B Newton's third law (equal and opposite force)

[Dale]

I've been trying to gain some intuition on internal and external forces, has Walter Lewin or Richard Feynman covered these topics? I wonder if there are some good resources out there, that will give me a working knowledge of these forces, so I can better understand how to identify them, and their implications in physics problems.

First you define your system. Then any force between two different parts of the system is an internal force. Any force between the system and something outside the system is an external force.

 

[Chenkel]

First you define your system. Then any force between two different parts of the system is an internal force. Any force between the system and something outside the system is an external force.

I noticed that Walter Lewin gave an example of a star system, he said that the attraction of two stars inside the system is an example of internal forces, and the internal forces cancel each other out when computing the total force. That makes sense; when we sum up the forces of stars attraction we get a grand total of 0, so the overall momentum of the system hasn't changed. He went so far to say, that if the stars explode the total momentum hasn't changed, and also if they collide, the total momentum hasn't changed, is this because Newton's third law? I can see the total force being 0 for two objects colliding, but what about an explosion? Is an explosion purely pair wise interaction of particles, with equal and opposite force, and when they interact, the forces cancel each other out? If I push on one of the stars from outside the system with my hand, that would make an external force, so the total momentum would change for both the external system and the internal system. I hope I'm seeing things clearly.

The hand is attached to the wrist. The wrist can produce large forces on the hand.

Yes, the force of the wrist on the hand is equal and opposite the force of the hand on the wrist

So there's an external force to the ball+hand system causing the ball, and the hand, to move to the left, I'm wondering how this external force can be maintained continuously as I move my arm from right to left, is it easy to maintain an external force? What is going on physically to create a sustained external force?

 

[Dale]

Is an explosion purely pair wise interaction of particles, with equal and opposite force, and when they interact, the forces cancel each other out?

Yes. That is correct

If I push on one of the stars from outside the system with my hand, that would make an external force, so the total momentum would change for both the external system and the internal system.

Yes.

What is going on physically to create a sustained external force?

There isn’t a single general answer to that question. For your case, the force is a contact force between the wrist and the hand. The continuous (for a brief time) force is achieved by deformation of the body. This deformation allows the short-range contact forces to continue even though the hand is accelerating.

 

[PeroK]

Also, so I understand the example better, is the force of the wrist on the hand a rotational force (torque) or a linear force?

It's all those things. If it wasn't for the rest of your body, your hand would fall to the ground under gravity.

It's nonsensical to treat your hand as an isolated rigid object.

Your whole body is needed to move a phone, the skin on your hand is just the contact point.

 

[PeroK]

Also, you are confusing yourself by using something so light that you can ignore certain aspects of the mechanics. Instead of a phone have a heavy table. Now it should be more obvious that you need your feet planted firmly on the ground to move it.

You can injure your back lifting a heavy object. Why is that? Your back is not touching the table when you lift it.

 

[Dale]

It's nonsensical to treat your hand as an isolated rigid object.

@Chenkel just in case you are unfamiliar with the terminology:

An isolated system is one that does not exchange either energy or matter with its surroundings

A closed system is one that does not exchange matter with its surroundings but can exchange energy

An open system is one that can exchange both matter and energy with it surroundings

The hand is an open system because it exchanges energy (the forces at the wrist and palm do work) and mass (blood flows in and out of the hand). For the purposes of the OP the blood flow is unimportant so although it is an open system you could get away with treating it as closed here. But as @PeroK says, it cannot be treated as isolated.

 

[sophiecentaur]

Yes, exactly. If the boat is very small, like a canoe, this can be significant and even dangerous.

@Chenkel Try to step ashore. You (high mass) move a short way forward and the (low mass) canoe moves a long way backward (momentum is conserved) you step into the gap between canoe and shore. One of the 'equal' third order pair acts on the boat and the other acts on you. But, more relevant, is Newton's second law. That says that the momentum imparted to you is the same as the momentum imparted to the boat and the (lighter) boat will move faster than you.

One way to deal with finding an apparent 'paradox' in basic Physics is to assume you were actually wrong. Then take the 'rules' you have been taught and apply them scrupulously, avoiding intuition. It's an internal process and it's seldom helped by pages and pages of 'helpful' comments and examples. That stuff can very often add further confusion. Proper understanding has to come from inside and acceptance that you need to change.
You are (almost certainly) not an Einstein so you have to accept (at least for now) those basic rules.

 

[James Brown]

I've been trying to gain some intuition on internal and external forces, has Walter Lewin or Richard Feynman covered these topics? I wonder if there are some good resources out there, that will give me a working knowledge of these forces, so I can better understand how to identify them, and their implications in physics problems.

Well I'd love to see Richard Feynman talking about this

 

[Herman Trivilino]

When my hand is accelerating my phone around why does my hand not bounce off the phone and move in the opposite direction? (The hand is pushing on phone, so the phone is pushing on hand)

When a force is exerted on an object, the object doesn't necessarily move in the same direction as the force. Look, for example, at projectile motion. The force of gravity is downward, but the direction of motion of the projectile is not necessarily downward.

If the two objects are attached to each other, and one acts on the other, don't the forces cancel each other out?

No. The forces are exerted on different objects. To "cancel" means to add up to zero. But you wouldn't add up the forces exerted on an object unless those forces are exerted on that object.

I'm a little confused about external vs internal forces. From what I read, first you define your system, and then you define what forces are internal or external to the system.

First of all, you define your system. Essentially what you are doing is creating an imaginary closed boundary. Everything that's inside the boundary is part of the system, everything outside the boundary is part of the environment.

The types of systems of interest here are systems that contain more than one object. If those objects exert forces on each other, those are internal forces. If the environment exerts a force on the system, then that is considered an external force. Only external forces can accelerate a system.

 

[Orodruin]

A closed system is one that does not exchange matter with its surroundings but can exchange energy

And momentum. A force primarily represents an exchange of momentum. It may also exchange energy if doing work (ie, moving contact point) but the primary transfer is that of momentum - as I recently discussed.

 

[Dale]

And momentum. A force primarily represents an exchange of momentum. It may also exchange energy if doing work (ie, moving contact point) but the primary transfer is that of momentum - as I recently diacussed.

Yes, that is correct, sorry about being unclear. Angular momentum isn't usually included in the list, but it probably should be.