FORCES Types of Friction, Newton’s Laws of Motion, Gravity, and Types of Forces FRICTION • Friction is all around us. Caused by the minor bumps and jagged edges in all surfaces there is no way for humans to escape it! • There are 4 types of friction: static, sliding, rolling and fluid. Each of these have their own unique application and advantages. ENERGIZER • In your own words, write a definition of the word friction. What is one way that friction can be increased? Friction: A Force Opposing Motion • Friction is a force that acts in a direction opposite to the motion of the moving object causing the moving object to slow down and finally stop. • Friction arises from the fact that objects and surfaces are not perfectly smooth. • The amount of friction between two surfaces depends on how hard the surfaces are forced together and on the materials of which the surfaces are made. Sliding friction • When solid objects slide over each other, they produce sliding friction. In order to move the object you must exert a force larger than the force of friction. • The amount of sliding friction depends on: • weight of the moving object. • type of surface that the object is sliding across. Rolling friction • When an object rolls over a surface rolling friction is produced • Produced by objects such as wheels and ball bearings • it tends to be less than that of sliding friction. Fluid friction • As an object moves through a fluid it produces fluid friction • fluid friction is less than sliding friction. • Air resistance is an example of fluid friction caused by the particles that make up air. • All liquids and gases are fluids • Friction also exists when an object moves across or through a fluid. • Substance called lubricants (oil, wax, and grease), change sliding friction to fluid friction, thus reducing friction. Static Friction • When a force is applied to an object but does not cause it to move. • The force of static friction balances the applied force. • Static friction disappears as soon as an object begins to move. Friction: Is it helpful or harmful? • An example: • In your car friction can be both helpful and harmful, here is how: • When you press on the breaks you cause friction between them which HELPS slow down your car • In this same car there is friction between the moving parts in the engine. This friction causes the parts to wear out over time and therefore they must be replaced. • Friction between your foot and shoe can make holes in your socks! • Friction between your shoe and the floor helps you to stand upright. • Friction is not always a problem . . . as a matter of fact, in many cases it can be helpful. •How might we lessen the amount of slide friction? ENERGIZER • Give an example of how friction can be harmful when running a car. Then give and example of how it could be helpful. Try to identify the type of friction involved in your example. Newton’s Laws of Motion Newton’s Laws of Motion • Research Newton’s 2nd and 3rd Laws of Motion. • Create a poster, skit, song or poem about each law. • A poster should include a definition, picture, formulas as appropriate and color • Skits should include a definition and a real life demonstration of each law • Songs and poems should include a definition and real life example of each law. • Have fun, be creative and be ready to present tomorrow! Newton’s First Law of Motion or The Law of Inertia An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted on by an unbalanced force. Newton’s Second Law • The acceleration of an object depends on the mass of the object and the amount of force applied. •F=mxa Part 1 • Acceleration Depends on Mass • More force is needed to accelerate objects that have more mass. • If the same force is applied to an object with a large mass vs. a small mass the smaller object will go farther because it is easier to accelerate. 40 N 40 N Part 2 • Acceleration Depends on Force • If you put more force on the same size object it will move faster. • An objects acceleration increases as the force on the object increases. • F=mxa Newton’s Third Law • Whenever one object exerts a force on another object, the second object exerts and equal and opposite force on the first. • All forces act in pairs. • When a force is exerted and equal and opposite force occurs (sitting in a chair). • Without action/reaction forces there would be no motion. • These 2 forces act on different objects. Action-reaction force pairs make it possible for fish (or humans) to swim. • A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite to the direction of the force on the fish (forwards). Types of Forces Contact forces are types of forces in which the two interacting objects are physically contacting each other. • Applied Force • Frictional Force • Tensional Force • Normal Force • Air Resistance Force (which is a type of frictional force) • Spring Force Action-at-a-distance forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite a physical separation. • • • • Gravitational Force Electrical Force Magnetic Force Centripetal Force APPLIED FORCE – PUSH OR PULL FRICTIONAL FORCE – GOES AGAINST THE DIRECTION OF THE MOTION TENSIONAL FORCE –WHEN THINGS ARE PULLING IN OPPOSITE DIRECTION NORMAL FORCE – OPPOSES THE GROUND AIR RESISTANCE FORCE – TYPE OF FRICTIONAL FORCE SPRING FORCE Recognizing Forces Choose from these forces • Applied Force • Frictional Force • Tensional Force • Normal Force • Air Resistance Force • Spring Force • Gravitational Force • A skydiver (who hasn't opened his parachute yet) falls at terminal velocity. Consider the forces acting on the skydiver. • A block rests on top of a table. Consider only the forces acting upon the block. • A block slides across the top of a table. Consider only the forces acting upon the block. Recognizing Forces Choose from these forces • Applied Force • Frictional Force • Tensional Force • Normal Force • Air Resistance Force • Spring Force • Gravitational Force • A block hangs at rest from the ceiling by a piece of rope. Consider the forces acting on the block. • A ball is shot into the air with a spring-loaded cannon. Consider the forces acting on the ball while it is in the air. • The wheels of a car are locked as it skids to a stop while moving across a level highway. • A car is attached by a cable to a moving truck and is being pulled along a level highway. Consider only the forces acting on the car. Force Diagram Force Diagrams Applied Force Force of Gravity Normal Force Force of Friction ENERGIZER ANSWER TODAY’S EQ IN A FEW SENTENCES AND EXPLAIN YOUR ANSWER: WHEN YOU ARE SKYDIVING ARE YOU IN FREE FALL? Gravity Myth or Fact?? 1. Gravity exists only on Earth. (Myth) (Fact) Gravity also affects objects in space. In fact, objects stay in orbit because of gravity. Without gravity, a satellite launched from the Earth would simply drift off endlessly into space, traveling in a straight line, instead of circling the planet. Gravity pulls objects toward the center of the planet, causing them to accelerate and drop toward the planet Gravity Myth or Fact?? 2. Weight and mass are the same. (Myth) (Fact) Weight and mass are not the same. Mass is a measure of a body's resistance to changes in its state of motion, which depends on the amount of matter it contains. Weight is the force of gravity exerted on a body due to its mass and its location near another, more massive object. An astronaut in space can be weightless but cannot be without mass. Gravity Clip • http://www.youtube.com/watch?v=HlAEGJ4TNm0 • How does this clip tie into gravity and relate to the Energizer and EQ with sky diving? Why does an apple fall towards the ground when dropped? • Isaac Newton asked this question in 1665. • The answer to this question gave birth to a law… The Law of Universal Gravitation • All objects in the universe attract each other through gravitational force. • The size of this force depends on the masses of the objects and the distance between them. • So… • Gravitational force increases as mass increases. • Gravitational force decreases as distance increases. How Mass Affects Gravitational Force: How Distance Affects Gravitational Force: • On Earth, all objects experience a "downward" force of gravity. • The force of gravity on an object is always equal to the weight of the object. • Gravity is of great importance in the interaction of all objects in the universe; • it binds us to the Earth, • holds the Earth and all planets in the solar system, Gravity is what holds the universe together Gravity in Motion • All objects fall to the ground at the same RATE because the acceleration due to gravity is the same for all objects. • This is because heavier objects experience more gravitational pull but are also harder to accelerate than a lighter object. Acceleration Due to Gravity • ALL objects accelerate toward earth at a rate of 9.8 m/s/s. • How do we calculate an objects change in velocity during free fall? • Change in velocity= g x t • g=acceleration due to gravity • t=time VELOCITY DUE TO GRAVITY PROBLEM •A boy standing on a high cliff dives into the ocean below and strikes the water after 3 seconds. What is the boy’s velocity when he hits the water? • V=gxt • V = 9.8 m/s/s x 3 s • V = 29.4 m/s downward One more • A foul ball is hit straight up in the air and falls from the top of its motion for 1.4 seconds before being caught by the catcher. What is the velocity of the ball as it hits the catcher’s glove? • V=g x t • V=9.8 m/s/s x 1.4 s • V= 13.72 m/s downward A few more things on acceleration due to gravity • All objects fall to the ground at the same rate; HOWEVER, differences in the size, shape and speed of an object can increase or decrease air resistance. • As speed increases so does air resistance • At a certain point the two will equal out and the object will fall at its terminal velocity (a constant rate). Quick Quiz 1. 2. 3. What is the difference between mass and weight? What must you know in order to calculate the gravitational force between two objects? Name three common items that could be used to increase friction. Weight Weight is • a measure of the force of gravity on an object • Metric or SI unit is the Newton (N) •Your weight varies according to the force of gravity pulling on you •The force of gravity varies according to the distance the object is from the center of the Earth. - You weigh less on top of a mountain than you do at sea level. ENERGIZER • A girl goes to the doctor and finds that she weighs 95 pounds. What is her weight in Newtons? • M = lb / 2.2 • M = 95/2.2 • M = 43.18 kg • W = kg*9.8 • W = 43.18*9.8 • W = 423.16 N Free Fall Terminal Velocity Air Resistance Gravity gravity Projectile Motion: Horizontal motion (inertia) Gravity Actual path of baseball ORBITING OBJECTS INVESTIGATION • Swing the stopper on a string around in a circle observing all of the forces acting on the stopper. (Make sure to put space between you and your neighbor.) • Draw a picture of the stopper being swung in a circle. • Label all forces with direction acting on the stopper from the moment it begins to swing. Orbital Motion: Forward motion (inertia) of the space shuttle Gravity Centripetal Force • Centripetal acceleration: acceleration toward the center of a curved or circular path • When the net force is toward the center of the curved path this is called centripetal force. • Gravity can be considered a centripetal force when objects are in orbital motion. Calculate your weight • Weight (N) = Mass (kg) x 9.8 • Your mass (kg) can be found in two ways…. • Mass = your pounds ÷ 2.2 • There are 2.2 lbs in 1 kg • Mass = your pounds x 0.4545 • There is about 0.4545 kg in 1 pound ENERGIZER If Wile E. Coyote and a boulder fall off a cliff at the same time, which do you think will hit the ground first? Label the forces involved in Tom Brady’s winning Touchdown throw!