Chapter4-Newton_s_Laws-The_Bascis

# Chapter4-Newton_s_Laws-The_Bascis - Newton’s Newton’s...

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Unformatted text preview: Newton’s Newton’s Laws of Motion I. Law of Inertia II. F=ma III. Action­Reaction While most people know what While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean). Newton’s Laws of Motion Newton’s Laws of Motion 1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2nd Law – Force equals mass times acceleration. 3rd Law – For every action there is an equal and opposite reaction. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by a force. 1 Law st Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force. 1 Law st Once airborne, Once unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop! 1 Law st Unless acted Unless upon by a force, this golf ball would sit on the tee forever. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? force? It’s a force we sometimes cannot see – It’s friction. friction. Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction. influence What is this unbalanced force that acts on an object in motion? There are four main types of friction: Sliding friction: ice skating Sliding Rolling friction: bowling Rolling Fluid friction (air or liquid): air or water resistance Fluid Static friction: initial friction when moving an Static object Slide a book Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force ­ that force being the force of friction ­ which brings the book to a rest position. In the absence of a force of friction, the book In would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.) Newtons’s 1 Law and You Newtons’s st Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour. 2 Law nd 2 Law nd The net force of an object is The equal to the product of its mass and acceleration, or F=ma. and 2 Law nd When mass is in kilograms and acceleration is When in m/s/s, the unit of force is in newtons (N). in One newton is equal to the force required to One accelerate one kilogram of mass at one meter/second/second. meter/second/second. If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration. nd Newton’s 2 Law proves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma F = ma 98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8 m/s/ s Check Your Understanding Check 1. What acceleration will result when a 12 N net force applied to a 3 kg 1. object? A 6 kg object? object? 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. Determine 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/ sec? Check Your Understanding Check 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 1. 12 N = 3 kg x 4 m/s/s 12 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the Determine mass. mass. 16 N = 3.2 kg x 5 m/s/s 16 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 66 kg-m/sec/sec or 66 N 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? 9800 kg-m/sec/sec or 9800 N 9800 3 Law rd For every action, there is an For equal and opposite reaction. equal 3 Law rd According to Newton, According whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. 3 Law rd There are two forces There resulting from this interaction - a force on the chair and a force on your body. These two forces are called action action and reaction forces. reaction Newton’s 3rd Law in Nature Newton’s 3rd Law in Nature Consider the propulsion of a Consider fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing reacts the fish forwards, propelling the fish through the water. the The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). force 3 Law rd Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. Consider the flying motion of birds. A bird flies by Consider use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the The force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). the Action-reaction force pairs make it possible for birds Action-reaction to fly. to Other examples of Newton’s Third Other examples of Newton’s Third Law The baseball forces the The bat to the left (an action); the bat forces the ball to the right (the reaction). 3 Law rd Consider the motion of Consider a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. backwards. 3 Law rd The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction. ...
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## This note was uploaded on 09/09/2011 for the course PHYS 6a taught by Professor Stanek during the Spring '07 term at UCSB.

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