Chapter 4: Forces

Force and Motion

Objectives Define a force and differentiate between contact force and long range force. Newton’s First Law of Motion equilibrium Explain the meaning of Newton’s First Law of Motion and describe an object in equilibrium. Newton’s Second Law of Motion Recognize the significance of Newton’s Second Law of Motion and use it to solve motion problems. Define a force and differentiate between contact force and long range force. Newton’s First Law of Motion equilibrium Explain the meaning of Newton’s First Law of Motion and describe an object in equilibrium. Newton’s Second Law of Motion Recognize the significance of Newton’s Second Law of Motion and use it to solve motion problems.

Activator:

An object that experiences a push or pull has a force exerted on it. Force: Latin: Fortis (strong)

Contact versus Long-range Forces  Contact Force - A force that acts on the object by touching it. It “makes contact” with it. Con - with; tact (tactile) - touch. Examples: desk on a book, floor on a person.  Long-range Force - A force that acts on the object without touching it. Examples: magnets, force of gravity.  Long-range Force - A force that acts on the object without touching it. Examples: magnets, force of gravity.

Types of Forces ForceSymbolDefinitionDirection FrictionFfFf Contact force Contact force that acts to oppose sliding motion between surfaces. Parallel to surface; opposite to sliding direction NormalFNFN Contact force Contact force exerted by surface on object. Perpendicular to and away from surface. SpringF sp A restoring force, push or pull spring exerts on object. Opposite the displacement of object at end of spring. TensionFTFT Pull exerted by string, rope, cable when attached and taut. Away from object, parallel to rope at point of attachment ThrustF thrust Forces that move rockets, planes, cars… Same direction as acceleration, travel WeightFgFg Long-range Long-range gravitational force. Down towards center of earth.

A Free-Body Diagram shows all the forces, in vector form, acting on an object. Physical Diagram Free-Body Diagram - AFree-Body Diagram - B Free-Body Diagram:

Newton’s First Law of Motion  An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero.  A.K.A. Law of Inertia. Inertia is the tendency of an object to resist change in its current motion. (Example: Car without gas moving or at rest)  Equilibrium: An object is said to be in equilibrium when the net force is zero. (net = sum of all vector forces)

Newton’s First Law of Motion

Newton’s Law of Inertia Paul G. Hewitt Disc One - Newton’s Law of Inertia

Law of Inertia Demo Paul G. Hewitt Disc One - The Old Table Cloth Trick

Cylinder Inertia Demo Paul G. Hewitt Disc One - Inertia of Cylinder

Doing the T.P. Roll Paul G. Hewitt Why you don’t have to hold the T.P. roll

Weight Mass Distinction Paul G. Hewitt Disc One - Weight-Mass Distinction

Newton’s Second Law of Motion  The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.  a  F net ; a  1 / m ; a = F net / m  Most Common Form: F net = m a Force = mass x acceleration  Most Common Form: F net = m a Force = mass x acceleration

Newton’s 2nd Law of Motion Paul G. Hewitt Disc One - Newton’s 2nd Law

All Together Now…. v = 0 m/s v  0 m/s 1)Depends directly on Net Force 2)Depends inversely on Mass

Using Newton’s Laws Sir Isaac Newton used prisms to show that sunlight was made up of all the colors of the rainbow. This proved that the ancient Greeks ideas about light were wrong.

Friction: What is it? the roughness of the surfaces.  Frictional resistance to the relative motion of two solid objects is usually proportional to the force which presses the surfaces together as well as the roughness of the surfaces.  Rub your hands together. Feel the heat that is generated. That is friction. Why do we put oil in our car engines? To overcome friction in the pistions. Why can ice skaters glide so easily? How do car brakes work?

Friction: Static and Kinetic

Friction

Gravity: Feather and Coin

Why do all objects fall at the same rate? Paul G. Hewitt Free-fall Acceleration Explained

Interaction Forces

Objectives  Define Newton’s Third Law of motion.  Explain the tensions in ropes and strings in terms of Newton’s third law.  Define the Normal force.  Determine the value of the normal force by applying Newton’s Second Law.  Define Newton’s Third Law of motion.  Explain the tensions in ropes and strings in terms of Newton’s third law.  Define the Normal force.  Determine the value of the normal force by applying Newton’s Second Law.

Newton’s Third Law of Motion  For every action, there is an equal and opposite reaction.  All forces come in pairs. These interaction pairs are equal in magnitude and opposite in direction.

Examples

Interaction Forces

Rifle Recoil

Tensions on Ropes In this simple rope tension diagram, how can you find the tension force in the rope? How does this relate to Newton’s Third Law?

Tensions on Ropes In this simple rope tension diagram, how can you find the tension force in the rope? How does this relate to Newton’s Third Law?

Normal Force The Normal Force is the perpendicular contact force that a supporting surface exerts on another object.

Normal Force The Normal Force is equal to the weight of the object if the supporting surface is horizontal. i.e. F N = F g = mg The Normal Force is equal to the weight of the object if the supporting surface is horizontal. i.e. F N = F g = mg

Pressure: The Bed of Nails Paul G. Hewitt Disc One - Pressure: The Bed of Nails

Pressure: The Bed of Nails Why doesn’t the assistant get hurt?

Einstein’s Angels Einstein's Angels

Summary  How would you summarize today’s lesson?  Forces come in interaction pairs that are equal and opposite.  Although the forces are the same in magnitude, the accelerations can vary greatly.  How would you summarize today’s lesson?  Forces come in interaction pairs that are equal and opposite.  Although the forces are the same in magnitude, the accelerations can vary greatly.

Assignments  Chapter 4 Review Questions 2-30 Evens only, p due Tuesday 11/2.  Study Guide due Thursday.  Quiz next Monday.  Chapter 4 Review Questions 2-30 Evens only, p due Tuesday 11/2.  Study Guide due Thursday.  Quiz next Monday.