Ch 04 Newton's Laws of Motion

# Ch 04 Newton's Laws of Motion - LECTURE NOTE 4 Newtons Laws...

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LECTURE NOTE 4 Newton’s Laws of Motion The concept of force * Classes of force: (A) Contact forces. (B) Field forces. Contact forces arise from physical contact . Action-at-a-distance (field) forces do not require contact and include gravity and electrical forces.

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Some Common Forces (1) The Weight Force : is the force of the earth’s gravitational attraction, which pulls the object toward the earth’s centre. W = m g m and W are two different quantities . (2) Tension Forces : exerted by strings , ropes , and wires when they are stretched tight. The direction of the force is such that the string pulls the object to which it is attached. T T T T (3) Spring forces : Hooke’s law: F = k s k is the“ stiffness constant or spring constant ”, s is the length change caused by stretch (or squish) of the spring from its natural unstretched length . The minus sign in Hooke’s law reminds us that the direction of the force exerted by the spring is opposite the displacement ( stretch or squish) that produces it ( restoring force ) . A force T is being applied to the right end of a rope. The force is transmitted to the box. Forces are applied to both ends of the rope. These forces have equal magnitudes and opposite directions.
When the spring is F s compressed , the s force increases in L o direct proportion to F s the amount of comp- s ression . L o A spring with natural (unstretched) length L o L o When the spring is F s stretched , it exerts a s “restoring force in L o direct proportion to F s the amount of stretch. s L o (4) Normal Forces: Whenever two surfaces are in contact they exert forces on each other. Such forces are known as “ contact forces ”. Normal Contact component force Normal N F component N f F f Contact Force For example, the contact force on your finger by the tabletop as you slide your finger along the rough surface. The “ normal force ” is the component of the contact force that is perpendicular to the surface .

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The “normal force” is a measure of how strongly the surfaces in contact are pressed together . The force you feel resisting your push ” is the normal force of the table pushing up on your hand. (This push by the table on your hand is a spring force of sorts .) Definition of the normal force The normal force F N 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 . Example: F N =26 N push=11 N pull=11 N F N =4 N W =15 N W =15 N (5) Frictional Forces : As shown in figure, the component of contact force parallel to the contact surface is generally called the “ frictional force ”.
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Ch 04 Newton's Laws of Motion - LECTURE NOTE 4 Newtons Laws...

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