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Forces

1. Forces. Dynamics. FORCEMAN. 2. What causes things to move?. What is a force? A push or a pull that one body exerts on another. 3. Balanced. No change in motion. 4. Question: Can there be balanced forces on a object and the object still have motion?. 5. Unbalanced

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Forces

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  1. 1 Forces Dynamics FORCEMAN

  2. 2 What causes things to move? • What is a force? • A push or a pull that one body exerts on another.

  3. 3 • Balanced No change in motion

  4. 4 Question: Can there be balanced forces on a object and the object still have motion?

  5. 5 Unbalanced • If the forces acting on a body are "unbalanced" this means that there is a "net" or "resultant" force. • The object will do one of the following: • Speed Up • Slow Down • Change the Direction of its Motion • In other words…. • The Body will Accelerate

  6. 6

  7. Forces that oppose motion 19 -Friction is a force that opposes motion between two surfaces that are touching each other. Friction Song 1 Friction Song 2 Acts in the opposite direction of the motion

  8. 20 Friction • What Causes Friction? • Friction acts on the SURFACE where objects are in CONTACT

  9. 4 types of Friction • Static Friction • Sliding Friction • Rolling Friction • Fluid Friction

  10. 21 Types of Friction • Types of friction: 1. Static – force required to overcome inertia of a stationary object

  11. Static friction- occurs between the surfaces of 2 objects that touch but do not move against each other. Static friction must be overcome for one of the objects to move.

  12. 22 Static Friction (FS)

  13. Not enough static friction

  14. 23 Types of Friction (cont’.) 2. Sliding – force required to keep an object sliding at a constant speed 3. Rolling – force required to keep an object rolling at a constant speed • To decrease friction, surfaces can be lubricated with a liquid such as oil.

  15. 24 CLIP The amount of force required to overcome static friction is always greater than the force required to overcome sliding or rolling friction.

  16. Fluid Friction • Opposes the motion of an object moving through fluid • Fluid Friction INCREASES as the speed of an object moving through the fluid INCREASES; Thus the FASTER you move the GREATER the friction

  17. Questions: How do we measure force? 7 Newtons • SI unit of force is the Newton (N). • Defined as the amount of force that when acting on a 1 kg object produces an acceleration of 1m/s2. • Therefore, 1N = 1 kg • m s2

  18. What will happen? • Abruptly stopping a cart with a suitcase on top…. • You set your coffee cup on the dash and then make a sharp left turn….. • You throw a tennis ball in outer space.

  19. 8 Newton’s First Law

  20. 10 Newton’s First Law • An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force. • ?What do you mean by “Net Force”? • Net means: Final Sum, so net external force is the sum of all the forces acting on the object.

  21. 11 Newton’s First Law • Newton’s First Law is also called the Law of Inertia. • Inertia is the tendency of an object to resist any change in its motion. Question: Does an object at rest still have inertia?

  22. 9 Newton’s First Law

  23. 12 Why does the ladder continue to move? Inertia!

  24. 13 Why does the person continue to move? Inertia! Clip

  25. 14 Newton’s Second Law • The acceleration of an object is directly proportional to the net external force acting on the object andinversely proportional to the object’s mass. • F=ma • Clip

  26. 16 Newton's 2nd Law of Motion: • F = m x a • Mass is in kg • Acceleration is in m/s2 • Force is in kg x m/ s2 (or N) Force = mass x acceleration Let’s do a sample problem…next slide 

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  28. 17 Force Problems: What force is needed to give an object with a mass of 25.0 kg an acceleration of 15.0 m/s2 ? Formula F = ma Unknown: Given: m = 25.0 kg a = 15.0 m/sec² F = ? F = (25 kg)(15 m/sec²) F = 375 kg•m/sec² = 375 N

  29. 34 Newton’s Third Law of Motion …to every action force there is an equal and opposite reaction force

  30. Newton’s Third Law "Every action has an equal and opposite reaction."

  31. Action-Reaction forces act on different objects 35

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  33. 18 Forces that oppose motion

  34. Forces that oppose motion -Air resistance is the force air exerts on a moving object

  35. GRAVITY!! 25 • The force of gravity is an attractive force between objects. • The amount of GRAVITATIONAL FORCE between objects depends upon their: • 1- Masses (greater mass means more gravity) • 2- Distance apart (greater distance means less gravity) *All objects fall toward the earth at the same rate of acceleration, regardless of their masses!

  36. Gravitation 27

  37. 30 Weight and Gravity • Weight is the force of gravity between the earth and a body on its surface. • Weight ≠ mass! • Weight is measured in Newtons (N). W =m x g BECAUSE IT’S A FORCE!

  38. 32 Weight • When the mass of an object and the acceleration due to gravity are known, the weight of an object can be calculated. W = (g)(m) Ex. Mass = 100kg Acc.= 9.81 m/s2 Weight = 981 N

  39. 29 Free fall and Terminal Velocity • The opposing force of air resistance increases with speed. • Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance. • This is the highest speed an object can reach. • Different objects have different terminal velocities.

  40. 31 Could your weight change by simply going to the top of a mountain?

  41. 37 Projectile Motion • Projectiles follow a curved path because of the Earth’s gravitational pull. • They have 2 types of motion that are independent of each other: 1. Horizontal motion 2. Vertical motion

  42. 33 Example Problem • What is your weight if your mass 4.52 kilograms? Given: Unknown: m = 4.52 kg W = ? g = a = 9.8 m/sec² Formula: W = mg • W = (4.52kg)(9.8 m/sec²) • W = 44.296 kg•m/sec² = 44.296 N

  43. 38 Projectile Motion Which will hit the ground first? A ball that is dropped straight down or a ball that is “shot” out horizontally? * An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!!!!

  44. 39 Another Example of Projectile Motion

  45. 40 Roller Coaster Clip Circular Motion • An object moving in a circular motion at the same speed is accelerating toward the center because its direction is constantly changing. • Centripetal acceleration is acceleration toward the center of a curved path. 'Centripetal' comes from the Latin word for 'center seeking' Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line.

  46. 41 Circular Motion cont.’ Without a centripetal force, an object in motion continues along a straight-line path. With a centripetal force, an object in motion will be accelerated and change its direction. • Centripetal force is the force that causes a moving object to move in a curved path

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  48. 43 How do rollercoasters work!?! Amusement Park Forces

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