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Acceleration and Force

Acceleration and Force. Engagement. Galileo’s Investigation. What are the similarities and differences between a basketball and a tennis ball? Make a T-Chart. Engagement. Galileo’s Investigation. What will happen if the balls are released from the same height above the floor?

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Acceleration and Force

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  1. Acceleration and Force

  2. Engagement Galileo’s Investigation • What are the similarities and differences between a basketball and a tennis ball? • Make a T-Chart

  3. Engagement Galileo’s Investigation • What will happen if the balls are released from the same height above the floor? • Try to use Physics vocabulary in your response.

  4. Engagement Galileo’s Investigation • What’s going on here, why did they move? • How would you describe the motion of the balls? • Path • Speed • Again, try to use Physics vocabulary in your response.

  5. Explanation Force of Gravity: Pull or Push? • Force can be described as a push or pull one object exerts on another. • One specific force is weight. Weight = the force of gravity on an object. • But what causes gravity? • The presence of mass

  6. Explanation Newton’s Universal Law of Gravitation HO 3.2 • Masses attract one another about their centers with a force proportional to the product of their masses and inversely proportional to the square of the distance between them (whew!). • Try to write that in an equation form.

  7. Explanation Newton’s First Law • Objects require a force to change their velocity. • Another term for this property of matter is inertia • What is the principle property of an object that determines its inertia? • mass

  8. Explanation Newton’s First Law Example • If we apply the same force (push) to a bus and a bicycle, which object will experience a greater change in velocity?

  9. Explanation Newton’s Second Law SF = ma • Force causes a change in motion • Change in motion means change in velocity • Change in velocity means??? • A change in velocity occurs in a certain amount of time. • AHA! That’s a rate.

  10. Explanation Acceleration is a Rate • The speedometer in your car indicates what property of motion? • What happens to the needle if you speed up or slow down? • The speed of the needle is a measure of your acceleration (in a straight line). • Acceleration is the rate of change in velocity.

  11. Explanation Newton’s Second Law Example #1 SF = ma • Important note: the acceleration vector is always in the same direction as the force vector. • If we apply a 100N force to a wagon full of rocks initially at rest with a mass of 50 kg, what is its acceleration? • What does this answer tell us? • What is the velocity of the object after 1 second? • 2 seconds? • 5 seconds? 100 N 2 m/s2

  12. Explanation Newton’s Second Law Example #2 • What happens when the acceleration vector points opposite to the velocity vector? 30 m/s F -5m/s2 How long will it take Mighty Mikey to stop the train?

  13. Explanation Newton’s Third Law • No single isolated force can exist • Equal and Opposite forces exist only between two objects WRONG! Push on water

  14. Explanation Newton’s Third Law Example • You push on the water • The water pushes back on you with an equal force in the opposite direction • So why do you move? Push on swimmer Push on water CORRECT!

  15. Explanation Acceleration Due to Gravity • The Tennis Ball and Basketball hit the ground at the same time because they accelerated at the same rate (~10 m/s2). • What force changed their velocities?...Weight. • Weight = mass X gravity (W=mg). Wbb>Wtb

  16. Explanation Acceleration Due to Gravity F = weight F = weight m = mass m= mass F/m=a F/m = a a = g regardless of mass!

  17. Elaboration Is there a Starbuck’s in the Leaning Tower of Pisa? • Observe the instructor drop a new set of objects. • As a class, discuss the similarities and differences in their motion. • Why is this different?

  18. Elaboration Paper Drop Challenge HO 3.3 • Materials per group: 2 sheets of paper, stopwatch, 8 paperclips, 30cm of tape, scissors, meter stick. • Have Materials Manager distribute supplies. • You have 10 minutes for this activity. • Make one whole piece of paper drop to the floor as fast as possible from a height of 2 meters and the other drop as slowly as possible from the same height. (Do not attach the scissors or meter stick to your paper.) • GO!

  19. Elaboration Paper Drop Challenge • As a group, describe your strategy for making the piece of paper: • Drop the slowest • Drop the fastest • Make a table of your drop times for comparison with the class • Explain why your two pieces of paper accelerate at different rates.

  20. Explanation Free Body Diagram HO 3.4 • A free body diagram is a pictorial representation of all forces acting on an object. • We consider the object to be isolated from the physical system and choose to examine only the forces directly acting ON the object, not forces applied BY the object.

  21. Explanation Free Body Diagram Example Label the forces acting on the block.

  22. Explanation Free Body Diagram Example Force on the block by the ground Force on the block by Antonio Force on the block by the ground Force on the block by the rope. Force on the block by the earth

  23. Explanation Free Body Diagram Example 2 • Forces on coffee filter before release (balanced forces) Support from my hand Weight

  24. Explanation Free Body Diagram Example 2 • Forces on coffee filter immediately after release (unbalanced force  acceleration) Weight

  25. Explanation Free Body Diagram Example 2 • Forces on coffee filter while falling Air resistance What factors influence the magnitude of the force of air resistance? Weight

  26. Explanation So Why Different Times? • Force due to gravity is different for the two objects • Their masses are different and therefore they have different inertias • They do not accelerate at the same rate …WHY??? • We must examine ALL the forces acting on the objects in order to determine the Net Force.

  27. Explanation Net Force SF • If you earn $2000/mo but taxes, ins, retirement and other reductions total $350/mo then your: • Gross Pay = $2000/mo • Net Pay = $1650/mo Net Force

  28. Explanation Net Force • The force of air resistance is greater for the basketball than the coffee filter. • However, the force of air resistance is negligible compared to the weight of the basketball, therefore the Net Force is toward the floor. Air resistance Weight

  29. Explanation So Why Different Times? • The force of air resistance is also acting on the coffee filter and is comparable in size to the weight. • In fact, at some point, the force of air resistance is equal to the weight of the coffee filter. • When this is the case: • There is NO net force • Therefore there is no acceleration • Therefore the object moves at a constant speed • This equilibrium condition is called terminal velocity

  30. Evaluation Sky Diver Physics HO 3.5 • A skydiver jumped out of a plane 1 sec ago. • Label the forces on the skydiver • After some time, the skydiver has zero acceleration • What does this indicate about her speed? • What is the name for this equilibrium condition of a falling object? • Label the forces on the skydiver • What factors influence the force of air resistance on the skydiver? • BONUS: What is the terminal velocity of a human?

  31. Summary Summary • Force • Gravity & Weight • Newton’s Laws • Air resistance • Free Body Diagrams • Net Force • So what? • This applies to transportation, moving objects, sports, etc.

  32. Homework Reading Assignments HO 3.6 • NSES • p. 123, 126-127; 149, 154-155; 176-181 • BSL • 87-92 • Integrated Science • Forces: p. 26-27, Falling Objects:p. 29-31, Laws of Motion:p. 32-38

  33. Homework Concepts and Questions • Integrated Science, Ch. 2 • Applying the Concepts • 1-6, 8-9 • Questions for Thought • 1, 5, 8

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