chapter04 - Force Newtons Three Laws of Motion The...

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1 •Force •Newton’s Three Laws of Motion •The Gravitational Force •Contact Forces (normal, friction, tension) •Application of Newton’s Second Law •Apparent Weight •Air Resistance •Fundamental Forces
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2 Isaac Newton was the first to discover that the laws that govern motions on the Earth also applied to celestial bodies. Over the next few chapters we will study how bodies interact with one another.
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3 Simply, a force is a “push” or “pull” on an object.
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4 How can a force be measured? One way is with a spring scale. By hanging masses on a spring we find that the spring stretch applied force. There are scales using springs The units of force are Newtons (N).
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5
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6 The net force is the vector sum of all the forces acting on a body. (principle of superposition) 3 2 1 F F F F F net
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8 Two forces are acting on an object. F 1 = 8 N, to the right ; F 2 = 6 N, up . What is the magnitude of the net force? A. 2 N B. 10 N C. 14 N
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9 Free Body Diagrams: FBD •Must be drawn for problems when forces are involved. •Must be large so that they are readable. •Draw an idealization of the body in question (a dot, a box,…). You will need one free body diagram for each body in the problem that will provide useful information for you to solve the given problem. •Indicate only the forces acting on the body. Label the forces appropriately. Do not include the forces that this body exerts on any other body.
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10 Free Body Diagrams continued : FBD •A coordinate system is a must. •Do not include fictitious forces. •You may indicate the direction of the body’s acceleration or direction of motion if you wish, but it must be done well off to the side of the free body diagram.
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11 Example FBD : The forces on an airplane in flight heading eastward are gravity = 16.0 kN (kiloNewton) downward; lift=16.0kN, upward, thrust = 1.8kN,east and drag=0.8kN west (Lift, thrust and drag are 3 forces that the air exerts on the plane ). What is the net force On the plane. Don’t forget: Forces are vectors they have direction
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12 Newton’s 1 st Law (The Law of Inertia ): If no force acts on an object, then its speed and direction of motion do not change. Inertia is a measure of an object’s resistance to changes in its motion .
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13 Example of Inertia : Shoveling Snow In order for the shoveling to be facilitated, there is a time when the snow is moving on its own. Without the shovel pushing it . First we swing the shovel forward Both the snow and the shovel move Forward Then suddenly the motion of the Shovel stops and the snow continues To move forward due to Inertia Another example : The body during a sudden change in velocity of the car.
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