2 inverts the ratio.
Key insight: Acceleration is the net force divided by mass, so the same force gives a lighter object a larger acceleration.
Question 2Foundational
A 5.0 kg box sits at rest on a level floor where the coefficient of static friction is 0.40. Using g = 10 m/s^2, what is the smallest horizontal force that will start the box sliding?
1. A2.0 N
2. B20 N
3. C50 N
4. D125 N
Show answer
Answer: B. 20 N
On a level floor the normal force equals mg = 50 N, so the maximum static friction is (0.40)(50 N) = 20 N and any larger force starts the motion. The 50 N option is the weight itself, and 125 N divides by the coefficient instead of multiplying.
Key insight: Friction is proportional to the normal force, not to the weight, and the two are equal only on a level surface with no vertical pull.
Question 3Intermediate
A 2.0 kg object hangs from a spring scale inside an elevator that is accelerating upward at 2.0 m/s^2. Using g = 10 m/s^2, what reading does the spring scale show?
1. A4.0 N
2. B16 N
3. C20 N
4. D24 N
Show answer
Answer: D. 24 N
The scale reads the tension, and Newton's second law gives T - mg = ma, so T = m(g + a) = (2.0 kg)(12 m/s^2) = 24 N. The 16 N choice subtracts the acceleration instead of adding it, which would describe an elevator accelerating downward.
Key insight: A scale reads the normal or tension force it supplies, which equals weight only when the acceleration is zero.
Question 4Intermediate
A 1200 kg car rounds a flat, level curve of radius 50 m at a constant speed of 15 m/s. Using g = 10 m/s^2, what is the minimum coefficient of static friction between the tires and the road that keeps the car on the curve?
1. A0.23
2. B0.45
3. C0.90
4. D4.5
Show answer
Answer: B. 0.45
Friction supplies the centripetal force, so mu(mg) = mv^2/r and the mass cancels, leaving mu = v^2/(rg) = 225/(50 x 10) = 0.45. The 4.5 choice forgets to divide by g, which also leaves the answer with units instead of being dimensionless.
Key insight: On a level curve the mass cancels, so the maximum safe speed depends only on the radius, g, and the coefficient of friction.
Question 5Advanced
A 5.0 kg block on a frictionless horizontal table is joined by a light string running over an ideal pulley at the table edge to a 3.0 kg block hanging freely. Using g = 10 m/s^2, what is the tension in the string after the blocks are released?
1. A11.25 N
2. B18.75 N
3. C30 N
4. D50 N
Show answer
Answer: B. 18.75 N
Treating both blocks as one system, a = (3.0 kg)(10 m/s^2)/(8.0 kg) = 3.75 m/s^2, and the only horizontal force on the table block is the tension, so T = (5.0 kg)(3.75 m/s^2) = 18.75 N. The 30 N choice is the hanging block's weight, which would be the tension only if the system did not accelerate.
Key insight: In an accelerating two-block system the tension is always less than the hanging weight, because the hanging block must have a net downward force.

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Unit 2: Force and Translational Dynamics