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.

Multiple-Choice Questions

Answers

- 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

• Intro to Normal Forces Part 1
• Intro to Normal Forces Part 2

• Khan Academy - Normal force (shoe on floor)
• Khan Academy - Normal force (shoe on wall)
• Organic Chemistry Tutor - Elevator Physics - Normal Force on a Scale & Apparent Weight

• Normal force

Other Resources

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.