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Forces

Forces. We use the word force all the time – what are some examples? When an object moves in a new direction, speeds up or slows down, it is because a force has been applied to the object!. Forces .

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Forces

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  1. Forces

  2. We use the word force all the time – what are some examples? When an object moves in a new direction, speeds up or slows down, it is because a force has been applied to the object!

  3. Forces • A force is a push or a pull exerted on an object in order to change the motion of the object. • All forces have two properties: Direction and Magnitude (size) • A Newton (N) is the SI unit used to describe the magnitude, or size, of a force.

  4. Forces cont. • Forces always act on objects. For any push to occur, something has to receive the push. • A force can act on an object without causing the object to move.

  5. Combined Forces • The net force is the combination of all the forces acting on an object. • When forces act in the same direction, you add the forces to determine the net force. • When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force.

  6. What is the Net Force?

  7. 1. No Forces Net force = 2. One Force Net force = 5. 2 Forces same direction Net force = 3. 2 Forces Opposite but equal Net force = 4. 2 Forces Opposite and unequal Net force =

  8. Net Force Problems • When two forces act in the same direction how do you calculate the net force? • A dog pulls on it’s leash forward with a force of 12 N, the owner pulls back with a force of 15 N. What is the net force? • Two students are pulling on a backpack in different directions. Student 1 pulls with a force of 10 N left, student 2 pulls with 5 N right. What is the net force? Which student will get the backpack? • What two things must all forces have?

  9. Balanced Forces • When the forces on an object produce a net force of 0 N, the forces are balanced. • Balanced forces do not cause a change in the motion of an object. • Objects that are not moving and have balanced forces acting on them are called static objects.

  10. Tension and Compression • Tension and compression are two forces that often act on static objects. • Tension is a force that is exerted when matter is pulled or stretched. • Compression is a force that is exerted when matter is pushed or squeezed. • Examine figure 4 on page 347. Write down the forces acting on: • The kittens • The bird feeder 3. The dog/walker

  11. Compression or Tension?

  12. Gravity and Normal Force Gravity – on Earth gravity is the force that pulls all things towards the center of the Earth Normal –opposite force of gravity, it prevents things from moving through each other into the center of the Earth.

  13. Unbalanced Forces • When the net force on an object is not 0 N, the forces on the object are unbalanced. • Unbalanced forces cause a change in the velocity of an object. • Unbalanced forces are called APPLIED FORCES • Objects do not always move in the direction of the unbalanced force.

  14. Balanced or Unbalanced?

  15. Quick Lab: identifying forces 1. Sketch these three items on your paper: a. pencil writing on a paper b. person sitting in a chair c. kite stuck in a tree 2. On each sketch, draw arrows identifying the forces acting on each object. 3. Answer these questions: Page 349 #s 2-9

  16. Imagine that you are trying to push a heavy desk across the floor. There are two opposing forces on the desk: the force you apply, and the force of friction between the desk and the floor. What will happen if the force of friction is the same magnitude as the force that you apply? What will happen if the force that you apply is greater than the force of friction?

  17. Quick Write • What is friction? • Tell me a story of a time you experienced friction for yourself. DON’T STOP WRITING UNTIL TIME IS UP!!!

  18. Sources of Friction • Read the paragraph ‘The Source of Friction’ on page 350, look at Figure 1 and answer: • Define Friction • What is figure 1 showing? • What are the hills and valleys? • Copy figure 1 into your notes When the hills and valleys of one surface stick to the hills and valleys of another surface, friction is created. • Friction is a force that opposes motion between two surfaces that are touching.

  19. Types of Friction • There are two types of friction: kinetic friction and static friction. • Kinetic friction is friction between moving surfaces. • Static friction balances an applied force and prevents changes in motion. • Some static objects do not move because the force of static friction balances the other forces acting on the object.

  20. FACTORS THAT AFFECT FRICTION 1. Mass of the object: heavier objects require stronger pushes to move them. • As the force pushing surfaces together increases, the friction between the surfaces increases. 2. Type of Surface: rough surfaces have more friction than smooth surfaces

  21. Air Resistance Air resistance is friction between an object and the air around the object The faster the movement of the object or the air, the more air resistance there is. Air resistance opposes motion of an object

  22. Harmful and Helpful Friction • Harmful friction can be decreased by: • Lubricants-Some examples of lubricants include motor oil, wax, and grease. • Rolling friction -happens between round objects like ball bearings. • Smooth out surfaces • Helpful friction can be increased by: • Make surfaces rougher • Increase the force pushing surfaces together

  23. Harmful or Helpful?

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