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Lecture05

# Lecture05 - PHYSICS 149 Lecture 5 Chapter 2 2.5 Newtons...

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PHYSICS 149: Lecture 5 • Chapter 2 – 2.5 Newton’s Third Law – 2.6 Gravitational Forces – 2.7 Contact Forces: Normal Force and Friction Lecture 5 Purdue University, Physics 149 1

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Newton s Third Law • All forces come in pairs • Third law forces involve TWO OBJECTS . • The two forces are: – the force object one exerts on object two – the force object two exerts on object one • Three ways to state the 3rd law: – Forces on each other are equal and opposite – For every action there is an equal and opposite reaction – You can t push on something without it pushing back on you Lecture 5 2 Purdue University, Physics 149
Newton’s Third Law of Motion • In an interaction between two objects, each object exerts a force on the other. These two forces are equal in magnitude and opposite in direction. – To every action, there is always opposed an equal reaction. – Forces always come in equal but opposite action- reaction pair. • Note that these two forces act on different objects; they do not cancel in any way. • Don’t forget that forces always exist in pairs. Lecture 5 Purdue University, Physics 149 3

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Free Body Diagram (FBD) • A simplified sketch of a single object with force vectors drawn to represent every force acting “on” that object. (It must not include any forces that act on other objects.) • FBD is useful to find the net force acting on an object. Lecture 5 4 Purdue University, Physics 149
Internal and External Forces • Internal Forces: Forces which act on one part of an object by another part of the same object • External Forces: Forces which act on an object by some other object. • Net force on a system = vector sum of internal forces + vector sum of external forces • But, vector sum of internal forces is zero because, from Newton’s third law, internal forces will occur in equal and opposite pairs and so they contribute nothing to the sum. They never influence the system’s motion. • Eventually, net force on a system = vector sum of external forces only. We need to consider external forces only in order to describe the motion of the system. Lecture 5 5 Purdue University, Physics 149

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Examples • Net force on a baseball = interaction with the Earth (gravity) + interaction with a bat + interaction with the air + interactions among protons, neutrons, and electrons in it
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Lecture05 - PHYSICS 149 Lecture 5 Chapter 2 2.5 Newtons...

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