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Transcript
Newton’s Laws Guided Notes Newton’s First Law An object at rest or an object in motion at constant speed will remain at rest or at constant speed in the absence of a resultant force. o A glass is placed on a board and the board is jerked quickly to the right. The glass tends to remain at rest while the board is removed. o Assume glass and board move together at constant speed. If the board stops suddenly, the glass tends to maintain its constant speed. Inertia Newton's first law is often referred to as the law of inertia. Inertia is the resistance or unwillingness of an object to accelerate (speed up, slow down, or change directions). The more mass an object has, the harder it is to accelerate. Thus, more mass equals more inertia. Understanding the First Law Discuss what the driver experiences when a car accelerates from rest and then applies the brakes. o The driver is forced to move forward. An object at rest tends to remain at rest. o Driver must resist the forward motion as brakes are applied. A moving object tends to remain in motion. Newton’s Second Law Whenever a resultant force acts on an object, it produces an acceleration that is directly proportional to the force and inversely proportional to the mass. o Formula and Units: Acceleration and Force Pushing a cart with twice the force produces twice the acceleration. Three times the force triples the acceleration. Newton: The Unit of Force One newton is the force required to give an acceleration of 1 m/s2 to a mass of 1 kg. What resultant force will give a 3 kg mass an acceleration of 4 m/s2? 1 Additional Notes: Newton’s Laws Guided Notes Example A 40 N resultant force causes a block to accelerate at 5 m/s2. What is the mass? Example A net force of 4.2 x 104 N acts on a 3.2 x 104 kg airplane during takeoff. What is the force on the plane’s 75-kg pilot? Example A 54-g tennis ball is in contact with the racket for a distance of 40 cm as it leaves with a velocity of 48 m/s. What is the average force on the ball? Weight and Mass Weight is the force an object applies as a result of gravity pulling it downward. It is directed downward and it varies with gravity. Mass is a universal constant which is a measure of the matter that makes up an object. It is always constant regardless of location Because weight is a force created by the downward acceleration of gravity, we can make this substitution. o W=mg Example What is the weight of a 10-kg block? 2 Additional Notes: Newton’s Laws Guided Notes Always Remember!! In Physics, the use of Newton’s second law and many other applications makes it absolutely necessary to distinguish between mass and weight. Use the correct units! o Metric SI units: Mass is in kg; weight is in N. o Pounds should never be used!!! Example A resultant force of 40 N gives a block an acceleration of 8 m/s2. What is the weight of the block near the surface of the Earth? Newton’s Third Law For every action force, there must be an equal and opposite reaction force. Forces occur in pairs. Action and Reaction Forces Use the words by and on to study action/reaction forces below as they relate to the hand and the bar: o The action force is exerted by the _____ on the _____. o The reaction force is exerted by the _____ on the _____. Forces If a bar of gold that has a mass of 1 kg is sitting at rest on a table, what forces are acting on it and to what magnitude? o Gravity acts on the object in the downward direction. However, because the bar is not moving downward, there must be another force counteracting it. o The normal force of an object pushes up with a force equal to the bar’s downward force. Because both forces are present, the bar remains at equilibrium. Free-body Diagrams A free body diagram is a diagram that shows all of the forces present on a given object. 3 Additional Notes: Newton’s Laws Guided Notes When drawing a free body diagram, objects are always represented as points or dots and forces are drawn as arrows pointing away from the object. For now the only forces we will consider are gravity, tension, normal, applied, and friction. Gravitational Force: This is the only force that will always be present and will always point straight down. Tension Force: This type of force is caused by a rope, chain, string, etc. and goes in the direction of the rope, chain, string, etc. Normal Force: This type of force is present when the object is in contact with a surface. It always points perpendicular to the surface. Applied Force: This type of force is used when an outside agent is acting on the object such as a person. Frictional Force: This type of force is the result of two surfaces being drug across each other, creating friction. It always points in the direction opposite of the movement. The size of the arrow should be proportional to the size of the force. Larger forces should have larger arrows!!! Examples F Example A cart and driver have a mass of 120 kg. What force F is required to give an acceleration of 6 m/s2 with no friction? 4 Additional Notes: Newton’s Laws Guided Notes Example What is the tension FT in the rope below if the block accelerates upward at 4 m/s2? (Draw sketch and free-body) Example Find the acceleration of the blocks and the tension force between the two blocks if there is no friction on the surfaces. 5 Additional Notes: