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Transcript
Chapter 10 Forces Chapter Preview Questions 1. How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d. It has a reference point. Chapter 10 Forces Chapter Preview Questions 1. How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d. It has a reference point. Chapter 10 Forces Chapter Preview Questions 2. What is velocity? a. speed b. speed in a given direction c. miles per hour d. change in speed over time Chapter 10 Forces Chapter Preview Questions 2. What is velocity? a. speed b. speed in a given direction c. miles per hour d. change in speed over time Chapter 10 Forces Chapter Preview Questions 3. Which is the best definition of acceleration? a. change in velocity b. change in reference point c. increase in speed d. decrease in speed Chapter 10 Forces Chapter Preview Questions 3. Which is the best definition of acceleration? a. change in velocity b. change in reference point c. increase in speed d. decrease in speed Chapter 10 Forces Chapter Preview Questions 4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed? a. -100 m/s b. 0 m/s c. 100 m/s d. 32 m/s Chapter 10 Forces Chapter Preview Questions 4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed? a. -100 m/s b. 0 m/s c. 100 m/s d. 32 m/s Chapter 10 Forces Section 1: Friction, Gravity, and Elastic Forces Standard 8.2.a Students know a force has both direction and magnitude. Standard 8.2.c Students know when the forces on an object are balanced, the motion of the object does not change. Chapter 10 Forces What is a Force? How is a force described? A force is described by its magnitude and by the direction in which it acts. Force A push or a pull Newton (N) The SI unit for the magnitude, or strength, of a force. The combination of all forces acting on an object. Net force Chapter 10 Forces Combining Force Vectors The strength and direction of the individual forces determine the net force. Chapter 10 Forces Unbalanced Forces Unbalanced forces acting on an object result in a net force and cause a change in the object’s velocity. Chapter 10 Forces Balanced Forces Balanced forces acting on an object do not change the object’s velocity. Chapter 10 Forces Section 2: Friction, Gravity, and Elastic Forces Standard 8.2.b Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. Standard 8.2.d 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. Chapter 10 Forces Friction What factors determine the strength of the friction force between two surfaces? The strength of the force of friction depends on the types of surfaces involved and on how hard the surfaces push together. friction A force that two surfaces exert on each other when they rub against each other Chapter 10 Forces Types of Friction Static Friction • • • Occurs with objects that are not moving. You must use extra force to start the motion of stationary objects. Once the object is moving there is no longer any static friction. Sliding Friction •Occurs when two solid surfaces slide over each other. •Examples: • Sticky powder on the soles of ballet slippers. • Rubber pads on bicycle brakes. Chapter 10 Forces Types of Friction Rolling Friction Fluid Friction •Occurs when an object rolls across a surface. •Rolling friction is less than sliding friction for similar materials. •Engineers use ball bearings to reduce friction in many products. •Occurs when a solid object moves through a fluid. •Fluid friction is usually less than sliding friction. Parts of machines are bathed in oil for this reason. •Fluid friction occurs between people and air. Chapter 10 Forces Gravity What factors affect the gravitational force between two objects? gravity Law of Universal Gravitation mass The force of gravity between objects increases with greater mass and decreases with greater distance. A force that pulls objects toward each other The force of gravity acts between all objects in the universe. The measure of the amount of matter in an object The SI unit of mass is the kilogram (kg). Chapter 10 Forces Gravity Between Objects The force of gravity between objects increases with greater mass and decreases with greater distance. Chapter 10 Forces Mass and Weight The gravitational force exerted on a person or object at the surface of a planet is known as weight. Weight = Mass X Acceleration due to Gravity Chapter 10 Forces Mass and Weight The weight of a person on the Moon is one-sixth of their weight on Earth. The mass of an object is the same on the Moon as it is on Earth. Example: Weight on Earth 1617 N Mass on Earth 165 kg Weight on Moon 270 N Mass on Moon 165 kg Chapter 10 Forces Gravity and Motion Why do objects accelerate during free fall? In free fall, the force of gravity alone causes an object to accelerate in the downward direction. •All objects in free fall accelerate at the same rate regardless of their masses. •The acceleration due to gravity is 9.8 meters per second for each second an object is falling. •An object’s velocity is 9.8 m/sec after 1 second •Its velocity is 19.6 m/sec after 2 seconds, etc. Chapter 10 Forces Free Fall The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions. Chapter 10 Forces Free Fall Interpreting Graphs: What is the speed of the object at 1 second? At 3 seconds? 9.8 m/s; 29.4 m/s. Chapter 10 Forces Free Fall Calculating: Calculate the slope of the graph. What does this number represent? The slope is 9.8. The speed increases by 9.8 m/s each second. Chapter 10 Forces Free Fall Predicting: Use the slope that you calculated in Step 2 to predict the object’s speed at 6 seconds. 58.8 m/s Chapter 10 Forces Free Fall Drawing Conclusions: The graph has a constant slope. What does the slope tell you about the object’s motion? The object’s speed increases at a constant rate. Chapter 10 Forces Air Resistance Falling objects with a greater surface area experience more air resistance. Air resistance is an upward force exerted on falling objects. Chapter 10 Forces Projectile Motion • • An object that is thrown is called a projectile. The path of a projectile has horizontal motion due to a lateral force and vertical motion due to gravitational force. Chapter 10 Forces Elastic Forces When is matter considered to be elastic? Two Types of Elastic Forces Matter is considered elastic if it returns to its original shape after it is squeezed or stretched. Compression •An elastic force that squeezes or pushes matter together. •Example: A cushion on a couch compresses due to a person’s weight until it comes close enough together to exert an equal compression force in the other direction. Chapter 10 Forces Elastic Forces Two Types of Elastic Forces Tension • An elastic force that stretches or pulls matter. • Example: The strings on a guitar. Chapter 10 Forces Newton’s First and Second Laws Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). Standard 8.2.f Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion. Chapter 10 Forces The First Law of Motion What is Newton’s First Law of Motion? An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force. •Inertia is the tendency of an object to resist a change in motion. •Another name for Newton’s First Law of Motion is the Law of Inertia. •The greater and object’s mass, the greater its inertia. Chapter 10 Forces The Second Law of Motion What is Newton’s Second Law of Motion? Acceleration depends on the net force acting on the object and on the object’s mass. Acceleration = Net force / Mass •Acceleration is measured in meters per second per second. •Net force is measured in Newtons (N), and mass is measured in kilograms (kg). The above formula can also be written as: Net Force = Mass X Acceleration Chapter 10 Forces Calculating Force A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s2 Chapter 10 Forces Calculating Force A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (Fnet) = __ What formula contains the given quantities and the unknown quantity? a = Fnet/m or Fnet = m x a Perform the calculation. Fnet = m x a = 55 kg x 2.0 m/s2 F = 110 kg • m/s2 F = 110 N Chapter 10 Forces Calculating Force A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required. This does not include the force that overcomes friction. Chapter 10 Forces Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s2? 3,000 N (1,000 kg x 3 m/s2) Chapter 10 Forces Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s2? 350 N (25 kg x 14 m/s2) Chapter 10 Forces Newton’s Third Law Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). Chapter 10 Forces Newton’s Third Law of Motion What is Newton’s Third Law of Motion? Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Chapter 10 Forces Newton’s Third Law of Motion How can you calculate the momentum of an object? A characteristic of a moving object that depends on both the mass and the velocity of the object. Momentum = Mass X Velocity The unit for momentum is measured in kilogram-meters per second (kg-m/s) Chapter 10 Forces Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s, or a 4.0-kg sledgehammer swung at 0.9 m/s? Read and Understand What information are you given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s Chapter 10 Forces Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Plan and Solve What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass x Velocity Perform the calculation. Smaller sledgehammer = 3.0 km x 1.5 m/s = 4.5 kg•m/s Larger sledgehammer = 4.0 km x 0.9 m/s = 3.6 kg•m/s Chapter 10 Forces Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Look Back and Check Does your answer make sense? It is possible for the 3.0-kg hammer to have more momentum than the 4.0-kg one because it has a greater velocity. Chapter 10 Forces Calculating Momentum Practice Problem A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum? Golf ball: 0.045 kg x 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg x 7 m/s = 0.98 kg•m/s The baseball has greater momentum. Chapter 10 Forces Calculating Momentum Practice Problem What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? 0.27 kg•m/s (0.018 kg x 15 m/s = 0.27 kg•m/s) Chapter 10 Forces Law of Conservation of Momentum What is the Law of Conservation of Momentum? The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Chapter-10 Forces Conservation of Momentum In the absence of friction, momentum is conserved when two train cars collide. Chapter 10 Forces Rockets and Satellites Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). Chapter 10 Forces Rockets How does a rocket lift off? • • A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket. As long as the upward force, called thrust, is greater than the downward pull of gravity there is an unbalanced force, and the rocket will accelerate toward space. Chapter 10 Forces Satellites What keeps a satellite in orbit? Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it •A satellite is a falling projectile that keeps missing the ground. •A satellite continues to move around the Earth due to its inertia. •Gravity continuously changes the satellite’s direction. •The speed an object must be thrown in order to orbit Earth is about 7,900 meters per second. Chapter 10 Forces What Is a Satellite? A projectile follows a curved path. The horizontal and vertical motions act independently. Chapter 10 Forces What Is a Satellite? The faster a projectile is thrown, the farther it travels before it hits the ground. A projectile with enough velocity moves in a circular orbit. Any force that causes an object to move in a circle is called a centripetal force. Chapter 10 Forces What Is a Satellite? Depending on their uses, artificial satellites orbit at different heights.