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“ If I have seen farther than others, it is because I have stood on the shoulders of giants.” Sir Isaac Newton (1642 – 1727) Physicist

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Forces and the Laws of Motion 4-1 Changes in Motion Force – a push or pull applied to an object Unbalanced forces cause objects to accelerate. Cause objects to start moving, stop moving or change direction. SI Unit of Force is the Newton – 1 N = the amount of force acting on a 1 kg mass, produces an acceleration of 1m/s 2 1 N = 1kg x m/s 2

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Forces can act through contact or without physical contact Contact force – force through touch (contact) Field Force – electric and magnetic fields, gravity

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FORCES COME IN PAIRS When a force is applied to an object, then the object applies a counterforce. Force is a Vector – magnitude and direction. Force Diagrams used to show all forces on object. Isolate the object – only show forces acting on it.

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F N – normal force (always perpendicular to the surface the object is touching) F g - force of gravity (weight of object – always directed straight down) F a – applied force (forced applied to object by person or another object) F f - force of friction (usually opposes motion of object) Free Body Diagrams – FBD’s Review: Free Body Diagrams

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4-2 Newton’s First Law “ An object at rest remains at rest and an object in motion remains in motion unless acted upon by an outside force” Sometimes called “Law of Inertia” – the mass of an object (not it weight) causes it to resist changes in motion. Inertia – “laziness”

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Newton’s 1 st Law

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1 Mass is a measure of inertia – easier to accelerate an object with less mass. Objects are in equilibrium when they are either at rest or moving at constant velocity - The net external forces are zero.

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Mass and Weight are different! Mass never changes Weight depends on the gravitational pull on 2 objects. Weight = mass of an object times the force of gravity (g) Formula: W = m x g Weight is a force so the units of weight are in Newton's (N) F g (force due to gravity) = m x g m.edu/ronh/weight/

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Astronauts on the orbiting space station are “weightless” because... a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum. c. space is a vacuum and there is no air resistance in a vacuum. d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.

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Astronauts on the orbiting space station are “weightless” because... a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum. c. space is a vacuum and there is no air resistance in a vacuum. d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.

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Find your mass in kilograms using the conversion: ___lbs = 1 kg 2.2 lbs Then Determine how much you weigh in Newton's. W = mass x gravity (m x g) = ______________ N

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Newton’s 2 nd Law A net Force on an object will cause it to accelerate. Formula: F = m x a Units of Force: = kg x m/s 2 = Newton (N)

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Use equation for weight (force of gravity): or Rearrange and get

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Falling and Air Resistance A. Air resistance decreases the net forces acting on a falling object 1. When air resistance equals downward force on falling object (force of gravity– also called weight) then net force is zero and no further acceleration occurs.

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terminal speed– when acceleration terminates we call this maximum speed terminal velocity FgFg F drag

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mass F g = mg F II F ϴ The F g (force in Newton’s due to gravity) can be shown as two useful vectors F II and F

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F perpendicular = F N (normal)

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Together these 2 forces replace the force of gravity

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mass F g = mg F II = m x g sin F = m x g cos ϴ ϴ Using the law of similar triangles and your trigonometric functions we can solve for F II and F Sin = opp / hyp = F II / m x g Cos = adj / hyp = F / m x g Hypotenuse = F g = m x g F II = m x g sin - parallel to the plane (x axis) F = m x g cos - the normal force that acts perpendicular to the plane (y axis)

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The net force is the vector sum of all the forces.

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Static friction = friction between surfaces - Force required to get something moving Each surface has its own “ coefficient of friction ” Coefficient of Static Friction ratio of the static friction to the normal force (perpendicular force) μ s = F s / F 2 Types of Friction: Static and Kinetic

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Kinetic Friction ( μ k ) = friction between moving surfaces. Examples: Sliding, rolling, fluid friction (air or liquid) Coefficient of Kinetic Friction ( μ k ) = ratio of kinetic friction to the normal force (perpendicular force) μ k = F k / F

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Coefficient of Friction – it’s a Decimal – percent of the weight required to move an object

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Example: How much force does it take to move a 100 N object that has a μ k =.33

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Example: How much force does it take to move a 100 N object that has a μ k =.33 Given: Weight: 100 N (F N ) = F µk =.33 Answer:. 33 x 100N = 33 N

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Newton’s 3 rd Law For every action there is an equal but opposite reaction

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