FULL REVIEW
Full Review: Newton's Laws and Free-Body Diagrams
Build Newton's laws from interactions and free-body diagrams: choose the system, identify external forces, and connect net force to changes in motion.
TIME
45–60 minutes
BEST FOR
A complete topic review
FINISH WITH
A readiness check
After this full 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 45–60 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
Recall Activity
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
Recall Activity
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
Recall Activity
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
Let's rebuild the key ideas one step at a time. Focus on understanding the relationships before worrying about solving problems.
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.
KEY CONCEPT 3
Newton's third law organizes interactions
Every force belongs to an interaction pair: A pushes B and B pushes A with equal magnitude and opposite direction. The pair acts on different systems. Contact forces such as normal force and friction depend on the interaction, not on a memorized value.
EXAMPLE Earth pulls a falling ball downward; the ball pulls Earth upward with the same gravitational force. Their accelerations differ because their masses differ enormously.
SENSEI NOTE The normal force is not automatically equal to weight, and friction is not automatically μN; first determine the actual contact situation.
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
Now it's time to apply what you've reviewed.
Work through each activity in order. The examples become gradually more challenging, and each one prepares you for the final readiness check.
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 3
Independent Problem
Combine interaction forces on one system.
Two students push a 25 kg cart. One pushes 45 N right and the other 20 N left. Ignore other horizontal forces. Find the cart's acceleration.
Reveal Answers
1.0 m/s² right.
Why it works: The net force is 45 - 20 = 25 N right. Then a = 25/25 = 1.0 m/s² right.
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.
QUICK 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.
QUICK 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.
QUICK CHECK 3
Third-law reasoning
Choose the force on the other object.
A truck collides with a small car. During the collision, which vehicle experiences the larger interaction force?
Reveal Answers
Neither; the forces have equal magnitude and opposite direction.
Why it works: Newton's third law fixes the interaction-force magnitudes. The smaller car can have the larger acceleration because a = F/m.
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.
KEY TAKEAWAY 3
Third-law forces act on different objects
Equal-and-opposite interaction forces do not cancel on a single-object 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
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