Newton's third law (equal and opposite force)

[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

 

[Mister T]

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.