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Newton’s First Law - Inertia Chapter Four

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Aristotle on Motion Greek scientist – 400 BC Two types of Motion –Natural – Straight-up motion (smoke) or straight-down motion (falling) or circular (planets) Objects would seek their “natural” resting places Not thought to be caused by forces

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Aristotle on Motion –Violent – imposed motion From push or pull External causes Examples – cart, tug-of- war Objects in natural resting positions would not move by themselves

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Copernicus and Moving Earth Astronomer – 1473-1543 Assumed earth and planets moved around the sun Used to think earth was the center of the universe Published his findings around the time of his death

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Galileo on Motion Believed Copernicus Was put under house arrest by the church Biggest contribution – force in NOT needed to keep an object moving

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Force and Friction Force – push or pull Friction – force between materials that touch –Caused by surface irregularities –Friction stops motion

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Galileo’s Experiments Only when friction is present is a force needed to get something moving Tested his idea with rolling balls on plane surfaces at different angles. Rolling down a hill, the speed increases

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Galileo’s experiments Rolling up a hill, speed decreases No slope, speed remains constant He reasoned the without friction – the ball would continue forever

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Galileo’s experiments The balls would rise up on each side the same height So what would happen if it was flat? The ball would go on forever….

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Inertia A moving body keeps moving Galileo –Concerned with how it worked – not why –Experiments (not logic) leads to knowledge

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Sir Isaac Newton Born Christmas day – in the same year as Galileo’s death Had made his famous laws by the time he was 24.

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1 st law of Motion - Inertia Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change by forces exerted on it Examples – coin on paper, dishes on table, hockey puck on air table, Pioneer and Voyager

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Question If the force of gravity between the sun and planets suddenly disappeared, what type of path would the planets follow? If the force of gravity between the sun and planets suddenly disappeared, what type of path would the planets follow? Is it correct to say that the reason an object resists change and persist in its state of motion is that it has inertia?

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Answer The planets would move in straight lines at constant speed (velocity) No, we don’t know the reason why objects persist in their motion when nothing acts on them, but we call this property inertia. We understand many things – but some things – gravity and inertia for example, are still unexplained.

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Inertia

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Mass – a Measure of Inertia Kicking a can –Empty –Filled with toothpicks –Filled with nails Amount of inertia is related to mass of object

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Mass is NOT volume Volume – is space –Cubic centimeters, cubic meters, liters Which weighs more –1 kg of feathers –1 kg of nails? Which has more volume?

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Mass is not Weight Mass – amount of matter, depends on number and kind of atoms Weight – force from gravity acting on it They are proportional to each other – but not equal to each other

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Question Does a 2 Kg iron block have twice as much inertia as a 1 kg block of iron? Twice as much mass? Twice as much volume? Twice as much weight (in the same location?)

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Answer Yes to all questions. It has twice as many atoms, so amount of matter, mass, weight will all increase.

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Question Does a 2-kg bunch of bananas have twice as much inertia as a 1 kg loaf of bread? Twice as much mass? Twice a much volume? Twice as much weight (when weighed in same location)

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Answer Two kilograms of anything will have twice the mass and inertia. In the same location, 2kg will have twice the weight as 1kg. However, volume and mass are related by density. Bananas are much more dense than bread, therefore they will occupy less space.

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Weight In USA – measured in pounds In World – measured in kilograms 1 Kg (mass) = 2.2 pounds (weight) How many kilograms do you weigh? –Your weight x 1 kg/2.2 pound =

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Force A push or a pull Measured in Newtons 1 N is about the weight of a cooked burger 1 kg weighs 9.8 N –1 kg = 9.8 N = 2.2 lbs (on earth) Weight = mass X acceleration F = ma or F = mg (gravity)

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Question 1 kg of nails weighs 9.8 N at Earth’s surface. Does 1 kg of yogurt also weigh 9.8 N at Earth’s surface?

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Answer Yes, anything that masses 1 kg will have a weight of 9.8 N.

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Net Force According to 1 st law – object either stay at rest or stay moving – unless a force changes it 5 N Is this block at rest?

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Net Force Combination of all forces acting on an object –If the net force is equal to zero then there is NO motion 5 N

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Net Force Combination The two applied forces work together to make a net force of 10 N 5 N 10 N

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Equilibrium When net forces equal zero Example-Forces on a book on a table –Gravity –Support force from the table Also called normal force Caused by the atoms in the table “pushing” back The book compresses the atoms (like a spring) –The table pushed back with the same force that gravity pulls down

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Equilibrium Example – Hanging from a rope –Atoms are not compressed – they are stretched –The force of tension = force of weight What if hanging by two ropes? –The tension is divided equally by the ropes

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Question When you step on a bathroom scale, the downward force by your feet and the upward force supplied by the floor compress a calibrated spring. In effect, the scale measures the floors support force. What will the scale read if you stand on 2 scales?

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Answer The scales will add up to your weight. If you balance your weight, the scales will read the same. If you shift your weight to one side, the scales will have different weights, but will still add up to your entire weight.

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Using force scales The scales are in equilibrium The resultant of the forces equal the downward pull 10 N 5 N

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Vector Addition of Forces Non-vertical orientation of forces –Tension is greater –Depend on angle from the vertical Use the same vector adding to calculate resultant forces

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Spring scales at an Angle 10 N 60

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Clothesline Why can a clothesline support you vertically but not horizontally? 50 N

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Clothesline Use the parallelogram method to find the force 50 N Resultant Note - as the rope becomes more horizontal resultant force would become close to infinity!!

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The Moving Earth When Copernicus first announced the idea that the Earth moved around the sun at 30 km/sec, people were skeptical Reason – How can a bird jump down out of a tree and catch a worm if the earth moves 30 km/sec while the bird is flying?

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Refute The whole environment is moving at 30 km/sec –Tree-Bird –Worm-Air 400 years ago people had trouble with high speeds Today we know the coins flip the same in a car or on the ground

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Conclusion Aristotle – didn’t recognize inertia because he thought there where different rules for heaven and earth Galileo and Newton – Recognized that all moving things follow the same rules –Things keep on moving unless a force (friction) stops it

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