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N t ’ L Newton’s Laws off M Motion ti Isaac Newton developed his 3 Laws of Motion (by the time he was 23!) to describe motion in the natural world. According to Newton Newton, why did the chicken cross the road? Chickens at rest tend to stay at rest and chickens in motion tend to cross the road N t ’ 1stt Law Newton’s L off Motion M ti Newton’s 1stt Law off Motion is a restatement of Galileo’s concept of inertia: “Every object “E bj continues i iin iits state of rest, or a uniform speed in a straight line, line unless acted on by a nonzero force.” N t ’ 1stt Law Newton’s L off Motion M ti Old tablecloth bl l h trick i k http://www.youtube.com/watch?v=cuG8sIi V8iQ The brief friction between the dishes and the fast-moving tablecloth starts the dishes moving but then the friction b t between th the dishes di h and d the th table t bl stops t the dishes before they slide very far. N t ’ 1stt Law Newton’s L off Motion M ti Force Block Diagrams { { Used to show the different forces acting on j an object. Helpful for finding the net force What forces are acting g on the green g box if it is still? Normal force Weight (Gravity) N t ’ 1stt Law Newton’s L off Motion M ti Force Block Diagrams What forces are acting on the green b box if someone is i pushing hi it? N Normal force Pushing Force Friction Weight eg t (Gravity) F P W N t ’ 1stt Law Newton’s L off Motion M ti Example: What is the force on the dog? When will the dog move? Wh t is What i Acceleration? A l ti ? Non-accelerated Motion Occurs when an object is moving at a constant velocity Problem 1 1m 5m The ball reaches the wall with a speed p of 1 m/s and takes a time of 5 seconds Wh t is What i Acceleration? A l ti ? Non-accelerated Motion Animal Distance Time Speed Cheetah 75 m 3s 25 m/s Greyhound 160 m 10 s Gazelle 1 km Turtle Speed = Distance / Time 100 km/h / 30 s 1 cm/sec Wh t is What i Acceleration? A l ti ? What about Accelerated Motion? Acceleration occurs when the object is not moving at a constant speed Time Acceleration Velocity v = at Distance d = ½ at2 0s 2 m/s2 0 m/s 0m 1s 2 m/s / 2 2s 2 m/s2 3s 2 m/s2 4s 2 m/s2 5s 2 m/s2 N t ’ 2ndd Law Newton’s L off Motion M ti Newton was the first scientist to recognize the connection between force and mass in producing p g acceleration. Newton’s 2nd Law of Motion states: “The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the t e mass ass o of the t e object.” object N t ’ 2ndd Law Newton’s L off Motion M ti A simpler way to state Newton’s 2ndd Law is: acceleration = net force mass N t ’ 2ndd Law Newton’s L off Motion M ti Question: Before we defined acceleration as the change h in speed d over time and d now we are saying that acceleration is the ratio of force to mass – which is it? a = v/t or a = f/m Acceleration is defined as that change in velocity over time but is caused by force applied to the mass CHECK YOUR NEIGHBOR Suppose in a high-flying f airplane the captain announces over the cabin public address system that the plane is flying at a constant 900 km/h and the thrust of the engines km/h, is a constant 80,000 newtons. What is the acceleration of the airplane? N t ’ 2ndd Law Newton’s L off Motion M ti Sample Problem Suppose you apply the same force (1 N)) to two d different ff carts, one with ha mass of 1 kg and the other with a mass of 2 kg. kg Which cart will accelerate more? 1N 1 Kg a= ? 1N 2 Kg a=? How are you feeling about this concept? t? { Reflection f How well do you understand Newton’s 1st Law? How about his 2nd Law? What can you do to improve your understanding?