Physics 151 Week 7 Day 2

Topics What is a Force? Newton’s 0th Law of Motion (Not in the book) Force Diagrams and

System Schemas (Not in the book) Newton’s 1st Law of Motion More vector math

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Vectors and Vector Components: ExampleDisplacement with 3 motion intervals

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Vectors and Vector Components: ExampleVelocity in 2D and vector equations

Clicker Question 11. A “net force” is

A. the sum of the magnitudes of all the forces acting on an object.

B. the difference between two forces that are acting on an object.

C. the vector sum of all the forces acting on an object.D. the force with the largest magnitude acting on an object.

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Answer1. A “net force” is

A. the sum of the magnitudes of all the forces acting on an object.

B. the difference between two forces that are acting on an object.

C. the vector sum of all the forces acting on an object.D. the force with the largest magnitude acting on an object.

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What Is a Force?

A force...

... is a push or pull. ... acts on an object.

... requires an agent.

... is a vector. ... is a contact force or a long-range force.

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Newton’s Zeroeth Law of Motion

Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what acts directly on them in a given moment.

Types of forces

Contact Forces vs. Non-contact Forces

Defining normal Forces and Friction Forces

Contact forces arise when two objects interact due to surfaces in contact. These forces can be broken into two components parallel and perpendicular to the surface

• The component parallel to the surface is the friction force

• The component perpendicular to the surface is the normal force. (normal is a mathematical term meaning perpendicular.)

Normal Force n

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Friction fk and fs

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Tension ForceT

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Drag D and Thrust Fthrust

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Force Vectors

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Free-Body Diagrams

You should always describe a force by identifying the type of force, the force agent and the object being acted on. For a force diagram label forces like this => Fg, Earth=>box

System Schema

Draw a system schema:• Draw a diagram where you write down the name of each object in

the system and then draw a solid circle drawn around it. • Draw two sided arrows like this between the object circles

of objects that interact (This illustrates all interactions between the objects in this diagram).

• Draw an additional dotted line around the block to indicate it is the object of interest. This diagram is called a system schema.

A system schema illustrates all the relevant interactions between the objects in a given physical situation

Each double-headed arrow represents an action-reaction pair of forcesA.K.A. a Newton’s 3rd Law pair

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Identifying Forces:Freebody (Force) Diagrams and System Schema

Michael Laura

Example Problem: One book, Two book

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Newton's First Law of Motion

DEMO - Pushing the cart on track

DEMO - Hoverpuck

=> How much force is need to make an object move with constant velocity?

Example ProblemThe hover puck

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What Causes Motion?

In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.”

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Newton's First Law of Motion

Every object continues in a state of rest or a state of motion with a constant speed in a straight line unless acted on by an unbalanced force.

or

Velocity = constant if and only if (IFF) Fnet = 0

Seat Belts: An Application of Newton’s First Law

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Newton's First Law of Motion Demonstrations

DEMO - Smash the HAND

DEMO - Tablecloth

An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. • Is T greater than, equal to, or less than w? Or is there not

enough information to tell?

• A => FT > Fg

• B => FT = Fg

• C => FT < Fg

Example Problem

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Summary

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