Newton’s 3rd Law is easy to say, but tough to apply. The problem is usually that many people know the catchy phrase, but don’t actually know the meaning of the words in the phrase.
Newton’s 3rd Law is easy to say, but tough to apply. The problem is usually that many people know the catchy phrase, but don’t actually know the meaning of the words in the phrase. (1:23) We explore the gravitational force between you and the Earth. (2:11) But, how does the 3rd law apply to running on a track (2:44) or a wagon being pulled by a tractor? (4:18) We look at all of these scenarios and still had time to discuss what makes a rocket capable of getting into orbit and beyond. (5:26)
The Question of the Day asks: (6:59)
How does the force that the Earth pulls on the International Space Station compare to the amount of force exerted on the Earth by the space station?
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Hi and welcome to the APsolute RecAP: Physics 1 Edition. Today’s episode will focus on Newton’s 3rd Law Pairs.
Let’s Zoom out:
Unit 2 – Dynamics
Topic - 2.5
Big Idea – Force Interactions
So… there you are sitting at your desk in school, or your desk in your bedroom, or at the kitchen table, or anywhere really… that isn’t the point! The point is that you are sitting stationary compared to the surface of the Earth. Certainly NOT accelerating. And you think to yourself, “Sure Earth is pulling me down, but what is the equal and opposite force acting on me that is referred to in Newton’s 3rd Law?” You, like many new Physics students, might think that it is the Normal Force of the chair pushing you up. If that sounds like something you would say, then you have fallen prey to one of the most common misconceptions in Physics 1.
Let’s Zoom in:
I like to say that Newton’s 3rd Law is the easiest to memorize, and the hardest to feel in your bones… likes deep down in the marrow. This is because it feels so counterintuitive. “Of course the Earth pulls on me with a greater amount of force, I mean look how big the Earth is compared to me!” Sure, it feels right, but is it? The short answer is “no.” The root of the problem with our 3rd Law understanding is that we remember it incorrectly. Instead of, “For every action force there is an equal and opposite reaction force.” We should remember, “If A pushes on B, then B pushes on A”. Where A and B are separate objects.
Let’s test our new method out. If the Earth pulls downward on you, then you must pull upward on the Earth. In this example, the Earth can be object B, and you are object A. The problem with the normal force of a chair pushing on you to keep you up is that both gravity and the normal force push on the same object, YOU, so it can’t be considered a 3rd Law pair. There is no object B. The Earth pulls on you, and the chair pushes on you. Not a 3rd Law Pair.
Let’s imagine we are sitting in the stands at a school track meet. The runners take their mark and the starting gun sounds. Bang! They are off! But why? The runner’s push backward off of the ground, why would they accelerate forwards if they are pushing backward? Because the friction with the track surface pushed them. Object A, the runner pushes back with friction on the running surface, and then object B, the surface pushes back on the runner allowing the runner to accelerate.
How about a tractor pulling a wagon? Surely the tractor must pull harder on the wagon right? RIGHT!??!! Nope again! The tractor pulls on the wagon, and the wagon pulls equally on the tractor but in the opposite direction. Then how does either object move? you might wonder. Well again, friction is the culprit. If you sketch the entire free body diagram of the tractor when it first starts to move you will have gravity down, normal force up (not a 3rd law pair), the wagon pulling back and then static friction from the ground forward. In order for the tractor to accelerate, it must be pushed forward by the friction force harder than the wagon is pulling backward. The wagon would then have a force of gravity down, a normal force up, a very small friction force due to the free axle of a wagon, and a force forward from the tractor that is equal to the force on the tractor. It is the difference in the static friction forces that allow the tractor to accelerate the entire system forward. The 3rd Law still holds up.
Have you ever wondered what makes a rocket able to accelerate? Well… BOOM! Now you have. Many students believe that it is the rocket pushing off of the ground and the ground pushing back on the rocket. While that does seem like a wonderful 3rd Law pair and frankly does happen, I challenge you to explain why rockets work once they are airborne, or better yet… in space? There isn’t anything for them to “push off” of, yet there they go, working as designed. Fortunately, Newton’s 3rd Law MUST work. It is a law after all. If you can think about the little molecules of gases that are created during the combustion of the propellants, then you will realize that a whole lot of those little molecules are accelerated out of the engine nozzles. In fact, you could say that the rocket... pushes on the exhaust molecules, and the molecules of exhaust in turn push on the rocket with equal and opposite force. 3rd Law for the win again.
To Recap…
If object A pushes on object B, then object B must apply an equal force on object A and in the opposite direction. Starting to feel it in your bones yet?
Coming up next on the APsolute RecAP Physics 1 Edition, we take a closer look at the dynamics of systems with more than one mass.
Today’s Question of the Day focuses on Newton’s 3rd Law.
Question:
How does the force that the Earth pulls on the International Space Station compare to the amount of force exerted on the Earth by the space station?