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Chapter 3-4

Chapter 3-4. Force, Mass and Acceleration. Force. Defined as a push or a pull . Net forces cause acceleration (change in motion) Unit of force is the Newton (N) Net force is the total amount of force acting on an object 1) Can be balanced or unbalanced

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Chapter 3-4

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  1. Chapter 3-4 Force, Mass and Acceleration

  2. Force • Defined as a push or a pull. • Net forces cause acceleration (change in motion) • Unit of force is the Newton (N) • Net force is the total amount of force acting on an object 1) Can be balanced or unbalanced a) balanced forces occur when the net force is equal to zero and do no change the motion of the object b) unbalanced forces when the net force is NOT equal to zero and do change the motion of the object.

  3. Calculating the Net Force • When the forces are acting in the same direction, add to get the net force. 2N + 2N = 4N The above object will accelerate because there is a net force acting on it.

  4. When the forces are acting in opposite directions, subtract to get the net force. 2N - 2N = 0N This object will not accelerate because the net force is equal to zero. Equilibrium occurs when the net force is zero, the object will not change its motion

  5. Calculate the net force 1) 3N 4N

  6. Calculate the net force 1) 3N 4N 7N to the left 2) 2N 2N

  7. Calculate the net force 1) 3N 4N 7N to the left 2) 2N 2N 4N up

  8. 3) 3N 4N

  9. 3N 4N 1N to the left 3N 4N 7N

  10. 3N 4N 1N to the left 3N 4N Force UP  3 + 4 = 7 Force Down = 7 Net Force = Force Up – Force Down 7 – 7 = 0N 7N

  11. 5) Which direction will the block move? 5N A 5N C B

  12. 5) Which direction will the block move? 5N A 5N C B

  13. 6) What is the net force on the block? 6N 9N • Between 6N and 9N • Less than 6N • Exactly 9N • Greater than 9N

  14. 6) What is the net force on the block? 6N To Solve: Use the Pythagorean theorem. 9N 6N • Between 6N and 9N 9N • Less than 6N a2 + b2= c2 • Exactly 9N 92 + 62 = c2 • Greater than 9 81 + 36 = 122 122 = 11 N

  15. Friction – A Force • Defined as a force that results from the movement of 2 objects in contact. • Caused by the surface irregularities between the 2 objects. rough surface smooth surface has high friction has low friction (carpet) (ice)

  16. Friction is what causes things to wear out Examples: break pads, erosion • Friction resists motion motion of block is to the right The force of friction acts to the left • Friction is what enables moving objects to come to rest

  17. Types of Friction • Static Friction occurs between surfaces that are not moving. This is the friction you must overcome in order to start motion. a) static friction is the most difficult friction to overcome. 2) Sliding (Rolling) Friction occurs between surfaces that are moving

  18. 3) Air Resistance a) friction due to air pushing up on a falling object b) depends on size, shape, and speed of the object air resistance gravity c) Terminal velocity occurs when the force of gravity down equals the air resistance up. 1) Highest velocity a falling object will reach

  19. The Absence of Air Resistance • In the absence of air resistance all objects will fall at the same rate. • If a hammer and a feather are dropped at the same time on the moon, both will land at the same time because there is no air resistance.

  20. Gravity – Another Force • Is a force that every object exerts on every other force. • Is the weakest force in nature • The Law of Universal Gravitation The force of gravity depends on the directly on the masses of the two objects and inversely on the square of the distance between the objects *Increase mass = increase gravity *Increase distance = decrease gravity

  21. Law of Universal Gravitation Formula Force ofmass 2 gravitymass 1 F = G (m1m2) d2 Universal constantdistance squared

  22. Gravity & Weight • Gravity is the force the earth exerts on you • The force of gravity changes with location • Weight is a measure of the force of gravity acting on a mass • Formula: Weight = mass X gravity W = m X g • Units: Weight = Newtons (N) Mass = kilogram (kg) Gravity = m/s2 on earth = 9.8m/s2 W g m

  23. Newton’s First Law • Two Parts: 1) An object at rest will stay at rest unless a net force acts on it. 2) An object in motion will continue in motion in a straight line at the same speed unless a net force acts on it. • In other words – objects resist changes in motion.

  24. Inertia is the ability of an object to resist changes in its motion. 1) depends on mass a) the more mass = the more inertia Tennis Ball Bowling Ball • Newton’s 1st Law is known as the Law of Inertia.

  25. Newton’s 2nd Law • Relates force and mass to acceleration. • Formula: acceleration = force a = F mass m • Units: Force (N) mass (kg) acceleration (m/s2) (We will solve some sample problems later ) F m a

  26. Formula tells us: 1) Net forces cause acceleration. a) force and acceleration are directly proportional. 1) F = A (Big Force = Big Acceleration) 2) f = a (Little Force = little Acc) 2) mass resists acceleration. a) mass and acceleration are inversely proportional 1) M = a (Big mass = little acceleration) 2) m = A (small mass = big acc)

  27. Newton’s 3rd Law • States for every action there is an equal and opposite reaction. • Relates forces as pairs. Each force pair is made up of an action force and a reaction force. Example: Student sitting in chair Action force: Student pushes down on chair Reaction force: Chair pushes up on student

  28. State the reaction force • Action force: Bat hits ball Reaction force:

  29. State the reaction force • Action force: Bat hits ball Reaction force: Ball hits bat • Action force: You push a box to the right Reaction force:

  30. State the reaction force • Action force: Bat hits ball Reaction force: Ball hits bat • Action force: You push a box to the right Reaction force: Box pushes you to the left • Action force: Book pushes down on table Reaction force:

  31. State the reaction force • Action force: Bat hits ball Reaction force: Ball hits bat • Action force: You push a box to the right Reaction force: Box pushes you to the left • Action force: Book pushes down on table Reaction force: Table pushes up on book

  32. Remember, Newton’s 3rd Law states that forces are EQUAL and opposite. • Since forces are equal, that means that you can only apply a force to an object that an object can apply to you.

  33. Check Understanding - Newton’s Three Laws • Imagine a place in the cosmos far from all gravitational and frictional forces. Suppose an astronaut in that place throws a rock. The rock will: a) gradually stop b) continue in the same direction and the same speed

  34. 2) A 2-kg object is moving horizontally with a speed of 4 m/s. How much force is required to keep the object moving with the same speed and in the same direction? Zero. Think about if friction could be eliminated~once an object is in motion, it will continue in motion at a constant velocity (same speed and straight line). Newton’s 1st Law INERTIA Remember Galileo postulated that if friction could be entirely eliminated an object in motion would continue to move

  35. 3) Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed, it will have greater inertia. Tosh argues that inertia does not depend on speed, but rather on mass. With whom do you agree? Tosh

  36. 4) If you were in a weightless environment in space, would it require a force to set an object in motion? Yes. Newton’s 1st law states: an object at rest will remain at rest unless a net force acts on it.

  37. 5) Mr. Wegley spends most Sunday afternoons at rest on the sofa watching football games and consuming large quantities of food. What effect (if any) does this situation have upon his inertia? His inertia will increase. As he eats, he will increase his mass. As his mass increases so does his inertia.

  38. 6) When a bug and a car have a collision, both experience the same force. Which object, the bug or car, will have little change in its motion? Why? The car will have very little change in its motion because it has more inertia.

  39. 7) While driving Anna Litical observed a bug striking the windshield of a car. The bug hits the windshield and the windshield hits the bug. Which of the two forces is greater: A) The force on the bug B) The force on the car C) Neither C. Newton’s 3rd Law – equal and opposite forces

  40. Momentum • Inertia in motion • Formula: momentum = mass X velocity p = m X v (momentum) (velocity) (mass) Units momentum = kg*m/s mass = kg velocity = m/s p m v

  41. Mass and momentum are directly proportional. 1) increase mass = increase momentum Explains why a bike is easier to stop than a car traveling at the same speed. Bike Car p = mv p = mv

  42. Velocity and momentum are directly proportional 1) increase velocity = increase momentum Explains why a slow moving bike is easier to stop than a fast moving bike Slow Bike Fast Bike p = mvp = mv

  43. The Law of Conservation of Momentum • States: As long as interacting objects are not influenced by outside forces (like friction), the total momentum does not change. Momentum is transferred between objects

  44. Collisions • Examples of the Law of Conservation of Momentum • Two types: 1) Elastic objects bounce off one another ex: billiard balls 2) Inelastic collisions  objects stick together ex: car crashes

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