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

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

2 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

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

4 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.

5 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

6 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”

7 Newton’s 1 st Law

8 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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10 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/

11 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.

12 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.

13 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

14 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)

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

16 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.

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

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19 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

20 F perpendicular = F N (normal)

21 Together these 2 forces replace the force of gravity

22 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)

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

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25 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

26 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

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

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

29 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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31 Newton’s 3 rd Law  For every action there is an equal but opposite reaction

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