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Chapter 5 Forces
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  1. Chapter 5Forces

  2. Force & Vectors

  3. Forces • Kinematics: Describes how an object moves • Dynamics: Describes why an object moves • Newton’s three laws of motion, describe the “dynamics” of why objects move

  4. Forces and Interaction • Force – a “push or pull” • Contact Force – you physically push on a wall • Long-range Force – like magnets or gravity • Force – a vector quantity. Has what two parts. • SI Unit – the Newton abbreviated as “N” • Superposition of Forces : forces combine according to vector addition

  5. 4 Types Forces • Gravitational forces • The Earth pulls and holds the moon in orbit • The moon pulls and causes tide changes • Electromagnetic forces • due to electric charges, both static and moving. • Strong Nuclear Forces • holds particles in the nucleus together.(Strongest of the 4) • Weak Nuclear Forces • Radioactive decay

  6. Physics = Phun Newton’s Three Laws of Physics Why did the apple accelerate? The apple was at rest, what started it in motion?

  7. Newton’s 1st Law

  8. Newton’s 1st Law • An object at rest will stay at rest unless a force acts on it. • Here, the girl is at rest until acted on by the force imposed by the cannonball.

  9. Newton’s 1st Law • An object at rest will stay at rest unless a force acts on it. • Here, the skateboard/log combination is at rest until acted on by the force imposed by the rocket.

  10. Newton’s 1st Law • An object in motion will continue in a straight line until acted on by some “outside” force. • Consider a baseball that is pitched to a batter. What force causes the ball to change direction? Hit

  11. Newton’s 1st Law • An object in motion will continue in a straight line until acted on by some “outside” force. • Consider the space shuttle turning while in space flight. What produces the forces?

  12. Newton’s 1st law • An object in motion will continue in a straight line until acted on by some “outside” force. • Consider the space shuttle when landing. What produces the forces?

  13. Multimedia • The car and the wall • The motorcyclist • The truck and the ladder

  14. Weight and Mass Mass: A measure of the amount of matter in an object has. Symbol (m) Unit (kg) Weight: The gravitational force exerted on a body Symbol (W) Unit (N Newton)

  15. Weight A Motor cycle weights 2450N. What is its mass?

  16. Homework • WS 5a 1-10 • WS 5a (b) 1-11

  17. Newton’s 2nd Law

  18. Newton’s 2nd Law • Acceleration is directly proportional to the magnitude of the net force. • Acceleration is inversely proportional to the mass of the object.

  19. Newton’s 2nd Law • Acceleration depends on both mass and the net force

  20. Acceleration depends on net force • A force of 10N accelerates the box

  21. Acceleration depends on net force • A force of 20N accelerates the box twice as fast

  22. Acceleration depends on mass • A force of 10N accelerates the box

  23. Acceleration depends on mass • A force of 10N accelerates the smaller box faster

  24. Free-body diagrams WS 5b #1 Draw the free-body diagram for a book is at rest on a table top.

  25. Free-body diagrams WS 5b #2 A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. A free-body diagram for this situation looks like this:

  26. Free-body diagrams WS 5b #3 An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:

  27. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

  28. If the net force is zero, there isno acceleration

  29. If the net force is not zero, there isacceleration

  30. Net Force • If there is no movement • The net force is zero • If there is no acceleration • The net force is zero • If there is acceleration • The net force is not zero.

  31. Homework • WS 5b (FBD) • 3-10

  32. Newton’s 1st Law Equilibrium Situation • An object at rest will stay at rest unless a force acts on it. • An object in motion will continue in a straight line until acted on by some “outside” force.

  33. Newton’s Second Law – Non-Equilibrium Situation • The result of a Non-Equilibrium situation is that the body, will begin to move.

  34. Newton’s Second Law – Non-Equilibrium Situation • Two 50g masses are placed 40cm on either side of a fulcrum. What is the Net force? • If left mass slides 20cm right, what happens? • If the left mass slides 20cm right and grows to 100g, what type of situation is this? Equilibrium Non-Equilibrium

  35. Newton’s 2nd law e.g. • What net force is required to accelerate a 1500kg race car at 3.0m/s2?

  36. 4 x 4 Newton’s 2nd law e.g. A truck with a mass of 710kg starts from rest and travels 40m in 3.0s What Fnet acts on the car?

  37. Newton’s 2nd law e.g. A 873kg dragster, starting from rest, accelerates to a speed of 26.3m/s in .59s a) Find the acceleration b) Find the average force on the dragster

  38. Newton’s 2nd law WS5d #1 • An artillery shell has a mass of 55kg. The shell leaves the 1.5m long barrel at a velocity of 770m/s. • What is the force the gun applies to the shell? • Step 1 Find the acceleration • Step 2 Find the Net Force

  39. Newton’s 2rd Law WS5d #1 • What is the force the gun applies to the shell?

  40. Newton’s 2rd Law WS 5c #5 • A car, mass 1225kg traveling at 105km/hr slows to a stop in 53m. • What is the size and direction of the force that acted on the car? • Step 1 Unit conversion km/hr to m/s • Step 2 Find the acceleration • Step3 Find the Force

  41. Newton’s 2nd Law WS 5d #5 Given: • mass 1225kg • velocity 105km/hr • distance 53m.

  42. Homework • WS 5c • 1-5 • WS 5d • 1,2

  43. Newton’s 3rd Law

  44. Newton’s 3rd Law • When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude but opposite in direction. • For every action there is an equal and opposite re-action. • Action – Reaction force pair

  45. Newton’s 3rd LawExample • A book rests on a table • The force from the weight of the book pushes down. • The table provides a supportive force up. • Normal force (N) is perpendicular to the surface

  46. Friction • Friction is a force like any other force • Friction acts on materials that are in contact with each other • Friction slows down motion • Forces due to friction are always in the opposite direction of the motion.

  47. Friction • There are two types of friction: • Kinetic (Sliding) friction: The force that opposes the motion of a moving object • Static friction: The force that oppose the start of motion.

  48. Friction • The force of friction depends on • The force pushing down (weight) • The surface materials in contact with each other

  49. Friction

  50. Friction Example • A horizontal force of 30N pushes a 12kg crate across a floor at a constant velocity. Find the coefficient of sliding friction.