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Forces

Learn about the effects of balanced and unbalanced forces on objects, including gravity, inertia, and friction. Explore Newton's Laws of Motion and projectile motion.

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Forces

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  1. Forces Standard: S8P3 b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

  2. Forces What are forces? Inertia Gravity Newton’s Laws Projectile motion

  3. FORCES

  4. What is a force? • A push or a pull • May be unbalanced or balanced

  5. What does a force do? • Gives energy to an object • Can set an object in motion, make it stop, or change its motion

  6. Unbalanced forces change motion

  7. Unbalanced forces change motion The truck motion is changed, but the ladder keeps going

  8. Unbalanced forces change motion crosswind tailwind headwind

  9. What are forces represented by?Forces are represented by vectors motion of book friction

  10. Forces are added with vectors

  11. What is friction? • A force which opposes motion, bringing objects to rest • Types: sliding fluid rolling

  12. Inertia

  13. What is inertia? I have more inertia • The tendency of an object to stay at rest or in motion. • Objects with greater mass have greater inertia but we fall at the same rate

  14. Inertia keeps astronauts in motion

  15. Seat belts protect against inertia

  16. Gravity

  17. Gravity The attractive force between all objects in the universe

  18. F = M1 M2 d2 • More mass means greater gravity • More distance means less gravity My gravitational attraction is small no matter what the distance! Massive objects have lots of gravity

  19. The earth exerts a force on the student and the student exerts a force on the earth because of GRAVITY!

  20. Gravity warps the fabric of space-time

  21. Gravity

  22. gravity pulls it in inertia keeps it going What forces are acting on the moon? Inertia

  23. Newton’s Laws of Motion 1. Inertia 2. F=ma 3. Action-reaction

  24. Newton’s First Law of Motion • The law of inertia: An object at rest stays at rest an object in motion stays in motion unless acted upon by an outside force.

  25. Inertia keeps astronauts in motion

  26. Inertia keeps satellites in orbit

  27. Inertia keeps you at rest

  28. Newton’s Second Law of Motion • Force = Mass x acceleration • A force is needed for acceleration • Acceleration will occur in the direction of the force.

  29. Which cars are accelerating? • If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.

  30. A force is needed for acceleration • Will these accelerate? • A skier going downhill? • What force is acting on the skier? • An astronaut on a spacewalk? • Are any forces acting on the astronaut?

  31. Which of the following will accelerate? • A bullet shot from a gun? • What forces are acting on the bullet? A hammer and feather dropped on the moon? • What force is acting?

  32. Newton’s 2nd Law Force = mass x acceleration • F_____ M A Force is measured in newtons. A newton is a kg m s2 • Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree?

  33. Answer: F_____ M A 1000kg * 50m/s/s = 50,000 kgm or 50,000N s2

  34. F = ma • A car is traveling at constant speed. Is a force acting on it? • Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.

  35. More 2nd Law Problems: • 1. What is the acceleration on a mass of 50kg if a force of 10N is applied? • 2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?

  36. Answers: • _F_ M A 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg 2. F______ 15kg 10m/s2 150 kgm/s2 or 150N

  37. Gravity is a force that causes an acceleration • On earth, ALL objects accelerate at 9.8m/s2 (ignoring air resistance) because of gravity. • No matter what the mass, ALL objects on earth accelerate at 9.8 m/s2

  38. Newton’s 2nd Law • F__ m a • An object of mass 3kg falls off a cliff. How much force is acting on the object? Answer: F_______ 3kg 9.8 m/s2 29.4N

  39. Free fall without air resistance Both the elephant and the feather land at the same time!

  40. Free fall with air resistance The feather takes lots longer to hit the ground!

  41. Newton’s Third Law • For every action, there is an equal and opposite reaction

  42. For every action, there is an equal and opposite reaction • In rocketry…… The gases being expelled is the action. The rocket propelling forward is the reaction.

  43. How do Newton’s laws describe the motorcycle rider? • 1st: Inertia keeps the rider moving forward. • 2nd: The force with which the rider hits the ground will be equal to his mass times his deceleration. • 3rd: The action of hitting the ground will equal the reaction of the dirt.

  44. Projectile motion

  45. Projectile motion • An object moves forward due to inertia and accelerates downward due to gravity.

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