# Newtons Laws Three Laws of Motion Aristotles Motion

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Newton’s Laws Three Laws of Motion

Aristotle’s Motion n Natural Motion is up or down u Down for falling objects u Up for smoke u Circular for heavenly bodies since without end Violent Motion u Due to imposed forces such as wind pushing a ship or someone pulling a cart Natural state of motion is rest u A force is needed to keep something moving

Aristotle’s Basic Error n Friction not understood as a force

Galileo’s Motion n n Force is a push or a pull Friction is a force that occurs when objects move past each other Friction due to tiny irregularities Only when friction is present is a force required to keep something moving

Galileo’s Inclined Planes n n Ball rolling downhill speeds up Ball rolling uphill slows down He asked about ball on smooth level surface Concluded it would roll forever in absence of friction

Inertia Resistance to change in state of motion n Galileo concluded all objects have inertia n Contradicted Aristotle’s theory of motion n No force required to keep Earth in motion around sun because no friction n

Newton n Born 1665 Built on Galileo’s ideas Proposed three laws of motion at age of 23

Newton’s First Law Ourtesy www. lakeheadu. ca/~alumni/ hockey. gif n n Every object continues in its state of rest, or of motion in a straight line at constant speed, unless compelled to change that state by forces exerted on it. Also called Law of Inertia: things move according to their own inertia Things keep on doing what they are doing Examples: Hockey puck on ice, rolling ball, ball in space

Mass Amount of inertia depends on amount of mass…or amount of material (number and kind of atoms) n Measured in kilograms n Question: Which has more mass, a kilogram of lead or a kilogram of feathers? n Mass vs. Volume: volume is how much space something occupies n

Experiencing Inertia is resistance to shaking n Which is easier to shake, a pen or a person? n Why is it so hard to stop a heavy boat? n

Mass vs. Weight Mass is intrinsic property of any object n Weight measures gravitational force on an object, usually due to a planet n Weight depends on location of object n Question 1: How does mass of a rock compare when on Earth and on moon? n Question 2: How does its weight compare? n

Review Mass vs. Weight n n What is mass? Answer: quantity of matter in something or a measure of its inertia What is weight? Answer: Force on a body due to gravity

Weight of 1 Kilogram 9. 8 Newtons n About 2. 2 pounds n Compare the weight of 1 kg nails with 1 kg styrofoam n Answer: Same n

Weight Examples What does a 70 kg person weigh? W = mg = 70 kg x 9. 80 N/m 2 = 686 N n An object weighs 9800 n on Earth. What is its mass? n m = W/g = 9800 / 9. 8 m/s 2 = 1000 kg n

Inertia in a Car n Discuss three examples of inertia in a car • Car hitting a wall • Car hit from behind by a truck • Car going around a corner

Newton’s Second Law of Acceleration n The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the body. n Acceleration = net force ÷mass n F =ma n Acceleration is in direction of net force n

Net Force means sum of all forces acting n Sum is Vector sum n F 2 F 1 Resultant force

Understanding the Second Law The cause of acceleration is… Force n _____ Mass resists acceleration n The greater the force, the ____ greater the acceleration less n The greater the mass, the _____ the acceleration. n

Units F = ma n Unit of force is the Newton (N) n 1 N = 1 kg m/s 2 n

F = ma is Three Equations F and a are vectors n So F = ma equation is really three SFx = max SFy = may SFz = maz n

Examples What force is required to accelerate a 1000 kg car at 2. 0 m/s 2 ? Answer: F = ma = 1000 kg x 2. 0 m/s 2 = 2000 N. n What is the acceleration of a 145 g baseball thrown with a force of 20. 0 N? a = F/m = 20/0. 145 kg = 138 m/s 2 n

Newton’s Third Law Forces always come in pairs n Two forces on different objects n Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first n Example: hammer hits nail n

Example: pushing on wall What are the forces when you push on a wall? n You exert force on wall n You accelerate in the opposite direction n Wall must have exerted a force on you in the direction you accelerated (by 2 nd Law) n

Example: person walking n n Foot exerts force backward on ground Ground exerts force forward on foot

Example: Throwing ball n n n Pitcher exerts force on ball Ball exerts equal and opposite force on pitcher Why doesn’t pitcher move?

Example: Rocket n n Rocket engine exerts rearward force on gas molecules Molecules exert forward force on rocket.

Horse and Cart n n n Horse exerts force on cart Cart exerts equal and opposite force on horse Net force = zero Cart can’t move Huh? The net force is NOT zero. Forces on different objects cannot be added to make zero

Book on Table The mass of the book is one kg. What is the force (magnitude and direction) on the book? n 9. 8 N upward n

More Examples n Can you think of some more examples of Newton’s Third Law in Action?

Review: Newton’s Laws of Motion n Newton’s First Law: Every object continues in its state of rest, or of motion in a straight line at constant speed, unless compelled to change that state by forces exerted on it. n Newton’s Second Law: The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the body. n Newton’s Third Law: Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first

Concept Check 1 n Which of these statements about an object in free fall is false? u (a) It accelerates u (b) It is in equilibrium u (c) It exerts an upward force on the Earth u (d) It’s mass equals the net force on it divided by its acceleration u (e) Only one force acts on it

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