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**Newton’s Third Law of Motion. Momentum.**

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**Newton’s Third Law of Motion**

A force is interaction between one object and another. For example, the interaction between a hammer and a nail: a hammer exerts a force on a nail and drives it into a board, and a nail exerts a force on a hammer. Newton’s Third Law of Motion: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. A ballon pushes escaping air down, and the escaping air pushes the ballon up.

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**Newton’s Third Law of Motion (continued)**

The Newton’s Third Law of Motion: “To every action there is always an equal and opposite reaction.” One force is called the action force. The other force is called the reaction force. In every interaction, the forces always occur in pairs. Interaction Rule: Object A exerts a force on object B. Object B exerts a force on object A

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**Newton’s Third Law of Motion (an example)**

An Example: A bullet is fired from a rifle: The force the rifle exerts on the bullet is exactly equal and opposite to the force the bullet exerts on the rifle, so the rifle “kicks”. Why the bullet moves so fast compared with the rifle? According to Newton’s Second Law of Motion, we must also consider the masses. F = m*a Bullet has less mass, and, as a result of it, more acceleration than a rifle. A given force exerted on a small mass produces a greater acceleration than the same force exerted on a large mass.

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**Do Action and Reaction Forces Cancel?**

The Horse-Cart Problem: An object accelerates (or not) because of the forces that push or pull on it. (Newton's 2nd Law) Only the forces that act on an object can cancel. Forces that act on different objects don't cancel - after all, they affect the motion of different objects! Identify the system in question. Restrict your attention only to the external forces that originate outside and act on the system, and not to the forces that the system may exert on external things.

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**The Horse-Cart Problem**

There are two Newton's Third Law force pairs in the diagram: “the horse pulls the cart" and “the cart pulls the horse“, and “the horse pushes the ground" and “the ground pushes the horse". The forces “the horse pulls the cart" and “the cart pulls the horse“, are equal in magnitude and opposite in direction. The forces “a horse pushes the ground" and “the ground pushes the horse" are also equal in magnitude and opposite in direction.

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**The Horse-Cart Problem (continued)**

Why does the cart accelerate? The cart accelerates because the horse pulls on it! The amount of acceleration equals the net force on the cart divided by its mass (Newton's Second Law). Why does the horse accelerate? There are 2 forces that push or pull on the horse in the diagram above. The cart pulls the horse backwards, and the ground pushes the horse forward. The net force is determined by the relative sizes of these two forces. If the ground pushes harder on the horse than the cart pulls, there is a net force in the forward direction, and the horse accelerates forward. If the cart pulls harder on the horse than the ground pushes, there is a net force in the backward direction, and the horse accelerates backward. (This wouldn't happen on level ground, but it could happen on a hill...). If the force that the cart exerts on the horse is the same size as the force that the ground exerts, the net force on the horse is zero, and the horse does not accelerate.

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Momentum Inertia: a measure of an object’s ability to resist a change in motion. Momentum: describes the quantity of motion, “inertia in motion”. It’s vector quantity, product of mass and velocity> p = m x v p – momentum, m – mass (kg), v- velocity ( 𝑚 𝑠 ). SI units of momentum: 𝑘𝑔∗𝑚 𝑠 . When velocity changes, acceleration occurs. Force produces acceleration.

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**Momentum (an example) An Example:**

Describe how an elephant and a golf ball can have the same momentum. p = m * v Elephant has a large mass and slow velocity. A golf ball has a small mass and high velocity. Both will have a similar effect on you.

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