#AP Physics 1: Newton's Third Law - Your Ultimate Guide 🚀

Hey there, future physics pro! Let's break down Newton's Third Law and get you feeling super confident for the exam. This guide is designed to be your go-to resource, especially the night before the test. Let's make sure everything clicks!

Newton's Third Law is a cornerstone of mechanics, and understanding it is crucial for success in AP Physics 1. It's not just a formula; it's a concept that explains how forces work in pairs. You'll see it everywhere, from simple object interactions to complex systems.

#1. Newton's Third Law: Action-Reaction Pairs 🤝

#1.1. The Core Principle

Newton's Third Law: For every action, there is an equal and opposite reaction. This means forces always come in pairs.
Think of it like a high-five: your hand exerts a force on your friend's, and their hand exerts an equal force back on yours. ✋
Key Point: Forces always act on different objects. This is why they don't cancel each other out!

#1.2. Mathematical Representation

The formula: $\vec{F}_{A \text{ on } B} = -\vec{F}_{B \text{ on } A}$
This means the force of A on B is equal in magnitude but opposite in direction to the force of B on A.

#1.3. Action-Reaction Examples

Book on a Table: The book pushes down on the table (action), and the table pushes up on the book (reaction).
Jumping off a Boat: You push the boat backward (action), and the boat pushes you forward (reaction).

Key Concept

Remember, action-reaction pairs are simultaneous. They happen at the exact same time and always involve two different objects. This is a common source of confusion, so make sure you've got it down!

Memory Aid

Think of it as a "push-back" scenario. If you push something, it pushes back on you with the same force. This helps visualize the equal and opposite nature of the forces.

#2. Internal Forces and Center of Mass ⚖️

#2.1. Internal vs. External Forces

Internal Forces: Forces between objects within a system. These always come in action-reaction pairs.
External Forces: Forces exerted on the system by objects outside the system. These can change the system's motion.

#2.2. Center of Mass (COM)

The COM is the point where all the system's mass can be considered concentrated. 📍
Key Insight: Internal forces do not change the motion of the system's COM. The net force from internal pairs is always zero.
Only external forces can alter the motion of the COM.

#2.3. Example: Collision

During a collision within a system, internal forces (action-reaction pairs) don't affect the COM's overall motion. The COM will continue moving as before unless an external force acts on the system.

Quick Fact

Internal forces are like a tug-of-war within the system. They don't move the whole system, just redistribute forces internally.

#3. Tension in Strings and Cables 🧵

#3.1. What is Tension?

Tension is the force transmitted along the length of a string, cable, or similar object. It's a result of the internal forces within the object.
Think of it as the pulling force that keeps the string taut when you pull on it.

#3.2. Ideal Strings

Properties: Negligible mass, do not stretch.
Key Result: Tension is uniform throughout the length of an ideal string. 📏
Ideal strings are a simplification, but they're super useful for solving problems!

#3.3. Non-Ideal Strings

Properties: Have mass, can stretch.
Key Result: Tension is not uniform. Tension is generally greater at the top of a hanging non-ideal string.

Common Mistake

Many students assume tension is always uniform, but it's only true for ideal strings. Always check if the problem specifies an ideal string!

#3.4. Ideal Pulleys

Properties: Negligible mass, no friction.
Key Result: Tension in the string is the same on both sides of an ideal pulley. 🔄
Pulleys change the direction of force but don't change the magnitude of tension (in ideal cases).

Memory Aid

Visualize tension as the tightness of a rope. In an ideal rope, the tightness is the same everywhere. In a real rope, the tightness might be greater at the top because it's supporting the weight of the rope itself.

Exam Tip

AP Physics 1 focuses on describing tension qualitatively in systems with mass. You won't need to calculate the exact variation, but you should understand that tension might vary along the length of a non-ideal string or cable.

#4. Boundary Statements

AP Physics 1 Focus: Interactions at a distance are limited to gravitational forces. 🍎
AP Physics 2: Includes gravitational, electric, and magnetic forces when considering interactions at a distance.

Practice Question

#Multiple Choice Questions

A 10 kg block rests on a table. Which of the following statements is true regarding action-reaction pairs? (A) The weight of the block and the normal force on the block are an action-reaction pair. (B) The weight of the block and the force of the block on the Earth are an action-reaction pair. (C) The normal force on the block and the force of the block on the table are an action-reaction pair. (D) The weight of the block and the force of the table on the floor are an action-reaction pair.
A person pulls a rope attached to a sled with a force of 100 N. According to Newton's third law, what is the reaction force? (A) The force of the sled on the rope, equal to 100 N. (B) The force of the rope on the person, equal to 100 N. (C) The force of the sled on the ground, equal to 100 N. (D) The force of the ground on the sled, equal to 100 N.
Two blocks are connected by a string over an ideal pulley. If the tension in the string is T, which statement is true? (A) The tension is different on each side of the pulley. (B) The tension is the same on each side of the pulley. (C) The tension is zero if the blocks are at rest. (D) The tension depends on the mass of the pulley.

#Free Response Question

Two blocks, A (2 kg) and B (3 kg), are connected by a light string that passes over an ideal, frictionless pulley. Block A is on a horizontal, frictionless surface, and block B hangs vertically. The system is released from rest.

(a) Draw free-body diagrams for both block A and block B. (2 points) (b) Write Newton's second law equations for both block A and block B. (2 points) (c) Calculate the acceleration of the system. (2 points) (d) Calculate the tension in the string. (2 points) (e) If the string was not ideal and had mass, how would the tension change along the string's length? Explain. (2 points)

Answer Key:

Multiple Choice:

(B) The weight of the block and the force of the block on the Earth are an action-reaction pair.
(A) The force of the sled on the rope, equal to 100 N.
(B) The tension is the same on each side of the pulley.

Free Response Question:

(a) Free Body Diagrams (2 points)

Block A: Tension (T) to the right, Normal force (N) upward, Weight (W_A) downward. (1 point)
Block B: Tension (T) upward, Weight (W_B) downward. (1 point)

(b) Newton's Second Law Equations (2 points)

Block A: T = m_A * a (1 point)
Block B: m_B * g - T = m_B * a (1 point)

(c) Acceleration of the System (2 points)

Combine equations: m_B * g = (m_A + m_B) * a
a = (m_B * g) / (m_A + m_B) = (3 kg * 9.8 m/s²) / (2 kg + 3 kg) = 5.88 m/s² (2 points)

(d) Tension in the String (2 points)

Using Block A: T = m_A * a = 2 kg * 5.88 m/s² = 11.76 N (2 points)

(e) Non-Ideal String Explanation (2 points) - The tension would not be uniform. (1 point) - The tension would be greater at the top of the string near the pulley and less at the bottom near block B, because the string's mass contributes to the force needed to lift the system. (1 point)

#Final Exam Focus 🎯

Okay, you've made it through! Here's what to really focus on for the exam:

Action-Reaction Pairs: Know that they act on different objects and are simultaneous. 💡
Internal vs. External Forces: Understand how they affect the center of mass.
Tension: Be clear on ideal vs. non-ideal strings and how tension varies.
Free Body Diagrams: Essential for solving force problems. Practice, practice, practice!

#Last-Minute Tips

Time Management: Don't get bogged down on one question. Move on and come back if you have time.
Read Carefully: Pay close attention to the details of each problem.
Show Your Work: Always show your reasoning and steps, even if you don't get the final answer. You can still get partial credit!
Stay Calm: You've got this! Take a deep breath and trust your preparation. 💪

Good luck, you're going to do great! 🎉

#AP Physics 1: Newton's Third Law - Your Ultimate Guide 🚀

#1. Newton's Third Law: Action-Reaction Pairs 🤝

#1.1. The Core Principle

Newton's Third Law: For every action, there is an equal and opposite reaction. This means forces always come in pairs.
Think of it like a high-five: your hand exerts a force on your friend's, and their hand exerts an equal force back on yours. ✋
Key Point: Forces always act on different objects. This is why they don't cancel each other out!

#1.2. Mathematical Representation

The formula: $\vec{F}_{A \text{ on } B} = -\vec{F}_{B \text{ on } A}$
This means the force of A on B is equal in magnitude but opposite in direction to the force of B on A.

#1.3. Action-Reaction Examples

Book on a Table: The book pushes down on the table (action), and the table pushes up on the book (reaction).
Jumping off a Boat: You push the boat backward (action), and the boat pushes you forward (reaction).

Key Concept

Remember, action-reaction pairs are simultaneous. They happen at the exact same time and always involve two different objects. This is a common source of confusion, so make sure you've got it down!

Memory Aid

Think of it as a "push-back" scenario. If you push something, it pushes back on you with the same force. This helps visualize the equal and opposite nature of the forces.

#2. Internal Forces and Center of Mass ⚖️

#2.1. Internal vs. External Forces

Internal Forces: Forces between objects within a system. These always come in action-reaction pairs.
External Forces: Forces exerted on the system by objects outside the system. These can change the system's motion.

#2.2. Center of Mass (COM)

The COM is the point where all the system's mass can be considered concentrated. 📍
Key Insight: Internal forces do not change the motion of the system's COM. The net force from internal pairs is always zero.
Only external forces can alter the motion of the COM.

#2.3. Example: Collision

During a collision within a system, internal forces (action-reaction pairs) don't affect the COM's overall motion. The COM will continue moving as before unless an external force acts on the system.

Quick Fact

Internal forces are like a tug-of-war within the system. They don't move the whole system, just redistribute forces internally.

#3. Tension in Strings and Cables 🧵

#3.1. What is Tension?

Tension is the force transmitted along the length of a string, cable, or similar object. It's a result of the internal forces within the object.
Think of it as the pulling force that keeps the string taut when you pull on it.

#3.2. Ideal Strings

Properties: Negligible mass, do not stretch.
Key Result: Tension is uniform throughout the length of an ideal string. 📏
Ideal strings are a simplification, but they're super useful for solving problems!

#3.3. Non-Ideal Strings

Properties: Have mass, can stretch.
Key Result: Tension is not uniform. Tension is generally greater at the top of a hanging non-ideal string.

Common Mistake

Many students assume tension is always uniform, but it's only true for ideal strings. Always check if the problem specifies an ideal string!

#3.4. Ideal Pulleys

Properties: Negligible mass, no friction.
Key Result: Tension in the string is the same on both sides of an ideal pulley. 🔄
Pulleys change the direction of force but don't change the magnitude of tension (in ideal cases).

Memory Aid

Exam Tip

#4. Boundary Statements

AP Physics 1 Focus: Interactions at a distance are limited to gravitational forces. 🍎
AP Physics 2: Includes gravitational, electric, and magnetic forces when considering interactions at a distance.

Practice Question

#Multiple Choice Questions

A 10 kg block rests on a table. Which of the following statements is true regarding action-reaction pairs? (A) The weight of the block and the normal force on the block are an action-reaction pair. (B) The weight of the block and the force of the block on the Earth are an action-reaction pair. (C) The normal force on the block and the force of the block on the table are an action-reaction pair. (D) The weight of the block and the force of the table on the floor are an action-reaction pair.
A person pulls a rope attached to a sled with a force of 100 N. According to Newton's third law, what is the reaction force? (A) The force of the sled on the rope, equal to 100 N. (B) The force of the rope on the person, equal to 100 N. (C) The force of the sled on the ground, equal to 100 N. (D) The force of the ground on the sled, equal to 100 N.
Two blocks are connected by a string over an ideal pulley. If the tension in the string is T, which statement is true? (A) The tension is different on each side of the pulley. (B) The tension is the same on each side of the pulley. (C) The tension is zero if the blocks are at rest. (D) The tension depends on the mass of the pulley.

#Free Response Question

Answer Key:

Multiple Choice:

(B) The weight of the block and the force of the block on the Earth are an action-reaction pair.
(A) The force of the sled on the rope, equal to 100 N.
(B) The tension is the same on each side of the pulley.

Free Response Question:

(a) Free Body Diagrams (2 points)

Block A: Tension (T) to the right, Normal force (N) upward, Weight (W_A) downward. (1 point)
Block B: Tension (T) upward, Weight (W_B) downward. (1 point)

(b) Newton's Second Law Equations (2 points)

Block A: T = m_A * a (1 point)
Block B: m_B * g - T = m_B * a (1 point)

(c) Acceleration of the System (2 points)

Combine equations: m_B * g = (m_A + m_B) * a
a = (m_B * g) / (m_A + m_B) = (3 kg * 9.8 m/s²) / (2 kg + 3 kg) = 5.88 m/s² (2 points)

(d) Tension in the String (2 points)

Using Block A: T = m_A * a = 2 kg * 5.88 m/s² = 11.76 N (2 points)

#Final Exam Focus 🎯

Okay, you've made it through! Here's what to really focus on for the exam:

Action-Reaction Pairs: Know that they act on different objects and are simultaneous. 💡
Internal vs. External Forces: Understand how they affect the center of mass.
Tension: Be clear on ideal vs. non-ideal strings and how tension varies.
Free Body Diagrams: Essential for solving force problems. Practice, practice, practice!

#Last-Minute Tips

Time Management: Don't get bogged down on one question. Move on and come back if you have time.
Read Carefully: Pay close attention to the details of each problem.
Show Your Work: Always show your reasoning and steps, even if you don't get the final answer. You can still get partial credit!
Stay Calm: You've got this! Take a deep breath and trust your preparation. 💪

Good luck, you're going to do great! 🎉

Newton's Third Law

Jackson Hernandez

#AP Physics 1: Newton's Third Law - Your Ultimate Guide 🚀

#1. Newton's Third Law: Action-Reaction Pairs 🤝

#1.1. The Core Principle

#1.2. Mathematical Representation

#1.3. Action-Reaction Examples

#2. Internal Forces and Center of Mass ⚖️

#2.1. Internal vs. External Forces

#2.2. Center of Mass (COM)

#2.3. Example: Collision

#3. Tension in Strings and Cables 🧵

#3.1. What is Tension?

#3.2. Ideal Strings

#3.3. Non-Ideal Strings

#3.4. Ideal Pulleys

#4. Boundary Statements

#Multiple Choice Questions

#Free Response Question

#Final Exam Focus 🎯

#Last-Minute Tips

Explore more resources

How are we doing?

Newton's Third Law

Jackson Hernandez

#AP Physics 1: Newton's Third Law - Your Ultimate Guide 🚀

#1. Newton's Third Law: Action-Reaction Pairs 🤝

#1.1. The Core Principle

#1.2. Mathematical Representation

#1.3. Action-Reaction Examples

#2. Internal Forces and Center of Mass ⚖️

#2.1. Internal vs. External Forces

#2.2. Center of Mass (COM)

#2.3. Example: Collision

#3. Tension in Strings and Cables 🧵

#3.1. What is Tension?

#3.2. Ideal Strings

#3.3. Non-Ideal Strings

#3.4. Ideal Pulleys

#4. Boundary Statements

#Multiple Choice Questions

#Free Response Question

#Final Exam Focus 🎯

#Last-Minute Tips

Explore more resources

How are we doing?