FOCUSED REVIEW
Focused Review: Newton's Laws and Free-Body Diagrams
Reinforce the highest-leverage force-modeling ideas and representative applications.
TIME
Approximately 15 minutes
BEST FOR
Targeted reinforcement
FINISH WITH
A readiness check
After this focused review, you'll be able to...
explain how forces arise from interactions, draw disciplined free-body diagrams, and use net force to predict how motion changes.
Choose how you want to review
Unit Alignment
This bundle is aligned to the approved Physics Sensei unit specification below. Use it to recover the unit structure, reinforce key decisions, and confirm readiness for the next study task.
ARCHITECTURE: Physics Sensei Independent Mechanics
UNIT: MEC-U05 — Newton's Laws and Free-Body Diagrams
SCOPE: Unit Review
PHYSICS LEVEL: Foundational
BEST USED
✓ When force diagrams feel confusing
✓ Before Newton's-law homework
✓ When you want the physical model before the algebra
Your Review Plan
Complete these six stages in order. Each stage builds on the previous one and prepares you for the final readiness check.
6 Stages • Approximately 15 minutes.
Warm-Up Check
Before you begin, take a moment to see what you already remember. Do not worry about getting everything right. This is only a starting point.
ACTIVITY 1
Key Ideas
Isolate one object before naming forces.
A book rests on a horizontal table. Name the external forces acting on the book and state whether the net force is zero.
Reveal Answers
Weight downward and the table's normal force upward; the net force is zero.
Why it works: The book has zero acceleration, so its external forces balance. The normal force is a contact interaction from the table; weight is the gravitational interaction with Earth.
ACTIVITY 2
Common Mistakes
Connect unbalanced force to acceleration.
A cart is moving to the right. Its forces suddenly become unbalanced with a net force to the left. What direction is its acceleration? Must it immediately move left?
Reveal Answers
Its acceleration is left. No; it can continue moving right while slowing down.
Why it works: Newton's second law links net force to acceleration, not directly to velocity. A leftward acceleration can reduce an existing rightward velocity before reversing it.
ACTIVITY 3
Quick Application
Keep action-reaction forces on different objects.
Your hand pushes on a wall. What force completes the Newton's-third-law pair?
Reveal Answers
The wall pushes on your hand with an equal-magnitude force in the opposite direction.
Why it works: The two forces belong to one interaction but act on different objects, so they never cancel on a single free-body diagram.
Ready to strengthen your understanding?
You've refreshed what you already know. Next, you'll reinforce the essential concepts that will help you solve problems with confidence. Need to see the learning path again?
Core Concepts
Reinforce the two highest-leverage relationships, then use them in representative situations.
KEY CONCEPT 1
A free-body diagram begins with the system, not with equations
Choose the object or system first. Draw only forces exerted on that system by objects outside it. Label force type and direction; do not include motion arrows as forces.
EXAMPLE A book on a table has weight downward and a normal force upward. If a hand pushes the book sideways, that applied force is added to the book's diagram.
SENSEI NOTE Ask 'who exerts this force on my system?' for every arrow. If you cannot name the interaction partner, reconsider the force.
KEY CONCEPT 2
Net force determines acceleration
Newton's second law says the vector sum of external forces determines acceleration. Balanced forces mean zero acceleration, which can mean rest or constant-velocity motion.
EXAMPLE If a 20 kg sled has 70 N right and 30 N left, the 40 N net force points right, so the acceleration points right.
SENSEI NOTE Do not confuse zero net force with zero velocity. A puck can move steadily while its forces balance.
FOCUSED STRATEGY
Choose the system first
A correct free-body diagram shows external forces on one defined system, each tied to a real interaction.
SENSEI NOTE Use this strategy as a setup check before writing equations.
Ready to apply these ideas?
You've reinforced the essential concepts. Now it's time to put them into practice by working through guided examples and building your problem-solving confidence. Need a quick reminder?
Guided Practice
Apply the reinforced ideas to two representative situations, then use the strategy card to check your setup.
PRACTICE 1
Worked Example
Use the force picture before calculating.
A 20 kg sled is pulled horizontally with 70 N while friction is 30 N opposite the motion. Find the net force and acceleration.
Reveal Answers
Net force = 40 N right; acceleration = 2.0 m/s² right.
Why it works: The horizontal forces are 70 N right and 30 N left, so ΣF = 40 N right. Newton's second law gives a = ΣF/m = 40/20 = 2.0 m/s².
PRACTICE 2
Guided Problem
Recognize balanced forces during motion.
A 60 kg rider in an elevator moves upward at constant speed. What is the rider's acceleration, and what normal force does the floor exert? Use g = 9.8 m/s².
Reveal Answers
Acceleration = 0; normal force = 588 N upward.
Why it works: Constant velocity means zero acceleration, so the net vertical force is zero. Therefore N = mg = (60)(9.8) = 588 N.
PRACTICE STRATEGY
Setup Check
Audit the model before calculating.
Before solving a Newton-law problem, what three setup decisions should be explicit?
Reveal Answers
Choose the system, draw all external forces, and declare axes/signs.
Why it works: Those decisions determine which forces belong in the model and how every vector component enters Newton’s second law.
Ready to check your understanding?
You've practiced the essential skills with guidance. Now it's time to solve a few short problems on your own and confirm you're ready to move forward. Need a quick reminder?
Confidence Check
You've rebuilt the key ideas and practiced them with guidance. Now try these short questions on your own to check your understanding before moving on.
CONFIDENCE CHECK 1
Balanced does not mean stopped
Interpret zero net force.
A hockey puck slides at constant velocity on nearly frictionless ice. What is the net force on it?
Reveal Answers
Approximately zero.
Why it works: Constant velocity means zero acceleration, so Newton's second law requires approximately zero net force.
CONFIDENCE CHECK 2
Normal force
Use the actual contact situation.
A 5.0 kg book rests on a table while a hand pushes downward on it with 15 N. What normal force does the table exert? Use g = 9.8 m/s².
Reveal Answers
64 N upward.
Why it works: Vertical acceleration is zero. The table must balance both weight (49 N) and the extra 15 N downward push, so N = 49 + 15 = 64 N.
CONFIDENCE CHECK 3
Force audit
Check each arrow against a real interaction.
If a force arrow cannot be tied to an interaction partner, what should you do?
Reveal Answers
Reconsider or remove that force from the free-body diagram.
Why it works: Every real force represents an interaction between the chosen system and something else.
How did it go?
You've checked your understanding. Take one final look at the essential ideas before deciding what to do next. Need a quick reminder?
Summary
Before moving on, take one final look at the most important ideas from this review.
KEY TAKEAWAY 1
Choose the system first
A correct free-body diagram shows external forces on one defined system, each tied to a real interaction.
KEY TAKEAWAY 2
Net force controls acceleration
Balanced forces give zero acceleration; an unbalanced net force points in the acceleration direction.
FOCUSED TAKEAWAY
Check the physical model
A correct equation set must match the forces, constraints, and assumed motion shown in the free-body diagram.
Ready for your next step?
You've reviewed the essential ideas one last time. Now choose the resource that best matches how confident you feel. Need a quick reminder?
Next Step
Great work!
You've completed this review. Choose the next resource that best matches how confident you feel.
I'm Still Unsure
Review the key ideas and examples again.
Review Again →
I Need More Practice
Continue with additional practice for this topic.
Go to Practice →
I'm Ready
Continue to the next recommended resource.
Continue →