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Download Newton`s Second Law of Motion Chapter 5 Force and Acceleration
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Transcript
Newton’s Second Law of Motion Chapter 5 Force and Acceleration Force causes Acceleration • Remember: change in velocity acceleration = time interval Acceleration is any change in velocity. 5.1 FORCE CAUSES ACCELERATION There is always MORE than one force acting on an object at one time. Net Forces Definition: The combination of all the forces that act on an object is the NET FORCE. The NET FORCE accelerates objects. ~ means “is proportional to” = means “is equal to” ACCELERATION ~ NET FORCE Examples of Proportion Proportion means that as one changes, the other changes at the same rate. EX: The length of your spine is PROPORTIONAL to your height. Think about a grocery cart… When the cart is empty, it is easy to push. The cart easily accelerates with simple pushing. What about when the cart is full and the mass is greater? The greater the mass of the cart, the harder you have to push. What is different with the 2 carts? MASS 5.2 Mass Resists Acceleration The more mass an object has, the less acceleration you can get – using the same force. In other words… Acceleration is INVERSELY proportional to mass, or… ACCELERATION ~ 1 / MASS 5.3 Newton’s Second Law “The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body.” 5.3 Newton’s Second Law ACCELERATION ~ However, . . . NET FORCE MASS If we use these units: Net force measured in Newtons Mass measured in kilograms Acceleration measured in m/s/s we can also say… Acceleration = Net Force Mass 5.3 Newton’s Second Law Using variables, we get the equation: A = F m Rearranging the equation, we can also say: Force = (mass) X (acceleration) or F = ma But remember… You must convert your data into the following units: kg for mass m/s/s for acceleration N for net force 5.4 Friction Definition: …the force that acts to resist the relative motion (or attempted motion) of objects or materials that are in contact. Fluids If an object is a “fluid” it is a liquid or gas. Fluids can flow. Air resistance Air resistance is any friction acting on something moving through the air. 5.4 Friction • Free body diagram – a diagram showing all the forces acting on an object. 5.5 Applying Force - Pressure Definition: …the force per unit of surface area where the force is perpendicular to the surface. Pressure is measured in PASCALS. In other words… Pressure = Force Area of application --or— Pressure = F (in Newtons) A (in meters2) Pressure Measured in “Newtons per Square Meter” Or N 2 m N 2 m = PASCALS or Pa 5.5 Pressure • Two books laying on a table: –One is upright; the other is face down Do they have the same force? Do they have the same pressure? (Fig. 5.7) 5.6 Free Fall Explained • Free fall = falling free of air resistance • Acceleration is CONSTANT does not change) • Remember: g = 10 m/s2 (it Ratios • Ratios are “in proportion” to each other. • F M F = M Example of a Ratios 12 24 = 6 12 = 2 4 = 1 2 This is why a 10 kg cannon ball flying out of a cannon has proportionate force (weight) / mass as a 1 kg baseball flying tossed by a child. Example page 48 5.7 Falling and Air Resistance TERMINAL SPEED – the speed at which the acceleration of a falling object terminates because friction balances the weight. TERMINAL VELOCITY – terminal speed together with the direction of motion (down for falling objects) 5.7 Falling and Air Resistance • A lighter object will meet terminal speed sooner than a heavier object. • A heavier object will continue to accelerate until it reaches a higher terminal speed. Chapter 5 Key Terms Air resistance Fluid Free-body diagram Inversely Newton’s Second Law Pascal Pressure Terminal speed Terminal velocity