Newton’s Laws of Motion

History Aristotle  First to study motion Galileo  Dropped big and small rock  Fell at same rate  If no interference – motion will go on forever

History Newton formalized ideas  At 23 (1665) he overthrew Aristotle’s 2000 year old ideas  LAWS OF MOTION

Newton’s First Law of Motion Law of Inertia  An object continues in its state of rest, or of uniform motion in a straight line, unless it is acted upon by an unbalanced force “Object” – any body “Continues” – keeps rest or moving “Unbalanced force” – net force, not in equilibrium

Law of Inertia Why Law of Inertia?  Tendency of a body to resist change of motion More matter = more inertia Ex. If chased by an elephant, how should you run away? Zig-zag!!

Mass and Inertia WoodLead Cardboard Same volumes  take up the same amount of space Which is harder to move? - More inertia - More mass (mass is a measure of inertia) VVVI I I M M M

Mass and Weight Does mass = weight?  Mass – quantity of matter in a body  Weight – measure of gravitational force which acts on a body Depends on where the object is Ex. Earth, Moon, Space g = 9.8 m/s 2 g = 1.6 m/s2 g = ?

Mass and Weight Gravity on the Moon  Gravity around the Solar System  /q0227.shtml /q0227.shtml

Mass and Weight How much would a 75 kg (165 lb) person weigh on the Moon? On Mars?  g moon = 1.62 m/s 2  g mars = 3.77 m/s 2  2.2 pound = 1 kilograms  1 pound = 4.45 Newtons Moon: N or 27.3 lbs Mars: N or 63.5 lbs

Examples of Inertia On bus (rest  rest)  Standing and bus moves forward  You seem to fall backward  Actually feet move forward with bus and body remains at rest  Bus stops?

Examples of Inertia Dishes on Table (rest  rest)  Snap a cloth from beneath  Friction between dishes and cloth not enough to move dishes

Examples of Inertia In stopping car (motion  motion)  Fly forward  Keep moving while car stops

Newton’s Second Law of Motion Deals with acceleration Acceleration = a = v / t  Ex. Car died  tow 5 N 50 N Which provides the greater acceleration? Which can get car from 0 mi/hr to 20 mi/hr faster? GREATER FORCE

Newton’s Second Law of Motion With same force, which can go from 0 mi/hr to 50 mi/hr faster? Mass = 1000 kg Mass = 5000 kg SMALLER MASS SMALLER INERTIA

Second Law Force = mass x acceleration (F = ma) The more force on an object, the more it accelerates. The more massive an object, the more it resists acceleration.

Second Law “The acceleration of a body 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 body” * “in the same direction as the net force”  a in the same direction of body’s motion  speed up  a in opposite direction of body’s motion  slow down  a at right angles to direction of body’s motion  deflect circular  Any other  change in speed and direction

Units mkscgsEnglish Length Mass Time Force Weight = W = g = Use ______________ as force units A force is a __________ or a __________ Now also say: Force is anything that can ____________ a body.

History Revisited Galileo gave no reason why bodies fall with an acceleration. Newton did.  Newton – a F pulls on a body and pulls it toward the Earth.  F = ma Heavier body attracted with more F. Heavier body has more m and…  More I  More resistance to change Ex. If 2 x F and 2 x m  same net acceleration

Newton’s Third Law of Motion Force:  Causes acceleration  A push or a pull  Always involves two objects  interaction Ex. Hammer and Nail  Hammer exerts force on nail  In process, nail slows down hammer  Therefore, nail exerts force on hammer Ex. Ice skater with rifle or with mitt

Newton’s Third Law of Motion To every action there is an equal and opposite reaction  One force is action force, other is reaction  Neither can exist without the other Action Weightlifter lifts on barbell Feet down on Earth You sit on chair Tires sit on road You push on wall Helical blades push down on particles Rifle pushes bullet Reaction Barbell pulls on weightlifter Earth pushes up on feet Chair pushes up on you Road pushes up on tires Wall pushes back on you Particles push up on blades Bullet pushes back on rifle (KICK) SAME FORCE: F/m b = aF/m r = a

Newton’s Third Law of Motion Sometimes not so obvious.  Reactions in free-fall Earth pulls on body = action Body pulls on Earth = reaction Therefore, can say boulder falls to Earth or Earth falls to boulder  F is equal but acceleration is very small When step off curb – street comes up to meet you Bug against windshield – deceleration of car small, deceleration of bug large

When objects are relatively the same mass, both forces are more easily noticed since both accelerations are about equal.