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Chapter 12

Chapter 12. Forces & Motion. Forces. “a push or a pull” A force can start an object in motion or change the motion of an object. A force gives energy to an object. Representing forces. We often use arrows. Larger the arrow, the greater the force Combined balanced forces equal zero

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Chapter 12

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  1. Chapter 12 Forces & Motion

  2. Forces • “a push or a pull” • A force can start an object in motion or change the motion of an object. • A force gives energy to an object

  3. Representing forces • We often use arrows. • Larger the arrow, the greater the force • Combined balanced forces equal zero • Normal force = the force acting perpendicular on an object in contact with another object.

  4. Balanced Forces • Forces opposite in direction and equal in size. • Net result: no change in motion.

  5. No change in motion! • Balanced forces will keep an object moving at a constant velocity ENGINE FRICTION

  6. Friction: • One of the most common forces. • Always acts in a direction opposite the direction of motion. • There are four main types of friction.

  7. Static Friction • The force that must one must overcome to get an object to move. • Examples: • Trying to move a heavy object • Tires on pavement when a car is moving • Walking

  8. Sliding Friction • Two surfaces sliding over each other. • Caused by two factors: • Weight of moving object • Types of surfaces in contact

  9. Rolling Friction • An object rolling over a surface • Not as strong as sliding • Can be reduced by using wheels and ball bearings.

  10. Fluid Friction • When an object is moving through a fluid (either gases or liquids) • ex. a fish swimming under water, air resistance

  11. Is friction ever helpful? • YES. Examples: • Stopping a car • Driving on ice • Walking

  12. GRAViTY • The pull of one body of mass on another.

  13. How fast do objects fall on earth? • Galileo 16th century • Dropped two metal spheres of different sizes off the Leaning Tower of Pisa • Which hit first? • They hit at the same time.

  14. Acceleration due to gravity • All objects fall at the same rate, regardless of their mass or weight • The acceleration due to gravity on earth is about 9.8 m/sec2. • This value will change with elevation and location on earth.

  15. Terminal Velocity • After a period of free fall, the force of gravity will be canceled by the force of air friction and objects will travel at a constant velocity

  16. Projectile motion • Any object thrown in the air becomes a projectile. • The object will move forward due to inertia, and downward due to gravity. • Always a curved path.

  17. Aristotle • 384 B.C. – 322 B.C. • Greek scientist and philosopher • First to work with force. • Incorrectly stated that a force is required to keep an object moving at constant speed

  18. Sir Isaac Newton • 1642-1727 • British Physicist • Considered the “Founder of modern physics”

  19. Newton’s First Law of motion • An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force. • Often referred to as the Law of Inertia. (the property of matter that resists any change in motion)

  20. Newton’s Second Law of Motion • The force of a moving object is directly proportional to the object’s mass and acceleration. • The most important of Newton’s laws. Force = mass x acceleration

  21. How do we label force??? • F = ma • Mass is measured in kg • Acceleration is measured in m/sec2 • If we multiply these two units : 1 kg m/sec2 = 1 newton (N)

  22. MASS The measure of inertia of an object. The amount of matter in an object Stays constant with changes in location A scalar WEIGHT A measure of the force of gravity acting on an object. Measured in newtons. Changes with changes in location. A vector Weight and Mass

  23. Newton’s Third Law of Motion • For every action there is an equal and opposite reaction, or • All forces occur in pairs • examples:

  24. Momentum • All moving objects have momentum. • It is the product of an object’s mass and velocity. • Which has more momentum? A moving car or a moving train? • Not enough information. • A 1500 kg car traveling 50 m/sec? Or a 48,000 kg train traveling 2 m/sec?

  25. Universal Forces • Electromagnetic force: • Forces that attract and repel.

  26. STRONG Responsible for holding the nucleus together Strongest known force WEAK Very small attractive force in the nucleus Nuclear forces

  27. Newton’s Law of Universal Gravitation • Every object in the universe attracts every other object. • Depends on two factors: • Distance between the objects • Mass of the objects

  28. The Earth, Moon and Tides • Earth’s gravitational pull on the moon combined with the inertia or momentum of the moon keeps the moon in orbit around the earth. • The pull of the moon and sun causes the tides

  29. Satellites • Satellites must be placed at a specific height above the earth’s surface in order to stay in orbit. • If it is too close, or moving too slow, it can spiral into the earth’s atmosphere and burn.

  30. Assignment: • Page 385-386 • 1-11, 16, 17, 18, 20, 23,27, 31,32,33 • 1-6 on page 387

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