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Forces and Motion

Forces and Motion. Three Incorrect laws of motion. http://www.youtube.com/watch?v=Yf0BN0kq7OU&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1. Why does the Earth Spin? http://www.youtube.com/watch?v=TQxeutcYP6I&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

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Forces and Motion

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  1. Forces and Motion

  2. Three Incorrect laws of motion. • http://www.youtube.com/watch?v=Yf0BN0kq7OU&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

  3. Why does the Earth Spin? • http://www.youtube.com/watch?v=TQxeutcYP6I&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1 • Second clip? • http://www.youtube.com/watch?v=9zso7ChaQXQ&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

  4. Vocabulary • Force - A push or pull on an object. • Net Force – vector sum of all forces • Equilibrium: • the state in which the net force on an object is zero or balanced • no change in velocity

  5. Sir Isaac Newton • 1643 - 1727

  6. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2.2.4 State Newton’s first law of motion. 2.2.5 Describe examples of Newton’s first law.

  7. Translation: Objects keep doing what they are doing, in a straight line. 1st Law of Motion (Law of Inertia)

  8. Newton’s 1st Law – Inertia • Inertia is a measurement of ______________ TO CHANGE • It is measured in terms of ______. The greater the _____, the greater the inertia. RESISTANCE mass mass

  9. What is Mass? • Mass is the amount of material a body/object contains • You are what you eat • Bigger does not equal more mass

  10. 5 N 10 N 6 N 2.2.2 - Identify the forces acting on an object and draw free-body diagrams representing the forces acting Free Body Diagrams • FBD’s are a neat way to show the forces acting on an object. • The object is drawn as a dot or a box, and then vectors are drawn to show the forces acting on it. • Ex:

  11. FBD’s • Can you draw the FBD for you sitting on your chair? • Draw the FBD for your friend pushing you in a rolling chair.

  12. 5 N 10 N 6 N FBD’s • Solve the net force for the following FBD.

  13. FBD • The picture at the bottom shows a snowman pulling a sled. Draw a free-body diagram for this sled. The magnitudes of the forces acting on the sled are 60N by the string, 130N from gravity, and 90N upward by the ground.

  14. FBD • Now try doing some of the free-body diagrams on • pg 124 Practice A • Pg 124 Section Review # 2-5

  15. 2.2.6 - State the condition for translational equilibrium.2.2.7 - Solve problems involving translational equilibrium. • Equilibrium in a straight line. • Fnet= 0 so there is constant velocity

  16. 2.2.6 - State the condition for translational equilibrium.2.2.7 - Solve problems involving translational equilibrium. • Derek leaves his physics book on top of a drafting table that is inclined at 35º angle. The FBD to the right shows the forces acting on the book. Find the net force acting on the book.

  17. Vocabulary • Force - A push or pull on an object. • Net Force – vector sum of all forces • Equilibrium: • the state in which the net force on an object is zero or balanced • no change in velocity

  18. 2.2.8 State Newton’s second law of motion. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

  19. Newton’s 2nd Law – Simplified • So, a little easier to read would be:a = ΣF OR ΣF = ma m • So… what are the units for force • Forces are measured in newtons  • N = (kg)(m/s2).

  20. What does F = ma mean? • Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force.

  21. What does F = ma mean? Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.

  22. What does F = ma mean? Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

  23. More about F = ma • If you double the mass, you double the force. If you double the acceleration, you double the force. What if you double the mass and the acceleration? (2m)(2a) = 4F Doubling the mass and the acceleration quadruples the force. So . . . what if you decrease the mass by half? How much force would the object have now?

  24. What does F = ma say? • F = ma basically means that the force of an object comes from its mass and its acceleration. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

  25. Newton’s 2nd Law – Vectors! • Remember, acceleration is a vector quantity, so Force will be too  • Some equations to remember: • Pythagorean Theorem • Sin θ • Cos θ • Tan θ

  26. 2.2.1 Calculate the weight of a body using the expression W = mg. Gravity causes weight • The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth. Gravitation force is equal to weight, therefore…

  27. How much does a person with a mass of 70.0 kg weigh on the earth? Weight = mass X 9.8 m/s2 Weight = 70.0 kg X 9.8 m/s2 Weight = 686 N Remember that 1 N = (1 kg) (1m/s2)

  28. Mass vs. Weight • Your MASS is the amount of material in your body. • Your MASS doesn’t depend on where you are. • Your WEIGHT is how much your body pushes down on a scale. • Your WEIGHT depends on how much MASS you have and where you are. • Weight is caused by gravity.

  29. Martym = 68 kg What does Marty weigh? • G(sun)= 275 m/s2 18,700 N G(pluto)= 0.654 m/s2 44 N

  30. 2.2.9 Solve problems involving Newton’s second law. • Determine the accelerations that result when a 12-N net force is applied to a 3-kg object. • A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. What is the mass of the encyclopedia?

  31. 2.2.9 Solve problems involving Newton’s second law. • Now try answering some problems on pg 132 practice C # 1-4

  32. There is a net force of 12N acting on a desk which has a gravitational force of 40N. How much is the desk accelerating?

  33. There is a net force of 12N acting on a desk which has a gravitational force of 40N. How much is the desk accelerating? • A 50N crate experiences a horizontally applied force of 10N from a student and a frictional force of 4N. What is the acceleration of the crate?

  34. Newton’s 2nd Law – Let’s practice • You have run out of gas and have to push your car. Your car weighs 560 N and you push with a force of 395 N, and your friend helps with 275 N. What is the acceleration of the car? • You and a friend push a 20kg rolling chair horizontally. You push with a force of 17 N. The chair has a frictional force of 5N. The chair accelerates at a rate of 1.35m/s2. How much force is your friend contributing?

  35. NET FORCE Practice • Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. Then click the button to view the answers.

  36. Newton’s 2nd Law – A little bit harder • You have run out of gas and have to push your car. Your car weighs 560 N and you push with a force of 395 N, and your friend helps with 275 N. What is the acceleration of the car? • You push a rolling chair (m=20kg) with a force of 17 N. A friend pushes with 15 N at 30o. What is the net force? What is the acceleration of the chair?

  37. Quick Quiz Consider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction? We’ll come back to this question.

  38. HWA • What is the net force acting on an object that has a force of 40N to the right and another 30N downward? • What are the components of a 20N force acting on a desk at an angle of 30º above horizontal?

  39. 2.2.14 State Newton’s third law of motion. 2.2.15 Discuss examples of Newton’s third law. http://www.youtube.com/watch?v=8bTdMmNZm2M&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

  40. For every action, there is an equal and opposite reaction. 3rd Law

  41. What does this mean? • For every force acting on an object, there is an equal force acting in the opposite direction. • Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. • There is a balanced force acting on you– gravity pulling down, your seat pushing up. • Put really simply: F = F

  42. Think about it . . . What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

  43. Newton’s Third Law • It states: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object • It requires two forces: • _____________ Force • ______________ Force Action Reaction

  44. Newton’s Third Law • Does it matter which force we call the ________ and which is the _________? • Forces always occur in ________. Therefore, it ________ matter which is the action and the reaction, as long as you can identify both. action reaction pairs doesn’t

  45. Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.

  46. Quick Quiz Consider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction? YES!!!! IT CAN!!!

  47. Quick Quiz Think about this situation. What are the forces on the boy on the sled between points B and C? I hope you got something that looked a lot like this.

  48. Quick Quiz This means a force is not required to keep a moving object in motion.

  49. Types of forces • Contact • Non-Contact Forces

  50. Contact • Normal • Frictional • Tension • Air Resistance • Spring Forces

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