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Newton’s First Law (Law of Inertia) ∑F = 0 An object will remain at rest or in a constant state of motion unless acted upon by net external forces. Newton’s First Law If ∑ F = 0 => No Change in Motion Dynamic Equilibrium Static Equilibrium Net Forces cause Acceleration Newton’s 2nd Law Fnet = ma Fnet a= m The acceleration of object is directly related to the net force acting on it and inversely proportional to its mass. Newton’s 2nd Law F = ma ⎡m⎤ = [ kg ] ⎢ 2 ⎥ = N ⎣s ⎦ Acceleration ~ Net Force Fnet a= m Acceleration ~ 1/Mass Fnet a= m Acceleration is in the direction of the net Force but not necessarily in the direction of velocity. Frictional Forces Static Friction Sliding Friction Frictional Forces Static Friction Fnet a= m Sliding Friction F− f a= m Question: Frictional Forces Static Friction F− f a= m Sliding Friction What net force does a sliding crate experience when you exert a force of 110 N and the sliding friction between the crate and the floor is 110 N? What can you say about the motion of the crate? Is it moving? Is it accelerating? (Workbook page 9) Mass & Weight F = ma W = mg g = 9.8m / s 2 Mass & Weight W = mg g = 9.8m / s 2 Calculate your weight in N. Calculate your mass in Kg. 1 lb = 4.45 N Free Fall Is the acceleration due to gravity zero? Free Fall You feel weightless in free fall because there is no floor pushing up against you. Rock & Feather A rock and feather fall with the same acceleration due to gravity. Is the force of gravity acting on them the same? Rock & Feather NO! The force of gravity acting on them is their weight! Rock & Feather Why does the rock and feather fall with the same acceleration due to gravity? Rock & Feather The acceleration due to gravity is the same for all object regardless of weight. More massive objects resist their change in motion more than less massive objects. The ratio F/m always turns out to be g. Free Fall Acceleration Air Resistance Air resistance is proportional to the size and speed of an object. WHY? Air Resistance Air resistance is proportional to the size and speed of an object. The bigger the object, the more air it has to push aside. The faster the object, the faster it has to push air aside. Terminal Velocity When the air resistance balances the weight (R = W), the object stops accelerating and it falls with constant velocity called the Terminal Velocity. Fnet a= m W −R a= m W −W = m =0 Terminal Velocity Which person will have the greatest terminal velocity. Why? Terminal Velocity The heavier an object, the greater the terminal velocity. WHY? Terminal Velocity The heavier object requires greater air resistance to balance its weight and therefore more speed so the heavier object accelerates longer and has a greater speed and greater terminal velocity. (Practice Book!) Newton’s Third Law Newton’s First Law (Law of Inertia) ∑F = 0 An object will remain at rest or in a constant state of motion unless acted upon by net external forces. Newton’s 2nd Law Fnet = ma Fnet a= m The acceleration of object is directly related to the net force acting on it and inversely proportional to its mass. Newton’s 3rd Law Fhand on wall = − Fwall on hand To every force there is an equal but opposite reaction force. Newton’s 3rd Law Fhand on wall = − Fwall on hand You can’t TOUCH without being TOUCHED back!! Newton’s 3rd Law Fhand on wall = − Fwall on hand This is an INTERACTIVE Universe. An interaction requires a pair of forces acting on two objects. Gravity is an Interaction FEarth on Rock = − FRock on Earth Gravity is an Interaction The Earth pulls on you, you pull on the Earth. You fall to the Earth, the Earth Falls to you. You accelerate towards the Earth with g = 10m/s2. With what acceleration is the Earth falling towards you? FEarth on You = − FYou on Earth mg = − M E aE mg aE = ME 662 N −22 2 aE = = 1.1x10 m / s 24 5.98 x10 kg Force is not Acceleration FEarth on You = − FYou on Earth The forces are equal but the accelerations are not! Bug Splat A bug and bus have a head on collision. Compared to the FORCE that acts on the bug, how much force acts on the bus? More Same Less Newton’s 3rd Law: Fbus −bug = − Fbug −bus Bug Splat Which undergoes the greater acceleration? Bug F a= m Which suffers the greatest damage? Bug Same Bus Same Bus Action Reaction Pairs kick Gun Pushes Bullet out. Bullet Pushes back on Gun (& Man) Rocket Thrust Rocket Pushes Gas Out. Gas Pushes Back on Rocket. Action-Reaction Pairs If ACTION is A acting on B, then REACTION is B acting on A. Action-Reaction Pairs Action-Reaction You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force on you. Doesn’t this mean the forces cancel one another, making acceleration impossible? Why or Why not? Action-Reaction pairs act on different objects. For F = ma, all the forces act on the same object.