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Forces and Motion

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  1. Forces and Motion

  2. 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

  3. Sir Isaac Newton • 1643 - 1727

  4. 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. 1st Law of Motion (Law of Inertia)

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

  6. 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

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

  8. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

  9. 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).

  10. 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.

  11. 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.

  12. 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.

  13. 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?

  14. 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.

  15. 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…

  16. 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)

  17. 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.

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

  19. Gravity causes weight What is the accelration due to gravity if Mr. Batchelder has a force of • 500N on Earth • 80N on the moon • 1000N on Saturn

  20. Newton’s 2nd Law – Let’s practice You push your 100kg friend in a rolling chair down the hall. They accelerate at a rate of 5 m/s2. How much force did you apply? You are texting while driving. You and your car have a mass of 500kg. You accidently hit someone while accelerating at 8 m/s2. What force did you hit them with?

  21. Newton’s 2nd Law – Let’s practice • 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

  22. 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 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?

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

  24. 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 What does this mean?

  25. 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).

  26. 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

  27. 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

  28. 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.

  29. 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: 5 N 10 N 6 N

  30. 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.

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

  32. 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.

  33. 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!!!

  34. 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.

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

  36. Types of forces Contact Non-Contact Forces

  37. Contact Normal Frictional Tension Air Resistance Spring Forces

  38. Non-Contact Electrical Magnetic Nuclear Forces

  39. The Normal Force • The normal force (FN) is one component of the force that a surface exerts on an object with which it is in contact • Simply, the component that is ____________ to the surface. Perpendicular

  40. The Normal Force • How do I find the normal force? • Easy, it is equal and opposite to the weight of an object if the object is on a horizontal surface. • The Fn is not always horizontal. • The Fn can be found by • Fn=mg cosΘ(where Θ is the angle between the contact surface and horizontal)

  41. The Normal Force • Where is the normal force?

  42. is a force resisting the relative motion of two surfaces in contact with each other. is the parallel component of the force on an object that is in contact with a surface. is a force that preventsmotion from occurring. Types Static Kinetic Fluid What is Friction?

  43. What causes friction? There are two factors which affect friction between two surfaces: Kind of surfaces in contact (rough or smooth) Amount of force pressing the surfaces together. The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

  44. From Glencoe Physical Science

  45. Static Friction- Friction force that acts on objects that are not moving Kinetic Friction - Friction force that opposes the direction of motion of an object as it slides over a surface. Static and Kinetic Friction I better be safe Ump!!

  46. Static Friction • Static Friction is the force that prevents the start of motion. • With no movement, Fs = Fapplied • Has a range of 0 – Fs,max

  47. Kinetic Friction • Once motion has begun, the static friction force is no longer a concern. • Instead, kinetic friction is important. • Kinetic friction is resistance to movement of an already moving object.

  48. Friction force that opposes the motion of an object through a fluid. Fluid Friction

  49. Frictional Forces • The frictional force will vary depending on the surface interactions. • Ex. Carpet vs tile • Friction can be calculated by using the coefficient of friction. • Use the basic formula of: • Ff = μFn • It can be rearranged to: • μk = (Fk/Fn) and μs = (Fs/Fn)

  50. Frictional Forces • Use the formula of: • Ff = μFn • Fk = μkFn and Fs = μsFn • A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.