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Friction, Gravity, and Elastic Forces PH 380 Previously, in physical science… • You learned about forces and combining forces. Now, you continue to combine forces and identify two or more forces acting on an object. Std 2d Students know how to… • Identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. 1. What is friction? The force that two surfaces exert on each other when they rub against each other. Byproduct of friction…Heat! 2. What direction does friction act? • Acts in direction opposite to the objects in motion. 3. How do you show friction as a force? • An arrow opposite of the object’s motion Friction force Motion 4. What determines the strength of friction? The types of surfaces (smooth or rough) and how hard the surfaces push together 5. What are 4 types of friction? a. Static friction: acts on object NOT moving: b. Sliding Friction Two solid surfaces slide over each other ex: book on desk c. Rolling Friction Object rolls across a surface ex: ball bearings d. Fluid Friction • Solid object moves through a fluid ex: oil, water, air (what do you use in your car to reduce friction?) • What are two ways in which friction can be useful? • allows objects to remain in place! • allows moving objects to stop to avoid colliding with other objects! 6. What is gravity? • The force that pulls objects toward each other 7. How do you show gravity as a force on Earth? • An arrow toward the center of Earth Gravity http://youtube.com/watch?v=L55jImEhNZ0 8. What is weight? • The gravitational force exerted on an object 9. What is the acceleration due to Earth’s gravity? • 9.8 m/s2 What is your weight (on Earth)? Weight = Mass × 9.8 m/s2 Weight = ? 10. What is free fall? • When the force of gravity alone causes an object to accelerate downward. • Causes an object to accelerate downward at 9.8 m/s2 11. What is air resistance? • Type of fluid friction as an object moves through air. • Depends on speed and surface area Air resistance Air resistance Free fall: Free fall: Gravity Net Force accelerate downward Air resistance Terminal velocity: Terminal velocity: Gravity 0 net force Move at constant speed Air resistance Gravity Net Force Air resistance Let’s drop coffee filters (again)! Leave one coffee filter open Crumple the other coffee filter Drop them at the same time, what happens? Draw a diagram of your observations Draw in your force arrows, including gravity and air resistance. Fluid friction Gravity Air resistance Rolling friction Gravity 12. When is matter elastic? • When it returns to its original shape after it is squeezed or stretched. …all matter has some degree of elasticity… 13. What are 2 types of elastic forces? • Compression: when an elastic force squeezes or pushes matter together. ex: couch cushion • Tension: when an elastic force stretches or pulls matter apart ex: guitar string Sometimes the tension forces can be great…like when catching a marlin… …and sometimes not so great. Compression forces squeeze together…like the shocks on a car Think of compression on a sponge…or a Twinkie