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Lecture

# Newtons Law's (Unit 4).pdf

17 Pages
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School
McMaster University
Department
Physics
Course
PHYSICS 1L03
Professor
Reza Nejat
Semester
Fall

Description
Biomechanics Sprinter Biomechanics Newton Laws Concepts: Newton’s Laws Free body diagram Application of Newton’s Laws Weight and Apparent Weight Chapter: 4 Pages: 77-103 •Suimolt. PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Biomechanics Biomechanics Questions: •Hwdeprqicai?n •owosertr? •Dpxrd by Sprinter on the starting block directly give him acceleration? •owwexcs? •Hdee)r •Wornwhtsre •Wadf? do? PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 1 Force Biomechanics What is force? (review) eitr •ir push or a pull. between different parts? •ir interaction between two objects •b,menlo •otn object. and bone •rin agent. •,illa •ir vector. transferred to the bone via a •ces acceleration. tendon. •ir contact force or a contact- free force (field force). PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Force Force ▯ F •Whdfro? •o: ▯ ▯ const. ▯ a •oae changes in velocity. F ▯ m •hn:t a –Force produces acceleration. –Theamountofaccelerationproducedis •eitrl proportional to the amount of force to acceleration is simply the mass of the object being accelerated: ▯ ▯ ▯ F ▯ ▯ F ▯ a ▯ ▯ ▯ const. F ▯ ma a • This is Newton’s second law PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 2 Newton’s Laws Newton’s Second Law: •etnetfs m , it produces an acceleration, the object such that: m . t c e j b o e h t f o s s a m ) l a i t r e n i ( e h t s •i PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Laws Newton’s Second Law: •hacliaoeto directly proportional to the net force acting on the object and inversely proportional to its mass: ▯ F ▯ m a ▯ ▯ a ▯ F m PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Law Example 1 Force and Motion (Discussion Point) •Wedtlt 5 kg object at 3 m/s ? •onotfeaiag moving object, can you tell in which direction the object is moving? PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Example 2 Example 2 •ltlla is in his hand for 0.5 s while being thrown and leaves his hand with a speed of 6.0 m/s. Estimate the force exerted on the ball by the child. Assume constant acceleration during throw. F ▯ m a net ▯v vf▯ v 1 a ▯ avg▯ ▯ ▯t ▯t PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 4 Newton’s Laws Newton’s Laws Discussion Point: Newton’s First Law: • While driving down the road, the driver •hnexenlcacioan of the car you are riding in hits the object is zero, or in the absence of brakes suddenly. As a result, your external forces, an object at rest body moves toward the front of the remains at rest, and an object in car. motion continues to move with a constant velocity in a straight line Why? (uniform motion). PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Laws Newton’s Laws Newton’s First Law (Application): • An isolated object, free from external forces, will continue moving at constant velocity, or remain at rest. •rit( net external force) also may move at constant velocity. • Motion at constant velocity doesn’t require any external causes (forces) • Only changes in velocityrequire external •Whfnawtr causes (forces). from the hair? PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 5 Newton’s Laws Newton’s Laws Newton’s First Law: ▯ ▯ Newton’s First Law: F ▯ F ▯ 0 •I net ▯ ▯ F ▯ 0 ▯ net,x Fnet ▯ 0 ▯ ▯ •Teethstomir ▯ Fnet,y▯ 0 motion (constant velocity), or at rest. ▯ ▯ ▯ Static Fnet ▯ 0 ▯ a ▯ 0 ▯ v ▯ const •Eqim: Dynamic PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Law Example Drag and terminal velocity orlrtAla in still air with a terminal speed of 0.3 mm/s. The •hmnf( the resistive force to the particle has a mass of 1g and a drag force of motion of an object in air) depends on the the form bv. What is the value of b? speed of the object. 2 R ▯ v or ▯ v •Whie) object become zero, the object reaches equilibrium, and moves with constant velocity (Terminal velocity. Fnte R ▯▯ Fi▯ 0 v ▯ const. ▯ terminal velocity Other PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 6 Example Newton’s Law dhifeys of 180 km/h with her arms and legs outspread. Inertial Reference Frame What is the magnitude of the upward drag force F d •ThereferenceframeinwhichNewton’s on the sky diver? first law is valid is called an inertial frame. •Allreferenceframesthatarestationary or have a uniform motion (ie: constant velocity) are inertial frames . PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Law Newton’s Law Discussion Point: Newton’s Third Law (interaction pair): •natatapcwal •ihaofir discovers that his jet pack no longer tIojtthtce works, leaving him stranded 50 m from the spacecraft. Assume the jet pack is exerted by object 1 on object 2 is equal in magnitude to and opposite in removable. How the astronaut can still use it to direction to the force exerted by object return to the ship? 2 on object 1. ▯ ▯ Explain! F ▯ ▯ F on 1by 2 on 2by1 PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 7 Newton’s Law Newton’s Third Law Newton’s Third Law (Action-Reaction): •eihy,tl •fAtsfooet, there is a trick: just flip the sentence! –Action:XexertsaforceonY object B exerts an equal, opposite force –Reaction:YexertsaforceonX back on A. ▯ ▯ –Ifthe exerts a force on the , the F ▯ ▯ F reaction pair for this action is the force of the on B by A on A by B on the ▯ ▯ – DO NOT get confused by jumping ahead: i.e. F BA ▯ ▯ F AB the action is gravity and the reaction is the normal force (wrong!) •F AB and F BA are called “interaction pair”. (wrong!) PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s third Law Newton’s third Law Example on Newton’s third law: Example on Newton’s third law: What is the “reaction” to the following forces? ▯ F What is the “reaction” to the following forces? FB T ▯ Earth pulls block ▯ Block pulls earth ▯Fg down (Gravity) Fg up (Gravity) Block pushes down on tableable pushes up on block You push on a block The block push back on you Balloon pushes Air outside pushes on air outside on balloon PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat 8 Newton’s Laws Newton’s Laws Discussion Point •Ail,’s pulls on the wagon, wagon pulls back on the horse with an equal force. Clearly, then, the net force is zero and the wagon will stay put. •haal!oyu •WclNfodedgse explain this? to shake water from its coat? PHYS 1L03, Fall 2013, McMaster University, R.NejatPHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Laws Newton’s Laws Two skaters of different masses push against one another. •Which one will push harder?
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