Chapter 4

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Information about Chapter 4

Published on January 4, 2008

Author: avsar


Chapter 4:  Chapter 4 Newton’s First Law of Motion—Inertia 4.1 Aristotle in Motion:  4.1 Aristotle in Motion Greek scientist; 4th century BC Studied motion and divided it into 2 types Natural Motion motion that is not caused by forces Violent Motion motion that is caused by forces, “imposed” motion 4.1 Aristotle in Motion:  4.1 Aristotle in Motion Natural Motion Objects seek their NATURAL resting place Heavy objects fall to the ground Light objects rise to the heavens Circular motion is natural for the heavens Believed that planets and stars moved in perfect circles around the Earth 4.1 Aristotle in Motion:  4.1 Aristotle in Motion Violent Motion The result of forces that push or pull A cart moves because it is pulled by a horse Caused by an external force Objects in their natural resting place cannot move themselves Aristotle in Motion:  Aristotle in Motion Aristotle’s beliefs were common for over 2000 years It was commonly believed that the proper state of an object is at rest Objects do not move, unless they are moving towards their natural resting place OR are being pushed or pulled Believed that the Earth was at rest Copernicus and the Moving Earth:  Copernicus and the Moving Earth Nicolaus Copernicus (1473-1543) Theory of Moving Earth Reasoned that the Earth (and other planets) move around the sun. This was controversial—it contradicted Aristotle. He kept his research a secret to escape persecution Galileo in Motion:  Galileo in Motion Supported Copernicus suffered house arrest due to his support Demolished the notion that a force is necessary to keep an object moving Force - any push or pull the SI unit of force is the newton (N = kg*m/s2) Friction - the force that acts between materials that touch as they moved past each other Forces:  Forces A force exerted on an object can usually change an object’s velocity with respect to time cause a stationary object to move throwing a ball cause a moving object to stop catching a ball can cause a moving object to change direction hitting a pitched baseball with a bat Forces:  Forces Forces can act through contact or at a distance Contact forces result in a physical contact between two objects catching a football; pulling a wagon Field forces do not involve in a physical contact between two objects gravitational force--when an object falls to the Earth, the object is accelerated by Earth’s gravity (Earth is exerting a force on the object with no immediate contact) Forces and Friction:  Forces and Friction Friction is caused by irregularities in the surfaces that objects are touching friction acts on each of two objects which are in contact when one object moves or when there is an attempt to move it relative to the other these forces are parallel to the surfaces in contact and they act in directions to oppose motion smooth surfaces have less friction than rough surfaces If friction were absent an object would not need force to keep moving Forces and Friction:  Forces and Friction Galileo’s ideas were supported by a variety of experiments A ball rolling down an inclined plane gains speed due to gravity A ball rolling up an inclined plane loses speed due to gravity On a level surface a ball would not roll with or against gravity, therefore speed should remain constant Galileo found that a ball moving on a smooth horizontal plane has almost constant velocity; if friction were entirely absent, a ball moving horizontally would move forever Force and Friction:  Force and Friction Galileo also experimented with two inclined planes to support his reasoning Forces and Friction:  Forces and Friction Newton’s Law of Inertia:  Newton’s Law of Inertia The tendency for a body to keep moving is natural and every object resists change to its state of motion INERTIA is the property of a body to resist change the tendency of a body not to accelerate Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with a constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force Mass--a Measure of Inertia:  Mass--a Measure of Inertia Suppose you kick an object. What makes an object easy to kick? Mass the lighter the object, the easier it is to kick the heavier the object, the harder it is to kick The amount of inertia an object has depends on its mass a brick has much more inertia than a piece of paper--because the brick has a much greater mass than the piece of paper Mass and Volume:  Mass and Volume Mass is not volume volume is the measure of space an object takes up; the units for volume are cubic meter, cubic inches, cubic feet, etc mass is measured in kg Mass does not depend on volume think of a bowling ball and a volleyball they have similar volumes, but the mass of a bowling ball is much greater than the mass of a volleyball Mass and Weight:  Mass and Weight Mass is not weight weight is determined by the force of gravity on an object mass is the quantity of matter in an object Mass and weight are proportional to each other in a given place--must be the same place like Earth Weight = mg Your weight on Earth is different than your weight on the moon; g = 9.8 m/s2 on earth and g = 1.6 m/s2 Mass, Weight, & Volume:  Mass, Weight, & Volume Does a 5 kg steel ball have 5 times as much inertia as a 1 kg block of steel? Yes--inertia = mass and they have the same mass 5 times as much volume? Yes--both are made of steel 5 times as much weight, when weighed in the same location? Yes--they have the same mass and weight is proportional to mass Mass, Weight & Volume:  Mass, Weight & Volume Does a 5 kg steel block have five times as much inertia as a 1 kg piece of paper? Yes--inertia = mass 5 times as much volume? No--steel and paper are different materials; steel is much more dense than paper; the volume of the paper would be much larger than the volume of the steel 5 times as much weight? Yes--they have the same mass and weight is proportional to mass Mass & Weight:  Mass & Weight One kilogram (kg) weighs 9.8 newtons (2.2 pounds) Felicia has a mass of 45.0 kg. What is Felicia’s weight on Earth? What is Felicia’s mass on Jupiter, where the acceleration due to gravity is 25.0 m/s2? What is Felicia’s weight on Jupiter? Free Body Diagrams:  Free Body Diagrams Force is a vector depends on both magnitude and direction Diagrams used to show forces acting upon a single object are called free body diagrams the tail of the vector is attached to the object on which the force is acting the force vector points in the direction of the force the force vector’s length is proportional to the magnitude of the force diagram will only show forces acting on one object (the object of interest) Free Body Diagrams:  Free Body Diagrams Free Body Diagrams:  Free Body Diagrams Draw free body diagrams for the following scenarios: A boat travels upstream with a constant velocity. A box is pushed across the floor with a rightward acceleration. A box sits at rest on an inclined plane. Net Force:  Net Force Net Force - the vector sum of all forces acting on an object the unbalanced force the resultant of all forces acting on a system You push an object with a force of 5N; at the same time your friend pushes the object, in the same direction, with a force of 7N. What is the net force? You push on the top of an object with a force of 5N; at the same time your friend pushed on the bottom of the same object with a force of 15N. What is the net force? Net Force:  Net Force 3. 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. 4. Derek leaves his physics book on top of a drawing table that in inclined at a 35° angle. The force of the table on the book is 18N. The force of gravity on the book is 22N. The force of friction on the book is 11N directly up the ramp. Find the net force on the book. Net Force:  Net Force Find the magnitudes missing forces for each object in the free body diagrams below Equilibrium:  Equilibrium Equilibrium--when net force equals zero objects that are at rest or are moving with a constant velocity (objects that are not accelerating) are in equilibrium Think of a 5 kg book sitting on a desk the book is at rest with respect to the table, therefore it is in equilibrium the force of gravity on the book is (5kg)(10m/s2) = 50 N down in order to be in equilibrium, the force of the table pushing back up on the book must be equal to 50 N up if something were between the book and the table, then it would feel like it was being smashed from both sides Equilibrium:  Equilibrium The force of the desk pushing back up on the book is called the normal or support force A car is traveling west with a constant velocity of 20 m/s, what is the net force on the car? If a car is accelerating downhill under a net force of 3674 N, what additional force would cause the car to have a constant velocity? Equilibrium:  Equilibrium Suppose you are hanging from a rope atoms are being stretched apart--this is called tension in the rope Tension is a force (unit—newton) How much tension is in the rope when you hang from it? If you are in equilibrium, the tension must equal your weight Suppose you hang on a bar from 2 ropes--neglecting the weight of the bar--what is the tension in one rope? Each rope supports 1/2 of your weight; therefore the tension must equal 1/2 your weight Equilibrium:  Equilibrium Statics - the study of forces in equilibrium Flip, an exhausted gymnast, hangs from a bar by both arms in an effort to catch his breath. If Flip has a mass of 65.0 kg, what is the tension in each of Flip’s arms as he hangs in place? Equilibrium:  Equilibrium 2. Jack and Jill lift upward on a 1.3 kg pail of water, with Jack exerting a force of 7.0 N and Jill exerting a force of 11 N. Jill’s force is exerted at an angle of 28° with the vertical. At what angle with respect to the vertical should Jack exert a force if the pail is to accelerate straight upward? Michelle likes to swing on a tire tied to a tree branch in her yard. If Michelle and the tire have a combined mass of 82.5 kg and Elwin pulls Michelle back far enough to make an angle of 30.0° with the vertical, what is the tension in the rope? Vector Addition of Forces:  Vector Addition of Forces Why can you hang on a clothesline when it is suspended vertically, but not when it is suspended horizontally? Vertically Horizontally Tension = weight Tension much greater than weight The Moving Earth Again:  The Moving Earth Again Copernicus proposed the idea of the moving Earth in the 16th century the Earth is actually moving 30 km/s we do not feel this movement because everything is moving with the Earth--even things suspended above the surface of the Earth Suppose you jump vertically next to the wall on the west side of the room since the Earth is moving so fast--you should slam into the wall but you land right where you took off although you changed your vertical motion; your horizontal motion remained at 30 km/s due to its horizontal inertia think of throwing something up in the air in a car that is traveling at a constant speed--the object will fall back to its original position

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