Physics – Newton’s Third Law of Motion
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Physics – Newton’s Third Law of Motion

The only question you need to ask yourself while studying the third law is. What happens when two objects interact with each other? What happens when you place an object on the table? The object exerts a force on the table. And the subject exert an upward force on the object. There are two forces that result from this interaction. A force on the table and a force on the object. The magnitudes of the two forces are the same, but the directions are opposite. These forces are called as action and reaction forces. The force exerted by the object on the table was an action force. And the force exerted by the table on the object is the reaction force. What we saw here is nothing but a display of the Newton’s third law of motion. Every action has an equal and opposite reaction. That’s how simple the third law is. Every action has an equal and opposite reaction. It means that when two objects interact there is a force on each object. The magnitude of force on the first object, equals the magnitude of force on the second one. And what about the direction? The direction of force on the first object is opposite to the direction of force on the second one. Let’s talk about more examples. While a frog is swimming it pushes the water back. and the water pushes its body forward. Now let me ask you a question. Say a bird is flying upwards, will the Newton’s third law apply here? Sometimes the third law is not so obvious. You’d probably say that the bird is the only object here, and there is no second object. But that’s not correct! The wings of the bird push the air downwards, and the air pushes the bird upwards. Action and reaction forces make it possible for birds to fly. We take one last example to understand something very interesting about the third law. What happens when a gun is fired? You’re right it exert the forward force on the bullet. And the bullet exerts an equal and opposite reaction force on the gun. And this exists in recoil of the gun. A perfect example of Newton’s third law! But hold on, if the force exerted by the gun on the bullet is the same as the force exerted by the bullet on the gun, then why doesn’t the gun recoil with the same acceleration as the bullet? The gun recoils, yes, but it doesn’t recoil at the same acceleration at the bullet, right? Same magnitude of forces but different accelerations. Why do you think this happens? Okay, so if your concept of the second law of motion was clear, you wouldn’t really be surprised with this. The second law of motion says that the force is the product of the object’s mass and acceleration. Or the acceleration of the object is equal to the force over its mass. What does this tell us? Yes, if the mass is more then the acceleration will be less. In this example the mass of the gun is much more than that of the bullet. Hence, the acceleration of the rifle is lesser than that of the bullet. If you’re in a boat and step out of it onto the land there are two forces involved. You pushing the boat behind and the boat pushing you forward. And that’s why you see the boat go behind when you go forward. Now let’s say a sailor jumps off a huge ship. The sailor does apply a force on the ship and the ship also applies a force on the sailor. But do you think the ship will accelerate as much as the sailor? Not really and I think you know why.


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