Newton's 3rd Law & Normal Force
So now we have an idea of what forces are and how they affect an object's motion from Newton's 1st and 2nd laws.
But how do two objects interact with each other? Why does an object stop falling when it hits the ground? Why don't you pass through a chair when you sit down? And if you lean against a wall, how does the wall know how much force to exert on you so you don't move?
In this lesson we'll learn about Newton's 3rd law of motion. We'll cover contact force pairs (forces between two objects that are touching) and non-contact force pairs (forces between two objects that are a distance apart).
We'll also learn why two solid objects don't pass through each other: the normal force. Don't let the name fool you, this is no ordinary force. It's called a "normal" force is because it always acts perpendicular to the surface and "normal" means perpendicular in geometry. A normal force is simple but also pretty interesting, because its magnitude can change due to other forces. It also affects the friction force acting on an object.
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Newton's 3rd law of motion
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Contact force examples
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Non-contact force examples
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Why don't the forces cancel out?
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What are equal and opposite forces?
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What is an action and a reaction?
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Why don't we always see the effects of Newton's 3rd law?
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What is a normal force?
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What does "normal" mean?
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A normal force changes based on other forces
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How do we draw a normal force in a FBD?
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Example problem 1: book on a table
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Example problem 2: book on a table with another force
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Example problem 3: two blocks accelerating
Answers
Free-Response Questions
Forces
- 2024 Q1 - - Block sliding on a track with loops, forces, FBDs, circular motion, energy
- 2024 Q3 - - Beam attached to a wall with a string, forces, FBDs, tension, rotational motion, torque
- 2024 Q5 - - Collision of two blocks, momentum, energy, forces
- 2022 Q1 - - Pulley system with 2 blocks and a spring, kinematics, forces, energy, work
- 2019 Q1 - - Plunger pushes block and sphere across surface, kinematics, forces, energy, work, torque, angular momentum
- 2019 Q2 - - Pulley system with 2 blocks, kinematics, forces, FBDs, tension, rotational motion
- 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation
- 2017 Q2 - - (Experimental design) Coefficient of friction between block and board, forces, friction
- 2016 Q1 - - Wheel rolls down an incline, forces, FBDs, energy, friction
- 2015 Q1 - - Pulley system with 2 hanging blocks, forces, FBDs, tension
Free body diagrams
- 2024 Q1 - - Block sliding on a track with loops, forces, FBDs, circular motion, energy
- 2024 Q3 - - Beam attached to a wall with a string, forces, FBDs, tension, rotational motion, torque
- 2023 Q3 - - Block and spring rotating about axle, circular motion, centripetal force, FBDs
- 2022 Q2 - - Gravitational force between planet and moons, law of gravitation, FBD's
- 2019 Q2 - - Pulley system with 2 blocks, kinematics, forces, FBDs, tension, rotational motion
- 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation
- 2016 Q1 - - Wheel rolls down an incline, forces, FBDs, energy, friction
- 2015 Q1 - - Pulley system with 2 hanging blocks, forces, FBDs, tension
- 2015 Q4 - - Projectile motion of two spheres, projectile motion, FBDs
- Newton's 3rd Law of Motion
- Professor Dave - Newton's Third Law of Motion: Action and Reaction
- Newton's Third Law of Motion Demonstrated in Space
- Normal Force
So now we have an idea of what forces are and how they affect an object's motion from Newton's 1st and 2nd laws.
But how do two objects interact with each other? Why does an object stop falling when it hits the ground? Why don't you pass through a chair when you sit down? And if you lean against a wall, how does the wall know how much force to exert on you so you don't move?
In this lesson we'll learn about Newton's 3rd law of motion. We'll cover contact force pairs (forces between two objects that are touching) and non-contact force pairs (forces between two objects that are a distance apart).
We'll also learn why two solid objects don't pass through each other: the normal force. Don't let the name fool you, this is no ordinary force. It's called a "normal" force is because it always acts perpendicular to the surface and "normal" means perpendicular in geometry. A normal force is simple but also pretty interesting, because its magnitude can change due to other forces. It also affects the friction force acting on an object.
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