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Physics Talk 2.3

Physics Talk 2.3. Newton’s Second Law. Newton’s Second Law of Motion. Do Now: How did the mass on the car change the force needed to push it ?. L.O. Students use F=ma to solve problems relating to Newton’s Second Law of Motion S.C. Identify the forces acting on an object.

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Physics Talk 2.3

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  1. Physics Talk 2.3 Newton’s Second Law

  2. Newton’s Second Law of Motion • Do Now: How did the mass on the car change the force needed to push it? • L.O. Students use F=ma to solve problems relating to Newton’s Second Law of Motion • S.C. • Identify the forces acting on an object. • Determine when the forces on an object are either balanced or unbalanced. • Compare amounts of acceleration semi-quantitatively. • Apply Newton’s Second Law of Motion. • Apply the definition of the Newton as a unit of force

  3. Physics Talk 2.3 • What is the equation for Newton’s Second Law? • What does each variable represent? a = acceleration (m/s2) F = force (Newton – N) m = mass (kg)

  4. Physics Talk 2.3 • What is a Newton? • What causes acceleration? • The Newton is the unit for force. 1 N is the force required to make on kg of mass accelerate at 1m/s2 • 1N = 1 kg*m/s2 • Unbalanced forces

  5. Physics Talk 2.3 • What are some examples of Newton’s second law? • Does Newton’s 2nd Law ever stop working? • If you push a small cart with a large force, it will accelerate a great deal. If you use the same force on a car, it will accelerate less. • No, there is always acceleration, it just may be too small to measure.

  6. Sample Problem 1 • A tennis ball with mass 58g accelerates at 430m/s2when it is served. What is the force responsible for this acceleration? • Given: • m =58 g = 0.058 kg • a = 430m/s2 • Unknown: Force • Tool: F = ma • Solution: • F = 0.058kg*430m/s2 • F = 24.95 kg*m/s2 • F ≈ 25N

  7. Sample Problem 1 • Could an identical force accelerate a 5.0 kg bowling ball at the same rate? • Given: • F=25N • m = 5.0kg • Unknown: acceleration • Tool: F=ma • Solution: • 25N=5kg*a m/s2 • 25N/5kg= a

  8. Sample Problem 1 • Could an identical force accelerate a 5.0 kg bowling ball at the same rate? • No, an identical force would not accelerate the bowling ball at the same rate. • Solution: • 25N=5kg*a m/s2 • 25N/5kg = a

  9. Sample Problem 2 • A tennis racket hit a sand-filled tennis ball with a force of 4 N. While the 275 g ball is in contact with the racket, what is its acceleration? • Given: • F=4N • m=275g = 0.275 kg • Unknown: acceleration • Tool: F=ma • Solution: • 4N=0.275kg*a m/s2

  10. Gravity, Mass, Weight, and Newton’s Second Law • What is the acceleration due to gravity? • What does this mean about the force of gravity? • What is weight? • 9.8 m/s2 • If you drop a 1kg mass, there is a force of 9.8N acting on the object • The vertical, downward force exerted on a mass as a result of gravity

  11. Gravity, Mass, Weight, and Newton’s Second Law • How do you calculate an object’s weight? • What do the variables mean? w = weight m = mass in kg g = acceleration due to gravity (9.8 m/s2)

  12. Balanced and Unbalanced Forces • What is a free-body diagram? • If an object is not accelerating, what will the free-body diagram show? What if the object is accelerating? • A diagram showing the forces acting on an object • The diagram will show opposing forces that are balanced. If the object is accelerating, the opposing forces will be unbalanced.

  13. Free-Body Diagram • Example of free body diagram for

  14. Free-Body Diagram • Example of a free-body diagram for a car moving on the road at a constant velocity. • Is the car accelerating? Since the car is traveling at a constant speed, it is not accelerating. This means that the force of the road on the tires is equal to the air resistance and we have balanced forces.

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