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EVERY-DAY FORCES Force of gravity Normal force Force of friction Universal force of gravity

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FORCE OF GRAVITY

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Newton ’ s Law of Universal Gravitation Every particle in the universe exerts an attractive force on every other particle. A particle is a piece of matter, small enough in size to be regarded as a mathematical point. The force that each exerts on the other is directed along the line joining the particles.

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For two particles that have masses m 1 and m 2 and are separated by a distance r, the force has a magnitude given by

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INVERSE PROPORTION

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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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MASS VS. WEIGHT

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Mass is a measure of the amount of “ stuff ” contained in an object. The weight of an object on or above the earth is the gravitational force that the earth exerts on the object. The weight always acts downwards, toward the center of the earth. On or above another astronomical body, the weight is the gravitational force exerted on the object by that body. SI Unit of Weight: newton (N)

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Relation Between Mass and Weight

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4.7 THE GRAVITATIONAL FORCE On the earth ’ s surface :

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Definition of the Normal Force The normal force is one component of the force that a surface exerts on an object with which it is in contact – namely, the component that is perpendicular to the surface.

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4.8 THE NORMAL FORCE

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Apparent Weight The apparent weight of an object is the reading of the scale. It is equal to the normal force the man exerts on the scale.

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EXAMPLE A box with a mass of 10.0 kg is being pulled by a string with a force of 40.0N A) horizontally B) at an angle of 30 degrees to the horizontal with a Neglecting friction find: the acceleration of the box the magnitude of the normal force

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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.

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Note that the magnitude of the frictional force does not depend on the contact area of the surfaces.

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Static friction opposes the impending relative motion between two objects. Kinetic friction opposes the relative sliding motion motions that actually does occur. is called the coefficient of kinetic friction.

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The sled comes to a halt because the kinetic frictional force opposes its motion and causes the sled to slow down.

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Suppose the coefficient of kinetic friction is 0.05 and the total mass is 40kg. What is the kinetic frictional force?

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