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# Lec4 - Physics 101 Today Chapter 5 Newtons Third Law First...

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Physics 101 Today Chapter 5: Newton’s Third Law

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First, let’s clarify notion of a force : Previously defined force as a push or pull. Better to think of force as an interaction between two objects. Eg. I push on the table, it pushes back on me with an equal and opposite force on me. If on ice (no friction), I’d slide backwards. This force pair constitutes a single interaction. (More egs very soon) You can’t push anything without it pushing back on you ! Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Newton’s 3 rd Law - often called “ action-reaction
Eg. Leaning against a wall. You push against the wall. The wall is also pushing on you, equally hard – normal/support force. Now place a piece of paper between the wall and hand. Push on it – it doesn’t accelerate must be zero net force. The wall is pushing equally as hard (normal force) on the paper in the opposite direction to your hand, resulting in zero Fnet . This is more evident when hold a balloon against the wall – it is squashed on both sides.

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Eg. You pull on a cart. It accelerates. The cart pulls back on you (you feel the rope get tighter). Can call your pull the “action” and cart’s pull the “reaction”. Or, the other way around. Newton’s 3 rd law means that forces always come in action-reaction pairs . It doesn’t matter which is called the action and which is called the reaction. Note: Action-reaction pairs never act on the same object
Examples of action-reaction force pairs In fact it is the road’s push that makes the car go forward. Same when we walk – push on floor, floor pushes us forward. (These forces are frictional). Fun Demo : blow up a balloon and release it. Same principle as rocket

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Clicker Question What is the reaction force to a bat striking a ball? A) the ball’s hit on the bat B) the muscular effort in the player’s arm C) the friction force between the ground and the player’s feet D) gravity E) None of these Answer: A Note that we could call either the action and the other the reaction – it doesn’t matter which one is which.
Another Question What are the action-reaction pairs once the ball is in the air? First note there are two interactions: (i) one with the earth’s gravity and (ii) the other with the air. (i) Action: Earth pulls down on ball (weight) Reaction: ball pulls up on Earth. (ii) Action: Air pushes ball backwards (air resistance) Reaction: Ball pushes air forwards

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Clicker Question Answer C: They act on different objects .
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