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Download 1 Newton`s Laws of Motion (Ch 5) Newton`s First Law
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Transcript
Newton’s Laws of Motion (Ch 5) • Isaac Newton • • • 1642-1727 English physicist & mathematician By the age of 31, discovered: • • • • laws of motion universal gravitation calculus Eccentric • read Coming of Age in the Milky Way, by Timothy Ferris Newton’s First Law of Motion Newton’s first law is often called the law of inertia. Force A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity. The magnitude of a force can be measured using a spring scale. Newton’s First Law Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it. An object in motion stays in motion and an object at rest stays at rest, unless acted upon by a force. 1 Newton’s First Law of Motion Inertial reference frames: An inertial reference frame is one in which Newton’s first law is valid. This excludes rotating and accelerating frames. ConcepTest A book is lying at rest on a table. The book will remain there at rest because: 1) there is a net force but the book has too much inertia 2) there are no forces acting on it at all 3) it does move, but too slowly to be seen 4) there is no net force on the book 5) there is a net force, but the book is too heavy to move ConcepTest A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? 1) more than its weight 2) equal to its weight ConcepTest You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why? 1) a net force acted on it 3) less than its weight but more than zero 2) no net force acted on it 4) depends on the speed of the puck 3) it remained at rest 5) zero 4) it did not move, but only seemed to 5) gravity briefly stopped acting on it 2 ConcepTest Mass You kick a smooth flat stone out on a frozen pond. • The stone slides, slows down and eventually stops. • You conclude that: 1) the force pushing the stone forward finally stopped pushing on it • • 2) no net force acted on the stone 3) a net force acted on it all along • 4) the stone simply “ran out of steam” 5) the stone has a natural tendency to be at rest Newton’s Second Law of Motion • • Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms. Mass is not weight: Newton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass. Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same. Newton’s Second Law • • • Force is a vector, so is true along each coordinate axis. The unit of force in the SI system is the newton (N). Note that the pound is a unit of force, not of mass, and can therefore be equated to newtons but not to kilograms. 3 ConcepTest From rest, we step on the gas of our Ferrari, providing a force F for 4 secs, speeding it up to a final speed v. If the applied force were only 1/2 F, how long would it have to be applied to reach the same final speed? 1) 16 s 2) 8 s 3) 4 s ConcepTest A force F acts on mass m 1 giving acceleration a 1. The same force acts on a different mass m 2 giving acceleration a 2 = 2a1. If m 1 and m2 are glued together and the same force F acts on this combination, what is the resulting acceleration? 1) 3/4 a1 3) 1/2 a1 5) 1 s 4) 4/3 a1 ConcepTest a1 m2 m2 m1 a2 = 2a1 a3 ConcepTest What can you say about the acceleration of gravity acting force of gravity Fg acting on a stone and a feather? 1) it is greater on the feather 1) Fg is greater on the feather 2) it is greater on the stone 2) Fg is greater on the stone 3) it is zero on both due to vacuum 3) Fg is zero on both due to vacuum 4) it is equal on both always m1 5) 2/3 a1 What can you say about the on a stone and a feather? F F 2) 3/2 a1 4) 2 s F 4) Fg is equal on both always 5) Fg is zero on both always 5) it is zero on both always 4 Newton’s Third Law of Motion More traditionally • Any time a force is exerted on an object, that force is caused by another object. For every action, there is an equal and opposite reaction. Newton’s third law: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. Newton’s Third Law of Motion A key to the correct application of the third law is that the forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object. Newton’s Third Law of Motion Rocket propulsion can also be explained using Newton’s third law: hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket. 5 ConcepTest A very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck? 1) it is too heavy, so it just sits there 2) it moves backward at const. speed 3) it accelerates backward 4) it moves forward at const. speed 5) it accelerates forward 6