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Unit 2

# 2.5 Newton's Third Law and Free-Body Diagrams

Peter Apps

### AP Physics 1Β π‘

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## Essential Knowledge 3.A.2 β‘οΈ

Forces are described by vectors. Key Vocabulary: Vector - quantities that are described by a size (magnitude) and a direction (ex. East, Up, Right, etc.)Β  Example: The gas station is five miles west of the car
• Force, Displacement, Velocity, and Acceleration are vector quantities
Vectors can also be represented by arrows, and the length of the arrow should represent the magnitude of the described quantity. From the image below you can see the 5m arrow is smaller in length than the 50m arrow to reflect the difference in magnitude of the two quantities.
A force can be simply described as a push or pull. We know that a push or pull has both magnitude and direction (therefore, it is a vector quantity) and can vary considerably in both regards.
Key Vocabulary: Free-Body Diagram - as we discussed in previous standards, this is a technique used to illustrate all the external forces (forces that affect motion) acting on a body. The body is represented by a single dot or box, and only those forces acting on the body from the outside are pictured. Free-body diagrams are helpful in analyzing all forces acting on a system and are often utilized in the study and application of Newtonβs laws of motion.

Image courtesy of physicsclassroom.com.

As pictured in the image above, Weight or Force of Gravity is always pointed downwards, Force of Normal is perpendicular to the surface, and Force of Friction is opposite motion. The applied force is in whatever direction the force is coming from (ie. the image could represent a hand pushing a box right across a level surface).Β  βΆ Still feeling a little confused about Free-Body Diagrams? Donβt worry! Check out this video from Khan Academy for more practice!Β

## Essential Knowledge 3.A.3 π

A force exerted on an object is always due to the interaction of that object with another object.
Force is always the result of an interaction of two or more objects. No object has force on its own. Therefore, no object can exert a force on itself. When you clap your hands, one hand exerts a force on the other. When you throw a ball, it exerts a force on your hand and your hand exerts a force on it.

Image courtesy of sayingimages.com.

## Essential Knowledge 3.A.4 π€

If one object exerts a force on a second object, the second object always exerts a force of equal magnitude on the first object in the opposite direction.
Newtonβs Third Law states, βFor every action, there is an equal and opposite reaction.β Simply put, in every interaction, there is a pair of forces acting on the two interacting objects. The magnitude of the force on the first object equals the magnitude of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.Β

Image courtesy of WikiHow.

Key Vocabulary: Action-Reaction Pair - the force exerted on an object is the action, and the force experienced by the object is the reaction. Action-Reaction pairs occur only when two objects interact. (Example: a foot pushing down on Earth <-> Earth pushing up on the foot.)
π₯Watch: AP Physics 1 - Unit 2 Streams

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