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Unit II Forces

Unit II Forces. Force = a push or pull on an object measured in Newtons (N). 4 Ways Force Effects an Object. Increase v 2. Decrease v 3. Change Direction 4. Change Shape. What are the forces action on box being pushed?. Which box is moving?. A. B.

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Unit II Forces

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  1. Unit II Forces

  2. Force = a push or pull on an objectmeasured in Newtons (N) 4 Ways Force Effects an Object • Increase v • 2. Decrease v • 3. Change Direction • 4. Change Shape

  3. What are the forces action on box being pushed?

  4. Which box is moving? A B

  5. Net Force = sum of forces acting on an object • Forces are vectors • Same direction add • Opposite direction subtract

  6. Balanced vs. Unbalanced Forces Unbalanced Forces: the forces are not equal and opposite -the object will change -the object will accelerate Balanced Forces: the forces on an object are equal and opposite. -no change to the motion -object does not accelerate

  7. Friction: force that opposes motion of an object 1. Static Friction 2. Rolling Friction 3. Sliding Friction 4. Fluid Friction

  8. Laws of Motion 12.2-12.3

  9. 1st Law of Motion • An Object in motion will remain in motion, or an object at rest will remain at rest unless acted upon by an unbalanced force teachertech.rice.edu/.../Newton/law1.html

  10. Inertia • The tendency of an object to keep doing what it is already doing

  11. 2nd Law of Motion • Force, mass and acceleration are all related

  12. 2nd Law Force = mass x acceleration F = ma

  13. 3rd Law of Motion Action and Reaction For every action there is an equal and opposite reaction http://upload.wikimedia.org/wikipedia/commons/f/f6/Skaters_showing_newtons_third_law.png

  14. How is weight different than mass? • Weight  pull of gravity acting on an object Weight = mass x acceleration due to gravity • Mass  amount of matter in an object

  15. Sample Problems 1. A boy pushes a cart of groceries forward with a total mass of 40 kg. What is the acceleration of the car if the net force on the cart is 60 N? 1.5 m/s²

  16. 2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10000 N act on it in an upward direction? 2 m/s²

  17. 3. An automobile with a mass of 1200 kg accelerates at a rate of 3 m/s/s in the forward direction. What is the net force acting on the automobile? 3600 N

  18. 4. A 25 N force accelerates a boy in a wheelchair at 0.5 m/s/s. What is the mass of the boy and the wheelchair? 50 kg

  19. Sample Problems 5. A girl pushes a box of books forward with a total mass of 80 kg. What is the acceleration of the box if the net force on the box is 40 N? 0.5 m/s²

  20. 6. An truck with a mass of 3600 kg accelerates at a rate of 2 m/s/s in the forward direction. What is the net force acting on the automobile? 7200 N

  21. 7. A 30 N force accelerates a boy on a tricycle at 0.5 m/s/s. What is the mass of the boy and the tricycle? 60 kg

  22. 14.1 Work • Definition: the product of distance and the force in the direction an object moves • W = F x d • SI unit for work: newton-meter (N·m) or joule (J) • If there is no movement, no work is done

  23. Power • What is power? • The rate of doing work • Doing work faster requires more power • Power = Work Time • SI Unit – Watt (W)

  24. Momentum Momentum = mass x velocity of an object Momentum is large if product of mass and velocity is large Objects with large momentum’s are hard to stop

  25. Momentum Momentum of an object at rest = 0 SI Units = kg*m s

  26. Momentum Law of Conservation of Momentum If no net force acts on the system, the total momentum of the system does not change. In a closed system, momentum is conserved.

  27. Momentum Problem Which has more momentum, a 0.046 kg golf ball with a velocity of 60 m/s or a 7.0 bowling ball with a velocity of 6 m/s? Remember: Momentum = mass x velocity

  28. 12.4 Universal Forces • What are the 4 universal forces? • Electromagnetic • Nuclear (Strong and Weak) • Gravity • How are these different from forces talked about in the 12.1-12.3? • All act over a distance between particles

  29. Describe each universal force and what makes them unique. • Electromagnetic Force • only force that can attract and repel • Gravitational force • An attractive force between 2 masses

  30. Describe each universal force and what makes them unique. • Nuclear force: act within nucleus to hold it together • Strong – acts only on neutrons and protons in the nucleus (100x stronger than charges) • Weak – attractive force that acts over a short range

  31. Force Can be represented by Can be Measured in units of unbalanced Causes the object to Causes no change to the object

  32. Force Can be represented by Can be arrows Measured in units of unbalanced Newtons balanced Causes the object to Causes no change to the object State of Motion Accelerate

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