More on Newton's third law | Forces and Newton's laws of motion | Physics | Khan Academy

Summary

This script delves into Newton's Third Law, clarifying common misconceptions. It emphasizes that the law involves equal and opposite forces acting on different objects, which cannot cancel each other out. The video explains why these forces, despite being equal, result in different accelerations due to mass differences. It also dispels the myth of a delay in the creation of reaction forces, illustrating that they are instantaneous. The script uses examples, such as a box on a table and gravitational forces, to demonstrate how to correctly identify Newton's Third Law partner forces.

Takeaways

• 🔄 Newton's Third Law is often misunderstood; it's more complex than the simple 'action-reaction' phrase suggests.
• 📏 The law states that for every force, there is an equal and opposite force, emphasizing that these are vector quantities with magnitude and direction.
• 📍 The forces described by Newton's Third Law act on different objects, which is crucial for understanding why they don't cancel each other out.
• 🌌 Forces in the universe do not cancel out because Newton's Third Law forces are exerted on different objects, not the same object.
• 👥 The law applies to interactions between two objects, such as object A exerting a force on object B, and vice versa.
• 🌍 Even vastly different-sized objects, like a planet and a star, exert forces of equal magnitude on each other according to Newton's Third Law.
• 🔄 The misconception that smaller objects exert less force on larger ones is incorrect; the forces are equal regardless of size.
• ⏱️ Newton's Third Law forces are instantaneous; there is no delay in the creation of the partner force when one force is applied.
• 📉 Forces being equal does not mean the results are equal; acceleration depends on the net force divided by mass, leading to different outcomes.
• 🧩 It's important to correctly identify third law partner forces by considering both interacting objects and not confusing them with other balanced forces.
• 🚫 Forces that are equal and opposite due to other reasons, like the second law (no acceleration, net force is zero), are not third law partner forces.

Q & A

What is the common misconception about Newton's Third Law of Motion?

The common misconception is that for every action, there's an equal and opposite reaction, which is often interpreted as forces canceling each other out universally.

What is a more accurate way to phrase Newton's Third Law?

A more accurate phrasing is that for every force, there's an equal and opposite force, emphasizing that these forces are equal in magnitude but opposite in direction.

Why doesn't Newton's Third Law imply that all forces in the universe cancel each other out?

It doesn't imply that because the forces are exerted on different objects; hence, they do not cancel out.

How are Newton's Third Law forces related to the objects they act upon?

The forces are exerted on different objects, with one being the action force on one object and the other being the reaction force on the second object.

Why do the forces exerted by two different objects on each other not cancel out?

They don't cancel out because they act on different objects, and Newton's Third Law ensures they are equal in magnitude and opposite in direction but do not negate each other due to acting on separate entities.

Can the magnitude of the forces in Newton's Third Law be different if the objects have different masses?

No, the forces must be of equal magnitude regardless of the masses of the objects involved.

Why does a planet orbit a star instead of the star moving around the planet, even though the forces they exert on each other are equal according to Newton's Third Law?

The reason is that while the forces are equal, the resulting accelerations are not, due to the difference in masses. The larger mass of the star results in a smaller acceleration compared to the planet.

Is there a delay in the creation of the reaction force in Newton's Third Law?

No, the reaction force is created instantaneously as soon as the action force is applied.

How can one identify the partner forces in Newton's Third Law?

One can identify the partner forces by listing both objects involved and reversing the labels to find the force on the second object exerted by the first.

Why are some forces equal and opposite but not considered Newton's Third Law partner forces?

Some forces may be equal and opposite due to other reasons, such as the second law of motion (net force being zero for no acceleration), and not because they are acting on different objects as required by the third law.

Can the force exerted by a table on a box be considered a Newton's Third Law partner force to the gravitational force acting on the box?

No, the force exerted by the table is not a Newton's Third Law partner force to the gravitational force. The partner force to gravity would be the force exerted by the box on the Earth.