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SPH3U – Unit 2 Newton’s Laws First… a quick video! http://www.youtube.com/watch?v=NWE_ aGqfUDs Newton & Galileo Newton and Galileo were very important to the development of physics. Together they discovered some very important principles of motion. In this course we will study: Newton’s first law of Motion Newton’s second law of Motion Newton’s third law of Motion Inertia During the 1600’s Galileo performed experiments with objects in motion and discovered that an object in motion would “like” to resist changes in its state of motion. He called this Inertia. Inertia Inertia the property of an object that resists change in its state of rest or motion. “Objects at rest tend to stay at rest” “Objects in motion tend to stay in motion” Newton’s First Law Later, Newton expanded on this idea and it became the first law: Newton’s first Law of Motion If the net force acting on an object is zero, the object will remain at rest or move at a constant velocity. Question Ex. Which object is in motion? F = 2N F = 4N F = 2N F = 2N Question Ex. Which object is in motion? F = 2N F = 4N The external net force is 2N right. Therefore it is actually accelerating. F = 2N F = 2N The external net force is 0. Therefore it could either be not moving or moving with a constant velocity Newton’s First Law Implications There must be an external force in order to change motion. (You can’t change the motion of a car by pushing on the seat while inside it – that’s an internal force, but you can change its motion by pushing on the outside of the car – that’s an external force). Newton’s First Law Implications Moving objects will continue to move in a straight line at a constant speed unless acted on by an external net force greater than zero. So… what happens if the external net force is not zero? Newton’s Second Law Newton’s Second Law If the external net force on an object is not zero, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the magnitude of the net force and is inversely proportional to the object’s mass. Newton’s Second Law Basically: Fnet ma or Fnet a m Newton’s Second Law Note: Many people believe that Force is related to velocity. They will say things like, “The more force you add, the faster the object will go”. Be careful about this! It is not accurate. What the person means to say is that, “The more force you add the greater the rate at which the object speeds up.” Force is related to acceleration. Zero net force is related to constant velocity! Newton’s Third Law Newton’s Third Law For every action force there is a reaction force equal in magnitude, but opposite in direction. Newton’s Third Law We can restate the Third Law as this: Every force is one of a pair of interaction forces. The two members of the interaction force pair act on different objects. The two members of the interaction force pair are equal in magnitude but opposite in direction. Newton’s Third Law That is FAonB FBonA Example 1 – Ball hits a bat Action Force: The ball pushes against bat (exerts a force on the bat) Reaction Force: Example 1 – Ball hits a bat Action Force: The ball pushes against bat (exerts a force on the bat) Reaction Force: The bat pushes against the ball (exerts a force on the ball) Together the Action Force and The Reaction force make an ‘interaction force pair’. Example 1 – Ball hits a bat Notice: There are two forces Both act on different objects (one on the bat, one on the ball) The two forces are actual equal in magnitude (same size) but opposite in direction. Example 2 – Push against wall Action Force: You push against a wall (exert a force against the wall) Reaction Force: Example 2 – Push against wall Action Force: You push against a wall (exert a force against the wall) Reaction Force: Wall pushes against you (exerts a force on you). Example 2 – Push against wall Action Force: You push against a wall (exert a force against the wall) Reaction Force: Wall pushes against you (exerts a force on you). Do you believe that the wall is exerting a force on you? Example 2 – Push against wall Consider this: Sit in a wheeled chair and push against the wall. What happens? Example 2 – Push against wall Consider this: Sit in a wheeled chair and push against the wall. What happens? You move backwards! (Why do you move?) As we’ve already discussed, you can’t have motion without a force. Therefore there must be a force pushing you back in the direction you are moving. This force is the Reaction force exerted by the wall! Try These Action Force: Ball exerts force on pin Reaction Force: Try These Action Force: Ball exerts force on pin Reaction Force: Pin exerts force on ball. Try These Action Force: Ball exerts force on glove Reaction Force: Try These Action Force: Ball exerts force on glove Reaction Force: Glove exerts force on ball. Discussion of Misconceptions Homework 1) Complete the online investigation of Newton’s Laws and self-quiz at: http://science.discovery.com/games-andinteractives/newtons-laws-of-motioninteractive.htm You are expected to do the full investigation, read about Newton’s bio, and complete the quiz. (It will be fun - trust me! You get to play with a cute little worm!) 2) Complete the following practice questions: pg. 129 # 2,4,12 pg. 136 # 1-4 pg. 141 #1,2,5