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Transcript
Chapter 12 Forces and Motion 12.1 Forces What is a force? Force-a push or pull that acts on an object A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction 12.1 Forces Measuring Force Tools are used to measure forces • Ex: a spring scale in a grocery store Units of force • Force is measured in newtons (N) • One Newton is the force that causes a 1.0 kg mass to accelerate at a rate of 1.0 m/s2 • 1.0 N = 1 kgm/s2 • The newton is named after Sir Isaac Newton (16421727), who explained how force, mass, and acceleration are related 12.1 Forces Representing Force Force is a vector • You can use an arrow to represent a force Combining Forces Forces acting in the same direction add together and forces acting in opposite directions subtract from one another • Net Force- the overall force acting on an object after all the forces are combined 12.1 Forces Combining Forces (cont.) Balanced Forces- are forces that combine to produce a net force of zero • When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion Unbalanced Forces- forces that result when the net force acting on an object is not equal to zero • When an unbalanced force acts on an object, the object accelerates 12.1 Forces Friction- a force that opposes the motion of objects that touch as they move past each other There are four main types of friction: • • • • Static friction Sliding friction Rolling friction Fluid friction 12.1 Forces Static Friction- the friction force that acts on objects that are not moving Always acts in the direction opposite to that of the applied force Once an object is moving, static friction no longer acts on the object Sliding Friction- a force that opposes the direction of motion of an object as it slides over a surface Sliding friction is less than static friction, so less force is needed to overcome it Less force is needed to keep an object moving than to start it moving 12.1 Forces Rolling Friction- the friction force that acts on rolling objects For a given set of materials, the force of rolling friction is 100-1000 times less than the force of static or sliding friction • Ball bearings are often used in machines to reduce the amount of friction acting on the parts • Rolling friction replaces sliding friction 12.1 Forces Fluid Friction- the force that opposes motion of an object through a fluid Fluid friction increases as the speed of the object moving through the fluid increases • The faster the object the greater the force of the fluid friction Air resistance- fluid friction acting on an object moving through the air 12.1 Forces Gravity- a force that acts between any two masses Gravity is an attractive force, that is it pulls objects together Earth’s gravity acts downward toward the center of the earth 12.1 Forces Gravity Falling Objects • Gravity causes objects to accelerate downward (at 9.8 m/s2), whereas air resistance acts in the direction opposite to the motion and reduces acceleration • The greater the surface area of an object the more influence air resistance will have on the object as it falls • Terminal velocity- the constant velocity of a falling object when the force of air resistance equals the force of gravity 12.1 Forces Projectile Motion- the motion of a falling object (projectile) after it is given an initial forward velocity Air resistance and gravity are the only forces acting on a projectile The combination of an initial velocity and the downward vertical force of gravity causes the ball to follow a curved path 12.2 Newton’s First and Second Laws of Motion Scientists that helped us understand the relationships between force and motion Aristotle • Ancient Greek philosopher (384 BC-322 BC) • Many discoveries through careful observation and logical reasoning • Not always correct 12.2 Newton’s First and Second Laws of Motion • Galileo • Italian scientist (1564-1642) • Experimented to answer questions • Newton • English scientist (1643-1727) • Built on the work of other scientists, like Galileo • Lots of work with force and motion 12.2 Newton’s First and Second Laws of Motion Newton’s First Law of Motion States that the state of motion of an object will remain the same until an outside, unbalanced force acts on it • An object at rest will stay at rest • An object in motion will stay in motion • Inertia- the tendency of an object to resist a change in its motion 12.2 Newton’s First and Second Laws of Motion Newton’s Second Law of Motion States that the acceleration of an object is equal to the net force acting on the object divided by the mass of the object • Acceleration = Net force / mass • a = F/m 12.2 Newton’s First and Second Laws of Motion Examples using Newton’s second Law formula An automobile with a mass of 2000 kg accelerates at a traffic light when it turns green. If the net force acting on the car is 4000 N, what is the car’s acceleration? • a = F/m • a= • F = 4000 N • m = 2000 kg 12.2 Newton’s First and Second Laws of Motion Weight and Mass Weight and mass are NOT the same thing! Weight- the measure of the force of gravity acting on an object Mass is a measure of the inertia of an object, or you can say that mass is the amount of matter in an object Weight is the product of an object’s mass and the acceleration of the object due to the force of gravity acting on it 12.2 Newton’s First and Second Laws of Motion Formula to calculate weight • Weight = mass (m) x acceleration due to gravity (g) • W = mg • The unit for weight must be in newtons • g = 9.8 m/s2 • Mass must be in kg 12.3 Newton’s Third Law of Motion and Momentum Newton’s Third Law States that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object • These forces are called action and reaction forces 12.3 Newton’s Third Law of Motion and Momentum Action and Reaction Forces • Examples • Walking/running • When you push off the floor with your foot your foot applies an action force on the floor, the floor pushes back on your foot with the reaction force • Swimming • Your arms push against the water--> action force The water pushes the swimmer forward--> reaction force • These forces are equal in value and opposite in direction 12.3 Newton’s Third Law of Motion and Momentum • Action-Reaction forces do not always cancel each other out • Action-reaction forces are equal and opposite-so why does the swimmer move? Why don’t the forces result in a net force of zero? -The action and reaction forces act on different objects -the action force acts on the water and the -reaction force acts on the swimmer 12.3 Newton’s Third Law of Motion and Momentum Momentum Momentum- the product of an object’s mass and its velocity • An object with a large momentum is hard to stop An object has a large momentum if the product of its mass and velocity is large Formula to calculate momentum • Momentum = mass x velocity • Mass must be in kg • Velocity must be in m/s 12.3 Newton’s Third Law of Motion and Momentum Example problem • Which has a larger momentum, a wagon full of logs (mass of 62 kg, speed of 12 m/s) or a 5th grader on a sled (mass of 47 kg, speed of 27 m/s)? • Momentum = mass x velocity • Momentum1 = Momentum = (62)(12) • Mass1 = 62 kg = 744 kgm/s • Velocity1 = 12 m/s Momentum = (47)(27) • Momentum2 = = 1269 kgm/s • Mass2 = 47 kg The kid on the sled has • Velocity2 = 27 m/s the greater momentum! 12.3 Newton’s Third Law of Motion and Momentum Conservation of Momentum Means overall momentum doesn’t increase or decrease A closed system must be in place for momentum to be conserved • Closed system means that other objects and/or forces can not enter or leave a system • Objects within the system can exert forces on each other 12.3 Newton’s Third Law of Motion and Momentum Law of Conservation of Momentum • States that if no net force acts on a system, then the total momentum of the system does not change In a closed system, the loss of momentum of one object equals the gain in momentum of another object--momentum is conserved 12.4 Universal Forces There is evidence that there are four forces that exist throughout the universe Electromagnetic Forces Nuclear Forces • Strong Force • Weak Force Gravitational Force All these forces act over a distance between particles (means particles don’t have to be touching for these forces to work) All these forces are affected by the distance between the particles 12.4 Universal Forces Electromagnetic Forces Electric and magnetic force are two aspects of the electromagnetic force • Electromagnetic force- is associated with charged particles • Electric force and magnetic force are the only forces that can both attract AND repel 12.4 Universal Forces • Electric Force • Act between charged objects • Opposite/unlike charges attract, like charges repel • Magnetic Force • Act on certain metals, on the poles of magnets, and on moving charges • Opposite/unlike poles attract, like poles repel • Magnets have two poles--north and south 12.4 Universal Forces Nuclear Forces Act on the nucleons (particles in the nucleus of an atom) in an atom Because of the fact that like charges repel (electric force-protons are positively charged) and the protons are close together, you may think that the nucleus couldn’t exist • Because the of the nuclear forces they can 12.4 Universal Forces Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together • Strong nuclear force overcomes the electric repulsion between the protons • Weak nuclear force is involved in radioactive decay The nuclear forces are attractive forces 12.4 Universal Forces Strong Nuclear Force • …is a powerful force of attraction that acts only on neutrons and protons in the nucleus, holding them together • The range over which this force acts is 10-15 m (about the diameter of a proton) • Acts over very short distances, but is 100 times stronger than the electric force of repulsion at that distance 12.4 Universal Forces • Weak Nuclear Force • …an attractive force that acts only over a short range (10-18 m) • Is involved in radioactive processes 12.4 Universal Forces Gravitational Force Weakest universal force at close distances, but most effective over great distances …an attractive force that act between any two masses Newton’s Law of Universal Gravitation states that every object in the universe attracts every other object 12.4 Universal Forces Gravity acts over large distances • The gravitational force between two objects is proportional to their masses and decreases rapidly as the distance between the masses increases • The greater the mass of the objects, the greater their gravitational forces 12.4 Universal Forces Centripetal force- a center-directed force that continuously changes the direction of an object to make it move in a circle • Results in a circular path The moon’s gravitational pull on the Earth results in two bulges in the earth’s oceans • One is on the side of the Earth closest to the moon and the second is on the side of the Earth farthest from the moon • Because the Earth rotates once in a day this results in two high and two low tides per day on Earth