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Forces Chapter 4

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Force & Motion Force-a push or a pull on an object System-the object(s) experiencing the force Environment-the world around the system

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Contact Forces & Field Forces How are all the ways you could move your textbook across the table top. Contact Forces Jump out of a plane. Is this a contact force? Field forces

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Free Body Diagrams Pictorial model of the forces

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Forces can be applied left, right, up or down Forces are vectors (both magnitude and direction)

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Combining Forces Forces can be added to find a Net force. If there is no net force the forces are balanced and then there is no change in motion If there is a net force then the forces are unbalanced and there will be a change in motion.

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Newton’s Second Law of Motion Experiments are necessary in order to study what happens when forces are exerted Important to eliminate friction and gravity Gravity doesn’t act in the horizontal direction

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Acceleration - Force If you apply 10 N to a 5 kg object and then 20 N to the same object, which would increase the motion more?

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Acceleration - Mass If you apply 10 N of force to a 5 kg object and 10 N of force to a 50 kg object, which will have more motion?

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Newton’s 2 nd Law Acceleration depends upon both force and mass

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a- acceleration, m/s 2 F net -the sum of the forces acting on the object m-mass, kg

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The Newton One unit of force causes a 1 kg mass to accelerate at 1 m/s 2 Defined as a Newton, N SI unit for force

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Newton’s First Law of Motion An object at rest stays at rest and an object moving will remain moving until a net force acts on the object.

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Inertia The tendency of an object to resist a change in motion First law is sometimes called the law of inertia

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Equilibrium The net force acting on an object is zero Equilibrium doesn’t mean the object is at rest Equilibrium means the acceleration is zero

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Common Misconceptions When a ball has been thrown, the force of the hand that threw it remains on it. A force is needed to keep an object moving.

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Using Newton’s Laws 4.2

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Early in the 20 th century physicists discovered that the second law doesn’t hold for velocities close to the speed of light or masses the size of atoms But everyday experiences are still governed by Newton’s laws

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Mass and Weight Galileo determined that all objects gain speed at the same rate no matter their size, shape or mass In other words, they have the same downward acceleration This has been tested and is true

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Weight A force due to gravity. On Earth g=9.8 m/s 2 Weight is different on different planets, but mass stays the same F g =mg

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Scales Scales have springs that compress when a mass is placed on them The compression depends upon the pull of gravity

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Apparent Weight The force exerted on a scale. The apparent weight changes if the object is accelerating up or down other than gravity. Example-elevator

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Weightlessness Doesn’t mean your weight is zero Means that there are no contact forces pushing up on you Your apparent weight is zero

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The Friction Force The force that opposes motion Measured in Newtons Always acts in the opposite direction to motion

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Terminal Velocity Unlike friction between surfaces, the air resistance (friction) depends upon the speed. Air resistance increases as the speed increases

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Eventually the upward force of air resistance equals the downward force of gravity Net force equals zero and the acceleration is zero and terminal velocity has been reached If the air resistance changes then the terminal velocity will also change

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