Forces and The Laws of Motion - Physics

Forces and The Laws of Motion { How force affects the motion of an object?  Forces describe { the interactions between an object and its environment. Force  { 1. Making a stationary object move 2. Making...

Forces and The Laws of Motion { How force affects the motion of an object?  Forces describe { the interactions between an object and its environment. Force  { 1. Making a stationary object move 2. Making a moving object stop 3. Changing the direction of a moving object Forces can change motion  1. The SI unit of force is the newton. 2. 1 N = 1 kgx1m/s2 F = ma 3. The newton (N) is defined as the amount of force that, when acting on a 1 kg mass, produces an acceleration of 1 m/s2. SI unit of force  🙐 If you pull on a spring, the spring stretches. If you pull on a wagon, the wagon moves. When a football is caught, its motion is stopped. 🙐 This are examples of contact forces. Forces can act through contact or at a distance  🙐 Field forces does not involve physical contact between two objects. 🙐 Whenever an object falls to Earth, the object is accelerated by Earth’s gravity. 🙐 Attraction and repulsion between electric charges. 🙐 Electromagnetic fields. Field Forces  🙐 The effect of a force depends on both magnitude and direction. 🙐 Force is a vector quantity. 🙐 In a force diagram, the object is centered, and all forces acting on the object are drawn with arrows. Force Diagrams 🙐 A free – body diagram helps analyze a situation or problem  🙐 Use three different types of balls and a book to model a car colliding with a brick wall. 🙐 Observe the motion of the balls before and after the crash. 🙐 Identify as many changes in its motion as you can, such as changes in speed or direction, differences between balls, etc. 🙐 Make a list of all the changes, and try to identify the forces that caused them. 🙐 Make a force diagram of the collision. 🙐 Turn in your comments and results. Quick Lab  🙐 Widely recognized as one of the most influential scientist of all time. 🙐 His book: Philosophiae Naturalis Principia Mathematica, 1687. 🙐 Laws of motion and universal gravitation. 🙐 He was born the same year that Galileo died. Isaac Newton (1642 – 1726)  🙐 An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force. 🙐 Inertia = a tendency to remain unchanged, or continue in an existing state. Inertia is a property of matter. Newton’s First Law (Law of Inertia) 🙐There is a natural tendency of objects to keep on doing what they are doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will mantain this state of motion.  🙐 First Newton Law https://www.youtube.com/watch?v=erghLWXD ScI Video 🙐 The net force is the vector sum of all forces acting on an object. 🙐 An external force is a single force that acts on an object as a result of the interaction between the object and its environment. The Net Force  Forced to Change Misconception: Any object that is set in motion will slow down on its own. Fact: A force is needed to change an object’s state of motion The Nature of Force  A Force is described by its strength and by the direction in which it acts. The direction and strength of a force can be represented by an arrow. The arrow points in the direction of the force. SI unit: Newton (N) How are Forces Described? Do Forces Affect Motion? 🙐 Often more than one force acts on an object at the same time. The combination of all the forces on an object is called the net force. 🙐 The net force will determine if and how an object will accelerate. 🙐 A nonzero net force causes a change in the object’s motion. 🙐 Force Diagrams Worksheet  1. Prepare and set the dynamometer as indicated by your instructor. 2. Measure the weight (in Newtons) of five different masses. Read the scale as precise as you can. Write your results in a chart. 3. Repeat the procedure but now each mass will have to be immerse in a beaker filled with water. Measure the weights again. Write your results in a chart. Think and analyze: 4. Is there a difference between the weight values suspended in the air and immerse in water? Explain. 5. What is the Archimedes Principle? Explain. 6. How is the Archimedes Principle related to our experiment? 7. Find the net force in each system with all 5 weights. Lab: Forces and Archimedes Principle  🙐 https://www.youtube.com/watch?v=IJWEtCR WGvI Video: Force Everyday Forces  🙐 Weight - Weight is a scalar quantity. It is the force exerted on a body by gravity. - Weight is expressed in the formula w = mg - Depends on location. - Fg = mg = w Weight  🙐 The normal force (N) is always perpendicular to the contact surface. The Normal Force  🙐 Is the force exerted by a surface as an object moves across it or makes an effort to move across it. 🙐 The friction force opposes the motion of an object. 🙐 There are 2 types of friction: - Static friction, Fs - Kinetic friction, Fk The Force of Friction Static and Kinetic Friction  🙐 Is a quantity that expresses the dependence of frictional forces on the particular surfaces in contact. 🙐 Greek letter, mu. ( μ ) 🙐 Ff = μFn Fn = Normal force Coefficient of Friction https://www.youtube.com/watch?v=IJWE tCRWGvI  🙐 The sensor in the torso of a crash-test dummy records the magnitude and direction of the net force acting on the dummy. If the dummy is thrown forward with a force of 130.0 N while simultaneously being hit from behind with a force of 4500.0 N, what force will the sensor report? 🙐 What force will the seat belt have to exert on the dummy to hold the dummy in the seat? Net Force with No Angles  🙐 Sebastian leaves his physics book on top of a drafting table that is inclined at a 35* angle. Draw the free- body diagram and find the net force acting on the book. 🙐 Force of the table on the book = 18 N 🙐 Force of gravity on the book = 22N 🙐 F friction = 11N Determining Net Force with an angle 🙐 1. A gust of wind blows an apple from a tree. As the apple falls, the gravitational force on the apple is 2.25N downward, and the force of the wind on the apple is 1.05N to the right. Find the magnitude and direction of the net force on the apple. 🙐 2. The wind exerts a force of 452 N north on a sailboat, while the water exerts a force of 325 N west on the sailboat. Find the magnitude and direction of the net force on the sailboat. 🙐 3. A man is pulling on his dog with a force of 70.0N directed at an angle of +30.0* to the horizontal. Find the x and y components of this force and the net force on the system. Practice and Solve  🙐 The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. Newton’s Second Law  🙐 Andrew and Jacqueline are studying across from each other at a wide table. Jacqueline slides a 2.2 kg book 🙐 Fnet = ma toward Andrew. If the net force acting on the book is 1.6 N to the right, What is the book’s acceleration? Sample Problem  1. The net force on the propeller of a 3.2 kg model airplane is 7.0 N forward. What is the acceleration of the airplane? Answer: 2.2 m/s2 2. The net force on a golf cart is 390 N north. If the cart has a total mass of 270 kg, what are the magnitude and direction of the cart’s acceleration? Answer: 1.4 m/s2, North 3. A car has a mass of 1.50X103 kg. If the force acting on the car is 6.75X103 N to the east, what is the car’s acceleration? Answer: 4.5 m/s2 4. A soccer ball kicked with a force of 13.5 N accelerates at 6.5 m/s2 to the right. What is the mass of the ball? Answer: 2.1 Kg. Practice!  🙐 If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forcers are opposite in direction. 🙐 For every action, there is an equal and opposite reaction. Newton’s Third Law  🙐 Answer Page 219 🙐 Page 220: Solve 1 – 3. Classwork!  1. Define weight and explain how you can calculate it. Explain the difference between weight and mass. Give an example. 2. What is the normal force? Explain and give an example. How do you find the direction and magnitude of normal forces? 3. What is the force of friction? What different types of friction can you see in objects? 4. How do you get the coefficient of friction? Write equations. Everyday Forces Review Read pages 222 – 228  Imagine a 5 kg box that you will push to the right. Describe the static and kinetic friction. Compare the coefficients of friction. Coefficient of Friction 1. A 15 kg box rests on a horizontal surface. a. What is the minimum horizontal force that is required to cause the box to begin to slide if the coefficient of static friction is 0.35? b. What is the acceleration of the system if a person pushes the box with a force of 90.0 N if the kinetic coefficient is 0.20? 2. A force of 65.0 N is needed to start a 8.00kg box moving across a horizontal surface. a. Calculate the coefficient of static friction. u s = 0.829 b. If the box continues to move with an acceleration of 1.4m/s2, what is the coefficient of kinetic friction? uk = 0.686  🙐 1. A 24 kg suitcase initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the suitcase and the floor. us = 0.32 🙐 2. A 25 kg chair initially at rest on a horizontal floor requires a 165N horizontal force to set in in motion. Once the chair is in motion, a 127N horizontal force keeps it moving at a constant velocity. a. Find the coefficient of static friction between the chair ant the floor. b. Find the coefficient of kinetic friction between the chair and the floor. Practice 3. A force of 150N pulls the 30kg box to the right with a rope that forms 30 degrees with the surface. If the coefficient of kinetic friction is 0.25, what is the horizontal acceleration of the box?  1. A student attaches a rope to a 20.0kg box of books. He pulls with a force of 90.0N at an angle of 30.0* with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.500. Find the acceleration of the box. 2. A box of books weighing 325 N moves at a constant velocity across the floor when the box is pushed with a force of 425 N exerted downward at an angle of 35.2* below the horizontal. Find u k between the box and the floor. Solve:  1. A 5.4 kg bag of groceries is in equilibrium on an incline of angle equal to 15 degrees. Find the magnitude of the normal force on the bag. 2. A basket is in equilibrium on a ramp with an incline of 25*. Find the friction force and the normal force on the basket. 3. A box slides down a 30.0* ramp with an acceleration of 1.20 m/s2. Determine the coefficient of kinetic friction between the box and the ramp. Hint: Write all the equations and see if you can cancel mass. FBD with Ramps and Angles Purpose: to measure the coefficient of static friction of different surfaces Materials - cube with different surfaces - incline plane - triple beam balance Procedure 1. Place the incline plane on a flat surface 2. As slowly as possible, rise the incline plane until the cube starts to move. 3. Measure the angle. 4. Repeat this measure three times and find the average. 5. Repeat for each different surface. Lab Analysis 🙐 Draw the free body diagram of the object 🙐 Find all forces in x and y 🙐 Is this friction coefficient a static or a kinetic coefficient of friction? Justify your answer. 🙐 Work out a formula for the coefficient of friction. 🙐 Make a table/chart with all data: type of surface, angle, mass, coefficient of friction. Challenge: Explain how the ABS system works in your car. Friction Lab 🙐 Solve Page 231. 🙐 Review for test: page 239: 7, 9, 12, 15, 20, 25, 29, 30, 36, 38.