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Phys141 Principles of Physical Science Chapter 3 Force and Motion Instructor: Li Ma Office: NBC 126 Phone: (713) Webpage: Department of Computer Science & Physics Texas Southern University, Houston Sept. 15, 2004

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Topics To Be Discussed Force and Net Force Newton’s First Law of Motion Newton’s Second Law of Motion Newton’s Third Law of Motion Newton’s Law of Gravitation Momentum

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Cause of Motion A push causes something to move This push is the application of a force Force and Motion: Cause and Effect Galileo did experiments on moving objects Newton formulated the laws of motion and explained the phenomena of moving objects on the Earth and the motions of planets

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Force Easy to describe force Define force in terms of what it does: –A force can produce changes in motion –A force can produce a change in velocity (speed and/or direction), or cause a acceleration –Observed motion is evidence of a force A force is a quantity that is capable of producing motion or a change in motion

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Net Force A force’s capability may be balanced or canceled by other force(s): the net effect is then zero More than one force acts on an object: –unbalanced/net force: tug of war Forces are vector quantities Only net force can cause change in motion

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Newton’s First Law of Motion The natural state of motion: –Aristotle: the natural state of an object is being at rest – no idea of friction –Galileo: objects could naturally remain in motion rather than come to rest Newton’s first law of motion –An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force

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Newton’s First Law of Motion (cont) External force: an applied force Internal force: can not change the state of motion Friction and Gravity on the Earth make it difficult to observe an object in a state of constant velocity

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Motion and Inertia Inertia: natural tendency of an object to remain in a state of rest or in uniform motion in a straight line - Galileo Mass is a measure of inertia – Newton –The greater the mass of an object, the greater is its inertia, the greater is its resistance to a change in motion Newton’s first law: Law of inertia

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Newton’s Assumptions of Acceleration The acceleration produced by an unbalance force acting on an object (or mass) is directly proportional to the magnitude of the force (a ∞ F) and in the direction of the force The acceleration of an object being acted on by an unbalance force is inversely proportional to the mass of the object (a ∞ 1/m)

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Newton’s Assumptions of Acceleration (cont) Combining these effects of force and mass on acceleration: unbalanced force acceleration ∞ mass or a ∞ (F / m)

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Newton’s Second Law of Motion a = F / m or F = m·a F is the net force m is the total mass Unit of the force is newton in metric system: 1 N = 1kg·m/s 2

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Example Given: –m 1 =1.0kg, F 1 =-5.0N (left, negative direction) ; –m 2 =1.0kg, F 2 =+8.0N (right, positive direction) ; Wanted: a (acceleration) Equation: a = (F 1 +F 2 )/(m 1 +m 2 ) a = (+8.0N-5.0N)/(1.0kg+1.0kg) = +1.5m/s 2 m kg m kg a F 1 = -5.0N F 2 = +8.0N

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Mass and Weight Mass is the amount of matter an object contains, or a measure of inertia Weight is related to the force of gravity (gravitational force acting on an object) They are related: weight = mass x acceleration due to gravity w = m·g

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Newton’s Third Law of Motion The law of action and reaction For every action there is an equal and opposite reaction Whenever one object exerts a force on a second object, the second object exerts an equal (in magnitude) and opposite (in direction) force on the first object action = opposite reaction F1 = - F2

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Comparing Newton’s Second & Third laws Newton’s third law relates two equal and opposite forces acting on two different objects Newton’s second law concerns how forces acting on a single object can cause an acceleration

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Newton’s Law of Universal Gravitation Gravity: a common fundamental force in nature Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them F ∞ (m 1 m 2 / r 2 )

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Newton’s Law of Gravitation (cont) F = (G m 1 m 2 / r 2 ) G is the universal gravitational constant G = 6.67 x N·m 2 /kg 2 Why objects fall to the ground of the Earth, Earth doesn’t move? Why we can’t feel attraction from book? Astronauts in space shuttle orbiting the Earth are weightless?

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Linear Momentum Product of mass and velocity Linear momentum is a vector, in direction of velocity If there is no external net force, linear momentum is conserved linear momentum = mass x velocity p = m·v

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Linear Momentum (cont) Law of conservation of linear momentum: –The total linear momentum of an isolated system remains the same if there is no external unbalanced force acting on the system Example of the conservation of linear momentum: man jump out from the boat

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Angular Momentum Angular momentum arises when objects go in the paths around a fixed point The angular momentum of a system can be changed by an external unbalanced torque L = m·v·r r = distance of object from center of motion

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Angular Momentum (cont) A torque is a twisting effect caused by one or more forces A torque tends to produce a rotational motion r F T = F·r v

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Angular Momentum (cont) Law of conservation of angular momentum: –The angular momentum of an object remains constant if there is no external unbalanced torque acting on it Example of the conservation of angular momentum: ice skaters spin on the ice

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