Phy 211: General Physics I - Portland Community College

Phy 211: General Physics I - Portland Community College

Phy 211: General Physics I Chapter 5: Force & Motion I Lecture Notes Sir Isaac Newton (1642-1727) 1. Greatest scientific mind of the late 17th century 2. Invented the calculus (independently but simultaneously with Gottfried Wilhelm Leibnitz) 3. Among his accomplishments included: A Particle Theory Light & Optics A Theory of Heat & Cooling 4. Established 3 Laws of Motion 5. Proposed the Law of Universal Gravitation (to settle a parlor bet) The bet was actually between Edward Haley & Robert Hooke

This theory successfully explained planetary motion and elliptical orbits If I have seen further it is by standing on the shoulders of giants. Newtons 1st Law Every object continues in its state of rest, or of motion unless compelled to change that state by forces impressed upon it. Also known as the Law of Inertia Key Points: When an object is moving in uniform motion it has no net force acting on it When there is no force acting on an object, it will stay at rest or maintain its constant speed in a straight line Or simply put, Nature is lazy! Fnet =0 a = 0 {v = constant}

Newtons 2nd Law 1. 2. When a net force is exerted on an object its velocity will change: dv Fnet = Fi F1 F2 F3 ... dt i The time rate of change of motion (acceleration) is related to: a. Proportional to the size of the net force b. Inversely proportional to the mass of the object (i.e. its inertia) 3. The relationship between them is

Fnet 1 a= = Fnet i + Fnet j Fnet =ma=m ax i + ay j or m m Thedirection of a will always correspond to the direction of Fnet 4. x y

Free-Body Diagrams 1. Free-body diagrams a simplified drawing of the forces acting on it specified, essentially force vector representations 2. The forces are drawn as vectors 3. Free-Body diagrams facilitate the application of Newtons 2nd Law Examples: Joey getting slapped Joey standing on a floor Joey in Free Fall Joey W Ffloor Joey W

while standing on a floor Ffloor Joey W Fslap Types of Forces In our world, forces can be categorized as one of 2 types: 1. Non-Contact: force is exerted over a distance of space with out direct contact (a.k.a. action-at-a-distance forces) 2. Contact: forces is exerted due to direct contact (Note: at the microscopic level, ALL forces are non-contact) In either case, Newtons 3rd law still applies to the forces present (more on this later) Examples of each type of force: Non-Contact: Gravitational

Electric Magnetic Contact: Normal Frictional Tension Weight The weight of an object is the gravitational force exerted on it by the gravitational attraction between the object and its environment: FG=maG=maGj On the surface of the Earth, the gravitational force is referred to as weight:

FG = W = (-mg)j Mass is a measure of an objects inertia (measured in kg) Independent of object location Weight is the effect of gravity on an objects mass (measured in N) Determined by the local gravitational acceleration surrounding the object Notes: Mass is a measure of an objects inertia (measured in kg) Independent of object location Weight is the effect of gravity on an objects mass (measured in N) Determined by the local gravitational acceleration surrounding the object

Normal Force The support force between 2 surfaces in contact Direction is always perpendicular (or normal) to the plane of the area of contact Example: the force of floor that supports your weight Consider standing on a scale on the floor of an elevator. The reading of the scale is equal to the normal force it exerts on you: Construct free body diagrams for the scale: At rest Constant velocity Accelerating upward Accelerating downward Examples of Normal Force Tension Force Force applied through a rope or cable When the rope or cable is massless (negligible compared to the bodies it is attached to) it can be treated as a connection between 2 bodies

No mass means no force needed to accelerate rope Force of pull transfers unchanged along the rope Action force at one end is the same as the Reaction force at the other end When attached to a pulley the tension force can be used to change the direction of force acting on a body Calculation of a tension force is usually an intermediate step to connecting the free-body diagrams between 2 attached objects Tension Applications M2 With Pulley (flat surface): M1 Inclined Plane: M 2 M1

Atwood Machine: M1 M2 Newtons 3rd Law When an object exerts a force on a second object, the second object exerts an equal but oppositely directed force on the first object Body 1 where: F2 on 1 F1 on 2 Body 2 F2 on 1 F1 on 2

Consequences: Forces always occur in action-reaction pairs (never by themselves) Each force in an action-reaction pair acts on a different object Important: Newtons 3rd law identifies the forces produced by interactions between bodies Newtons 2nd law defines the accelerations that each object undergoes

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