Newton's Third Law

Summary

In this AP Physics essentials video, Mr. Andersen explores Newton's third law, the law of action and reaction. He explains how forces are reciprocal, using examples like a cannon firing and a person pushing a wall or a bowling ball. The video emphasizes the importance of free-body diagrams to visualize and understand action-reaction pairs and calculate overall acceleration. It illustrates scenarios both on Earth and in space, highlighting how the absence of a reaction force, like the ground's normal force, affects motion. The lesson aims to help viewers grasp Newton's third law and apply it to solve physics problems.

Takeaways

• 🔵 Newton's third law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction.
• 🎯 When a force is applied to an object, such as launching a cannonball, the object exerts an equal and opposite force on the source of the action, causing recoil.
• 🧩 Action-reaction pairs are always equal in magnitude and opposite in direction, and can be identified using free body diagrams.
• 🚫 Free body diagrams should not include internal forces within the body or forces exerted by the body on other objects.
• 👤 Pushing against a wall results in an equal and opposite force from the wall, but since there's friction with the ground, there's no net acceleration.
• 🌌 In space, without a ground to push against, applying a force to an object results in both the object and the person applying the force accelerating away from each other.
• 🏋️‍♂️ The mass of an object affects how it accelerates in response to a force; a person with a greater mass will accelerate less than a lighter object like a bowling ball.
• 🚶‍♂️ When walking, the force applied to the ground is met with an equal and opposite force, allowing for movement due to the interaction with the ground.
• 📊 Free body diagrams are essential for visualizing and calculating the net force and acceleration on an object by considering only external forces.
• 📚 Understanding Newton's third law and constructing free body diagrams are crucial for analyzing the dynamics of physical systems and solving physics problems.

Q & A

What is Newton's third law of motion?

Newton's third law of motion, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Why does a cannon recoil when it fires a cannonball?

A cannon recoils when it fires a cannonball because of Newton's third law. The force exerted by the cannon on the cannonball is met with an equal and opposite force exerted by the cannonball on the cannon, causing it to move backward.

What is the significance of action-reaction pairs in physics?

Action-reaction pairs are significant in physics because they illustrate the mutual nature of forces between objects. These pairs are always equal in magnitude and opposite in direction, and they help in understanding the dynamics of how objects interact.

How can a free-body diagram help in identifying forces and acceleration?

A free-body diagram is a visual tool used in physics to represent all the external forces acting on an object. It helps in identifying where these forces are applied and calculating the overall acceleration of the object by considering the net force.

What should be included in a free-body diagram according to the script?

In a free-body diagram, one should include the body itself and all the external forces acting on it. It's important to note that internal forces, forces exerted by the body, and other bodies should not be included.

Why doesn't a person move when they push against a wall?

A person doesn't move when they push against a wall because the wall exerts an equal and opposite reaction force. Additionally, the person's feet apply a force on the ground, and the ground provides an equal and opposite force, resulting in no net force and no acceleration.

What happens when a person pushes a bowling ball in space, where there is no ground?

In space, where there is no ground, when a person pushes a bowling ball, both the person and the ball will accelerate away from each other due to the action-reaction forces. The person's acceleration will be less than the ball's because of their greater mass.

How does walking or running across the floor relate to Newton's third law?

When walking or running, a person applies a force into the floor, and the floor applies an equal and opposite force back. This interaction allows the person to move forward, demonstrating the action-reaction principle in everyday motion.

Why is it important to consider the mass of objects when analyzing action-reaction pairs?

The mass of objects is important when analyzing action-reaction pairs because it affects the acceleration of the objects. According to Newton's second law, F = ma, where F is the force, m is the mass, and a is the acceleration. A greater mass results in less acceleration for the same force.

What is the role of friction in the scenarios described in the script?

Friction plays a crucial role in scenarios by providing a resistive force that opposes the motion of objects. In the script, friction between the person's feet and the ground prevents them from moving when pushing against a wall, demonstrating the balance of forces.