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For a mass on a string to travel in a circle, a force must act along the string to overcome inertia. Without that force, inertia makes the mass move in a straight line. Newton's First Law of Motion “The Law of Laziness” Newton's first law of motion is often stated as: • An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Newton's First Law of Motion There are two parts to this statement – one which predicts the behavior of stationary objects and the other which predicts the behavior of moving objects. These two parts are summarized in the following diagram: Pass the Water exercise • the container is at rest and you attempt to move it • the container is in motion and you attempt to stop it • the container is moving in one direction and you attempt to change its direction. The Car and The Wall The Motorcyclist Newton's First Law of Motion • blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. • the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. • a brick is painlessly broken over the hand of a physics teacher by slamming the brick with a hammer. (CAUTION: Do not attempt this at home!) • to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted. • headrests are placed in cars to prevent whiplash injuries during rearend collisions. • while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard. Newton's First Law of Motion Now it’s your turn! Inertia and Mass: Things in motion stay in motion – forever! • Inertia is the resistance an object has to a change in its state of motion. Friction slows things down… Isaac Newton built on Galileo's thoughts about motion. Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a stop. Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). Check Your Understanding • Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will … • An 2-kg object is moving horizontally with an acceleration of 4 m/s2. What is the net force acting on the object? • If you were in a weightless environment in space, would it require a force to set an object in motion? Newton's First Law of Motion State of Motion Inertia is the tendency of an object to resist changes in its velocity. The Truck and Ladder Inertia is the tendency of an object to resist accelerations. Mass – A measure of inertia • Mass is not Volume • Mass is not weight 1 kg = 9.8 Newtons • Weight is the force of gravity on something that has mass: – Weight (N) = mass x a (gravity) = m (9.8) kg-m/s2 • Mr. Wegley spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Newton's First Law of Motion Balanced and Unbalanced Forces Newton's First Law of Motion Balanced and Unbalanced Forces Types of Forces All forces (interactions) between objects can be placed into two broad categories: – contact forces, and – Long-Range forces resulting from action-at-a-distance Force is a quantity which is measured using a standard metric unit known as the Newton. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s2. Contact Forces Action-at-aDistance Forces Frictional Force Gravitational Force Tensional Force Electrical Force Normal Force Magnetic Force Air Resistance Force Applied Force Spring Force For each situation, determine the net force acting upon the object. Newton's Second Law of Motion Check Your Understanding 1. What acceleration will result when a 12-N net force is applied to a 3-kg object? A 6-kg object? 2. A net force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass. 3. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the object is doubled, what is the new acceleration? 4. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the object is halved, what is the new acceleration? Free Fall with no Air Resistance Free Fall with Air Resistance • As an object falls through air, it usually encounters some degree of air resistance. • Air resistance is the result of collisions of the object's leading surface with air molecules. • The actual amount of air resistance encountered by an object depends upon a variety of factors. Free Fall with Air Resistance Why?? • The elephant encounters a smaller force of air resistance than the feather and therefore falls faster. • The elephant has a greater acceleration of gravity than the feather and therefore falls faster. • Both elephant and feather have the same force of gravity, yet the acceleration of gravity is greatest for the elephant. • Both elephant and feather have the same force of gravity, yet the feather experiences a greater air resistance. Free Fall with Air Resistance Skydiving The amount of air resistance is dependent upon two variables: •the speed of the skydiver, and •the cross-sectional area of the skydiver. Newton’s Third Law: Action-Reaction A force is a push or a pull upon an object which results from its interaction with another object. Forces result from interactions! You can’t touch without being touched… Newton’s Third Law: Action-Reaction "For every action, there is an equal and opposite reaction.“ Forces always come in pairs – equal and opposite actionreaction force pairs. Newton’s Third Law: Action-Reaction In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. Newton’s Third Law: Action-Reaction According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs — known as "actionreaction force pairs." Newton’s Third Law: Action-Reaction Identify at least five pairs of action-reaction forces in the following diagram. Friction Force • Friction acts in opposite direction of applied force. • Friction force increases as applied force increases. • Friction caused by molecular interaction between two rough surfaces. Ffriction = μ x Fnorm • μk = coefficient of kinetic friction • μs = coefficient of static friction Static friction/Kinetic friction • Static friction- Force required to get an object moving • Kinetic friction-Force necessary to keep an object moving • Static friction is greater than kinetic friction • Videos- phone book friction, why bart and lisa need school Friction Force A shoe (mass of 1.5kg) is pulled at constant speed across concrete by its laces (µ= .65). The shoes has a rubber sole. What force is needed? 1. 2. 3. 4. 5. Draw free body diagram List “knowns” Find the correct (μ) if needed. Solve by plug and chug. Double check for direction and magnitude. Friction practice • A boy exerts a 36-N horizontal force as he pulls a 52 N sled across a cement sidewalk at constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners? • Suppose the sled runs on packed snow. The coefficient of friction is now only 0.12. If a person weighing 650N sits on the sled, what force is needed to pull the sled across the snow at constant speed? Now you try • What force is needed to start moving a wooden packing crate on a wooden jetty? The crate has a mass of 225 kg(μs =.50 μk = .20) • • What force is needed to keep the packing crate moving at a constant speed on the wooden jetty? The crate has a mass of 225 kg. Check your understanding… • When you walk along a floor, what pushes you along? • When you jump up, what is the action reaction pair? • How do rockets travel in space when there is not “air to push on”? • Why can’t you hit a feather in midair with a force of 200 N? • What would a convict’s last wish before a firing squad? Think and explain…. • An aircraft gains speed during takeoff due to the constant thrust of its engines. When is the acceleration during takeoff greatest – at the beginning of the run along the runway or just before liftoff? • Why does a sharp knife cut better than a dull knife? • Why is it easier to walk on a carpeted floor than on a smooth, polished floor?