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FORCES AND LAWS OF MOTION

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FORCE

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EXAMPLES OF FORCES: Close rangeLong Range Pulling the handle of the door Pushing a stroller Hitting a tennis ball with a racquet Bouncing a soccer ball Force of gravity Attraction of negative and positive particles Magnetism

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FREE BODY DIAGRAM

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Only forces ON THE OBJECT matter

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Aristotle 384 – 322 BC Believed that no motion is possible without a force. As soon as a force stops, motion stops.

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1564 - 1642 Galileo Galilei 1564 - 1642 It is as natural for an object to keep moving once it’s in motion as it is to remain at rest once it’s at rest A force will accelerate an object

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1643 - 1727 ISAAC NEWTON Sir Isaac Newton 1643-1727 Summarized motion in his three laws. Famous for his work on gravity, light and chemistry. Invented calculus.

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Which FBD represents the following: a)a hockey puck sliding on frictionless ice b)a hockey puck sliding on real ice What would the third FBD represent?

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An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. The net force is the vector sum of all of the forces acting on an object. Newton’s First Law

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Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line (constant velocity). The mass of an object is a quantitative measure of inertia. SI Unit of Mass: kilogram (kg)

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An object is resting on a flat surface An object is being pulled on a flat surface, but is not moving The very same diagram would illustrate an object that is already in motion and keeps moving at a constant speed. There are two forces applied to the object: force of gravity, directed downward, and the force from the surface. They are equal in magnitude and opposite in direction, so Force of pull is equal to force of friction.

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NET FORCE

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NEWTON’S II LAW When a net external force acts on an object with mass m, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the force.

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[NEWTON]= [N] [N] = [kg ·m/s 2 ] A net force of 1N exerted on an object with a mass of 1 kg will result in the object moving with an acceleration of 1 m/s 2

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EXAMPLE Space-shuttle astronauts experience accelerations of about 35 m/s 2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?

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EXAMPLE A block with a mass of 5 kg is being pulled across a tabletop by a force of 10 N applied by a string tied to the front end of the block. The table exerts a 2-N frictional force on the block. What is the acceleration of the block?

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NEWTON’S THIRD LAW When one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. Every action has an equal opposite reaction.

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3 RD LAW: ACTION- REACTION

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MULE - CART If a mule is pulling on a cart with a force F and there is a counter force –F, according to Newton’s third law, why does the cart start moving ?

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EDUCATED APPRENTICE

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COMPARE FORCES ON THE OBJECT Newton’s III law is about TWO objects and TWO forces that the TWO exert on EACH OTHER. Newton’s I (and II) law is about ONE object and ALL the forces exerted ON IT ALONE. So, when working on problems and drawing FBD, isolate appropriate forces to determine the nature of motion.

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GRAVITY Another example of N’s III law: two bodies, two equal and opposite forces.

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WEIGHT OF AN OBJECT The weight of an object on or above earth is the gravitational force that the earth exerts on the object. On or above another astronomical body, the weight is the gravitational force exerted on the object by that body. Weight = mg, where m- mass of the body, g – acceleration due to gravity (-9.8 m/s 2 )

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Relation Between Mass and Weight

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MASS VS. WEIGHT Mass depends on the amount of matter. Mass can change if the amount of matter changes. Weight depends on the strength of gravitational pull and it changes from planet to planet. (g is different on different planets)

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WEIGHT CAN CHANGE:

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WEIGHTLESSNESS

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NORMAL FORCE Normal force is the force that a surface exerts on the object, always to the surface (normal is not ‘natural!)

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EXAMPLES OF NORMAL FORCE Normal force = weight only if an object is at rest on a flat surface. Any additional force or motion with acceleration can change (increase or decrease) normal force. Normal force is the reaction of the surface to the force applied to it by the object.

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EXAMPLE A 10.0-kg box is resting on a 20.0 o incline. Determine the normal force acting on the incline. Find the force of friction acting on the incline. If the incline is frictionless, find the acceleration of the box.

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FORCE OF FRICTION When an object is in contact with a surface there is a force acting on that object. The component of this force that is parallel to the surface is called the frictional force.

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STATIC FRICTION When the two surfaces are not sliding across one another the friction is called static friction. Static friction is a ‘lazy’ force: it will only be as large as it needs to. Static friction has a maximum value. If pull >>, the object will move

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Note that the magnitude of the frictional force does not depend on the contact area of the surfaces. It depends on the roughness / smoothness of the two surfaces in contact.

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