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Newton’s Second Law of Motion How fast does it accelerate? Acceleration • The rate at which velocity changes In other words… Acceleration is a change in velocity – Speeding up • Positive acceleration – Slowing down • Negative acceleration • Deceleration – Changing direction Acceleration • An unbalanced force causes something to accelerate. Acceleration • Acceleration is directly related to the size of the force and the direction of the force. • An object accelerates in the direction you push or pull it. In other words…. Large Force F = Large Acceleration a In other words…. Small Force F = Small Acceleration a So….if you push twice as hard, it accelerates twice as much. But there is a twist…. • Acceleration is INVERSELY related to the mass of the object. In other words…..using the same amount of force…. F Small acceleration Large Mass a Large acceleration F Small Mass a Newton’s Second Law • Newton, observed these “rules” of acceleration and came up with his second law of motion. It is both a formula & a law. Newton’s Second Law • The acceleration of an object is directly proportional to the net force & inversely proportional to it’s mass. • F = ma • Force = Mass x Acceleration Calculating Acceleration • Acceleration Equation (final speed (in m/s) – initial speed (in m/s)) Acceleration = Time (in seconds) A= (sf-si) t A few more things to know… • First, you need to know the units of Force, Mass & Acceleration. – The units used for force are Newtons (N) – The units used for mass are kilograms (kg) – The acceleration units are meters per second squared (m/sec2). What is a Newton? 1 Newton is the force it would take to accelerate a 1 kilogram mass to 1 m/s2 Let’s practice… • Rob is really bored one Saturday night and goes outside to study the nocturnal habits of mice in the hayfield. He sees a mouse sniffing along at 0.1 m/s. but it hears Rob and starts to scurry for safety. In just 3.7 s it accelerates to 0.9 m/s. Find its acceleration. Calculating Acceleration • Acceleration Equation (final speed (in m/s) – initial speed (in m/s)) Acceleration = Time (in seconds) A= (sf-si) t Oh… I get it now! Let’s Practice (final speed (in m/s) – initial speed (in m/s)) Acceleration = A= A= (sf-si) t 0.9m/s – 0.1 m/s 3.7s A= 0.8m/s 3.7s A= 0.22 m/s2 Time (in seconds) Let’s Practice again… • A roach moves down the hall at 1.2 m/s. When he sees the janitor coming down the hall, he begins to run. After 3.2 s, he is moving at 3.6 m/s. What is his acceleration? Now that we’re pros at Acceleration… A quick review problem of Net Force: The net force in this case is: 275 N + 395 N – 560 N = 110 N to the right