Newton’s Laws of Motion The “Cause” Behind Kinematics

Aristotle’s Ideas Aristotle believed that objects had two different types of motion “Natural” motion – up and down Objects return “home” “Violent” motion – horizontal motion Motion due to a push or pull Also believed that objects moved only if a force was continually applied Therefore, it was “natural” for objects to be at rest Oct. 31, 2007 AF Carpinelli

Galileo’s Contribution Challenged many of Aristotle’s ideas Law of Falling Bodies (for example) Realized that objects did not need a continual application of force to remain in motion It only APPEARS that this is the case due to friction Tested this will extremely smooth surfaces and inclined planes Saw that it was NOT the nature of the objects to come to rest Oct. 31, 2007 AF Carpinelli

Galileo’s Contribution Realized that in the absence of friction, objects keep moving at constant velocity Said that instead of a natural rest state, objects have a natural resistance to a change in their motion – inertia This idea became Newton’s First Law of Motion Oct. 31, 2007 AF Carpinelli

Sir Isaac Newton Most famous physicist of all time Most of the laws of physics can be traced back to Newton’s work Landmark book – The Principia Invented Calculus (although this is disputed – could also have been von Leibniz) Worked in many areas of science, including optics, chemistry, botany, and astronomy Oct. 31, 2007 AF Carpinelli

Newton’s First Law of Motion Law of Inertia Objects at rest remain at rest, objects in motion remain in motion at constant velocity unless acted upon by an outside unbalanced force. KEY POINTS Remain – objects cannot change their own motion Constant velocity – no change in speed or direction with the application of a force Outside unbalanced force – a force that is applied to the object AND not canceled out by another force Oct. 31, 2007 AF Carpinelli

Newton’s First Law of Motion A balanced force 5 N 5 N No change in motion An unbalanced force 3 N 5 N Change in motion Oct. 31, 2007 AF Carpinelli

Newton’s 2nd Law of Motion Law of Acceleration The acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass. Basically, the greater the force, the greater the acceleration BUT the greater the mass, the less the acceleration Expressed mathematically as F=ma [Force = mass X acceleration] Oct. 31, 2007 AF Carpinelli

Newton’s 3rd Law of Motion The “Action-Reaction Law” If Object A exerts a force on Object B, Object B exerts the same force on Object A but in the opposite direction. Known as Action-Reaction Pairs Although the force on the pairs is the same, the acceleration of the objects will be different, depending on their masses For example, the gravitational force that the earth exerts on you when you jump is the same as the force you exert on the earth, but because the earth is so much more massive, its acceleration is essentially zero compared to yours Oct. 31, 2007 AF Carpinelli

Working with Newton’s Laws New Quantities, Symbols and Units: Mass (m) – unit is the kilogram (kg) Force (F) – unit is the Newton (N) Conversions – 1 kg = 2.225 lbs 1 kg = 9.8 N 1 N = 0.225 lbs Oct. 31, 2007 AF Carpinelli

Conversion practice!! Convert the following – 3.2 kg = ____________ N 120 lbs = ___________ kg 65 kg = ____________ lbs 45,000 g = __________ kg 82,420 g = __________ N 72,500 g = __________ lbs Oct. 31, 2007 AF Carpinelli

Conversions Answers Convert the following – 3.2 kg = 31.4 N 120 lbs = 54.4 kg 65 kg = 143 lbs 45,000 g = 45 kg 82,420 g = 808 N 72,500 g = 160 lbs Oct. 31, 2007 AF Carpinelli

2nd Law Sample Problem A force of 10 N is applied to a 20 kg crate. What is its acceleration? Oct. 31, 2007 AF Carpinelli

Net Force The net force is the sum of all the forces acting on an object. Direction of the forces is very important. For example: 4 N 3 N 2 N Oct. 31, 2007 AF Carpinelli

Net Force Problem A force of 17 N directed east is applied to a 5 kg brick. At the same time, a force of 22 N directed to the west is applied. What is the net force on the brick? In what direction will it move? What is its acceleration? Oct. 31, 2007 AF Carpinelli

Newton’s 2nd and Kinematics A 1250 kg car brakes from 20 m/s to rest in 5 s. What force did the brakes exert on the car? Oct. 31, 2007 AF Carpinelli