Newton's Third Law | Common Misunderstandings | Doc Physics

Summary

In this video, Newton's third law is explored through a practical demonstration using spring scales and a loop, illustrating how forces are always equal and opposite. The presenter explains the concept of equilibrium, showing how the forces on two scales remain the same when a force is applied. The importance of distinguishing between Newton's second and third laws is emphasized, dispelling common misconceptions. The video also touches on the tactile experience of forces, such as when touching an object, highlighting the ubiquitous application of Newton's third law in everyday interactions.

Takeaways

• 😀 Newton's third law states that for every action, there is an equal and opposite reaction.
• 😀 A spring scale experiment can be used to demonstrate Newton's third law by exerting forces on a loop of string, where both scales read equal forces in opposite directions.
• 😀 The forces exerted by the two spring scales in the loop experiment are always equal when the system is in equilibrium.
• 😀 Newton’s third law applies at every instant—if one scale pulls on the loop, the loop pulls on the scale with an equal force.
• 😀 Newton's third law should not be confused with Newton's second law, which relates to the net force and acceleration of an object.
• 😀 The normal force on an object (like an orange on a stool) is not explained by Newton's third law but by Newton's second law, depending on whether the object is accelerating or in equilibrium.
• 😀 The relationship between the normal force and gravitational force is only true when the object is stationary (no acceleration).
• 😀 An object in equilibrium has no net force acting on it, meaning forces in opposite directions (like the forces on the orange) are balanced.
• 😀 Real-world examples, like touching a table or a piece of paper, also show Newton’s third law in action—when you exert force on an object, the object exerts an equal and opposite force on you.
• 😀 Newton’s third law applies universally, whether in simple interactions (like pressing on a paper) or complex systems (like the spring scale setup).

Q & A

What is Newton's third law of motion?

Newton's third law states that for every action, there is an equal and opposite reaction. This means that if an object exerts a force on another object, the second object exerts a force of equal magnitude but in the opposite direction on the first object.

How can we test Newton's third law using spring scales?

To test Newton's third law, you can use two spring scales connected by a loop. If a force of 3 Newtons is exerted on one scale, the other scale will also register 3 Newtons but in the opposite direction. This demonstrates that the forces are equal and opposite, which aligns with the third law.

What does it mean for the loop in the experiment to be in equilibrium?

For the loop to be in equilibrium, the net force acting on it must be zero, meaning the forces acting on it balance each other out. This is why when a 3 Newton force is applied to the loop, the other scale reads 3 Newtons in the opposite direction.

Why does the spring scale always show the same reading on both sides in the equilibrium case?

The spring scales show the same reading because the forces acting on the loop are equal in magnitude but opposite in direction. This happens because the spring constant in both scales is the same, ensuring that both scales are calibrated to read the same force.

Why should Newton's third law not be confused with Newton's second law?

Newton's third law involves the interaction between two objects, where each exerts an equal and opposite force on the other. In contrast, Newton's second law deals with the relationship between the force on an object and its acceleration (F = ma), which can lead to a net force that causes motion, unlike the third law.

What misunderstanding about Newton's laws is highlighted in the script?

A common misunderstanding is that the forces acting on an object, such as an orange on a stool, are equal due to Newton's third law. This is incorrect because those forces are a result of Newton's second law, not the third. The normal force and gravity are equal only when there is no acceleration (i.e., when the object is at rest).

How does Newton's third law apply when an object is not in equilibrium?

Even when an object is not in equilibrium and is accelerating, Newton's third law still applies. For example, if you exert a force on an object, it will exert an equal and opposite force back on you, but the object will accelerate due to a net force acting on it (according to Newton's second law).

Can you give an example where Newton's third law applies to everyday objects?

An example of Newton's third law in everyday life is when you press your finger against a desk. Even though it may seem like the desk isn't touching you, the desk is exerting an equal and opposite force on your finger, which you can feel as pressure. This is an application of the third law.

What happens when the upward force on an orange on a stool is greater than its weight?

If the upward force on the orange exceeds its weight (mg), the orange will accelerate upwards. This happens when the normal force applied by the stool is greater than the gravitational force acting on the orange.

Why is it important to understand Newton's third law?

Understanding Newton's third law is essential because it helps explain interactions between all objects. Misunderstanding it can lead to confusion, especially in real-world applications where forces between objects are always equal and opposite, such as in mechanical systems, vehicle collisions, and more.