3of6 newtons 3rd law

Summary

The script delves into Newton's Third Law, emphasizing the interaction between two bodies exerting equal and opposite forces without needing physical contact, exemplified by magnets. It clarifies misconceptions using a farmer and donkey story, highlighting the importance of applying the law correctly by considering forces on different bodies. Further examples include a block on a table and a hose emitting water, illustrating the law's application in real-world scenarios and the distinction between Newton's Third Law and free body diagrams.

Takeaways

• 🔄 Newton's Third Law states that for every action, there is an equal and opposite reaction, but it's more accurately defined as 'when two bodies interact, they exert equal and opposite forces on one another'.
• 🧲 Interaction between two bodies doesn't require physical contact, as demonstrated by the force between two magnets that don't touch.
• 🌐 Even if you jump off a building and are in free fall, you are still interacting with the Earth due to gravity pulling you down, illustrating Newton's Third Law.
• 🐴 The example of a farmer and a donkey humorously highlights the misunderstanding of Newton's Third Law, where the donkey argues that it can't pull a cart because the forces would cancel out, which is a misapplication of the law.
• 📚 Newton's Third Law is often misunderstood as being about forces acting on the same body, but it actually refers to forces acting on different bodies.
• 📐 Free body diagrams are useful for visualizing all the forces acting on a single body, but they do not illustrate Newton's Third Law, which is about interactions between two different bodies.
• 💧 The reaction force of water shooting out of a hose demonstrates Newton's Third Law, where the hose pushes the water forward and the water pushes back on the hose.
• 🔗 When two objects are connected and a force is applied to one, the force that one object exerts on the other is not necessarily the same as the applied force, as it depends on their masses and the net force.
• 📉 Newton's Second Law (F=ma) can be used in conjunction with Newton's Third Law to solve for the forces between two interacting bodies when their masses and accelerations are known.
• 🔄 The forces described by Newton's Third Law are equal in magnitude and opposite in direction, but they do not necessarily cancel each other out unless they act on the same body along the same line.

Q & A

What is the complete definition of Newton's Third Law?

When two bodies interact, they exert equal and opposite forces on one another.

Do two bodies have to touch to interact according to Newton's Third Law?

No, they do not have to touch. An example is two magnets that exert forces on each other without touching.

How does Newton's Third Law apply when you jump off a building?

Even in free fall, you are still interacting with the Earth because the Earth pulls you down, and according to Newton's Third Law, you must pull the Earth up with the same force.

What is the mistake in the donkey's argument about not being able to pull a cart according to Newton's Third Law?

The mistake is that the donkey considers itself and the cart as one body, but Newton's Third Law requires considering the forces on separate bodies.

Why is the force that the cart exerts on the donkey not necessarily zero even if the cart doesn't move?

The cart may not move because the force exerted by the donkey is balanced by the frictional force at the wheels, not because the forces cancel each other out.

What is the difference between Newton's Third Law and a free body diagram?

Newton's Third Law involves forces acting on opposite bodies, while a free body diagram represents all forces acting on a single body.

Why is the normal reaction force not an example of Newton's Third Law when a block is resting on a table?

The normal reaction force and the weight of the block act on the same body (the block), so they do not represent the action-reaction pair of Newton's Third Law.

How does Newton's Third Law explain the hose jerking backward when water is released from a faucet?

The hose pushes the water forward, and according to Newton's Third Law, the water pushes back on the hose with an equal and opposite force, causing the hose to jerk backward.

If you push object A with 12 newtons, what is the force that object B exerts back on A?

Object B will push back on A with a force equal in magnitude but opposite in direction, but it will not be exactly 12 newtons because the force that A exerts on B is part of the net force moving both objects.

How can you determine the force that object B exerts on object A when they are connected and a force is applied to A?

You can combine the masses of A and B and use Newton's second law to find the acceleration, then use the force that A exerts on B to find the force that B exerts on A.