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Forces and Motion

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  1. Forces and Motion

  2. Force • FORCE is a push or a pull applied to an object that will cause it to start moving, stop moving or change its speed or direction • Demonstration

  3. Force • Force = Mass x Acceleration • F = MA • Force is measured in Newtons (N) which is one kilogram meter per second squared • N = kg x m/s2

  4. Newton’s First Law (law of inertia) • MASS is the measure of the amount of matter in an object • measured in grams (g) or kilograms (kg)

  5. Newton’s First Law (law of inertia) • WEIGHT is a measure of the force of gravity on the mass of an object • measured in Newtons (N)

  6. Force • But weight, what’s my mass? • Please do not confuse the two. • Weight is determined by the acceleration due to gravity. • If you were on another planet that had less gravity, you would weigh less.

  7. m m m m m m NET FORCE • In order for motion to occur, the net force must be >0 10 N = 20 N 10 N = 10 N 10 N 0 N = 20 N 10 N 10 N

  8. THE EQUILIBRIUM RULE Scales pushing up Examples of Mechanical Equilibrium: • Computer setting on a table • Weighing yourself on a set of scales • Hanging from a tree • Car parked on an incline Normal up Weight down Tree pulling up Weight down Normal Friction Weight down Weight down

  9. The Equilibrium Rule ΣF=0

  10. Scales pushing up Normal up Weight down Weight down SUPPORT FORCE • In the first example of mechanical equilibrium the table supplied a force upward that was called the normal force. It is a support force. • Consider the second example of mechanical equilibrium. The scales supply a support force on the man.

  11. EQUILIBRIUM OF MOVING THINGS • Equilibrium is a state of no change. • If an object moves in a straight line with no change in speed or direction, it is in equilibrium. Examples: Driving at constant velocity Normal up Air resistance Air Resistance Force from road Weight down Terminal velocity in parachuting Weight down

  12. What do you weigh? • Weight on Other Planets

  13. Force Problems • Let’s start with an easy one, your weight. • Remember that weight is relative – your mass isn’t changing (the amount of matter in you) but you weigh different amounts because of gravity • Gravity’s acceleration is 9.8m/s2 • On earth you take your weight to be what it is

  14. Force Problems • If you lived on another planet, such as mars for example, the acceleration due to gravity is 3.8m/s2 • In order to find out weight, we use the following formula • w = m x g

  15. Force Problems • Since gravity is a force (pulling you towards the center of the planet) this is technically a force problem • My weight on earth is 185lbs, or 84kg (just divide your weight by 2.2) • That means that if we stick my weight in and we know the acceleration due to gravity here on earth, we can find out my mass

  16. Force Problems • w = m x g • 84kg x m/s2 = m x 9.8m/s2 • M = 8.57kg • Kg x m/s2.. That’s also called a Newton! • So my mass is 8.57kg.

  17. Force Problems • But what if we were on another planet? • Well, we use w = m x g • W = 8.57kg x 3.8m/s2 • W = 32.57N • As a reminder, weight is m x g, so it equals a kg m/s2, or a N. • Mass is measured in kg.

  18. Force Problems • Ok, now you try one • What would be your weight on Jupiter, where gravity is 22.88m/s2? • What would be your weight on the sun, 274.4m/s2? That’s assuming you could stand on it

  19. And now for something completely different… • The Galaxy Song

  20. Force Problems • Ok, let’s move on to earthly stuff. • We remember that f = ma • What would be the force exerted by a truck with a mass of 1818kg accelerating at 15m/s2? • f = ma • F = 1818kg x 15m/s2 • F = 750N

  21. Force Problems • If you accelerate a rocket with a mass of 300kg at Taber’s face at 500m/s2 with what amount of force will it hit him? • F = ma • F = 300kg x 500m/s2 • F = 150,000

  22. More Practice • Troy Polamalu, with a mass of 115kg, hits Adrian Peterson with a force of 2300N. With what acceleration does Troy hit Adrian? What force does Adrian exert on Troy? • F = ma • 2300N = 115kg x a • A = 2300N / 115kg • A = 20m/s

  23. One More • A 20g sparrow mistakes a pane of glass for air and slams into a window with a force of 2N. What is the bird’s acceleration? • F = ma • 2N = .02kg x a • A = 100m/s2 or 10g’s!!

  24. Oh yeah, one more. • Suppose your car is parked on an incline of 10 degrees. If the parking brake lets go and your car starts rolling, with what force are you going down the hill? What is your force on the ground? Assume the car weighs 1500kg.

  25. Friction • FRICTION is the force that acts in the opposite direction of the motion of the object

  26. Types of Friction • Static Friction – Friction due to gravity when an object is at rest. • Demonstration • Sliding Friction – Friction while an object is at motion. • Example • Rolling Friction – Similar to sliding friction, but the object is on wheels or castors to reduce the sliding friction. • Fluid Friction – Friction through water or air • Terminal Velocity

  27. Types of Friction

  28. Sliding Friction Ffriction = µFnormal µ = the coefficient of sliding friction (has no units) product of the friction b/w materials and amount of force 1. Ben is walking through the school cafeteria but does not realize that the person in front of him has just spilled his glass of chocolate milk. As Ben, who weighs 420 N, steps in the milk, the coefficient of sliding friction between Ben and the floor is suddenly reduced to 0.040. What is the sliding force of friction between Ben and the slippery floor?

  29. Friction • While redecorating her apartment, Kelly slowly pushes an 82 kg china cabinet across the wooden dining room floor, which resists motion with a force of 320 N. What is the coefficient of sliding friction between the china cabinet and the floor? 3. A rightward force is applied to a 10-kg object to move it across a rough surface at constant velocity. The coefficient of friction, µ, between the object and the surface is 0.2. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force. (Neglect air resistance.)

  30. Terminal Velocity

  31. Projectile Motion • What is a projectile? – Throw ball • Projectiles near the surface of Earth follow a curved path • This path is relatively simple when viewed from its horizontal and vertical component separately • The vertical component is like the free fall motion we already covered • The horizontal component is completely independent of the vertical component (roll ball) • These two independent variables combined make a curved path!

  32. Projectile Motion

  33. Projectile Motion No Gravity With Gravity

  34. Projectile Motion

  35. Time Horizontal Displacement (x) 0s 0m 1s 25m 2s 50m 3s 75m 4s 100m 5s 125m Ts vxt Vertical Displacement (y) 0m 25m 50m 75m 100m 125m ½ gt2 Horizontally Launched Projectile(initial speed (vx) = 25 m//s)

  36. Horizontally Launched Projectiles • What will hit the ground first, a projectile launched horizontally, a projectile dropped straight down, or a project fired up?

  37. The Plane and the Package

  38. Projectile Motion • Remember that nothing is accelerating the projectile after it leaves • The only thing accelerating the projectile after launch is gravity • The two vectors can be separated into the velocity at launch and the acceleration of gravity

  39. Truck and Ball • Imagine a pickup truck moving with a constant speed along a city street. In the course of its motion, a ball is projected straight upwards by a launcher located in the bed of the truck. Imagine as well that the ball does not encounter a significant amount of air resistance. What will be the path of the ball and where will it be located with respect to the pickup truck?

  40. Fast-Moving Projectiles—Satellites • What if a ball were thrown so fast that the curvature of Earth came into play? • If the ball was thrown fast enough to exactly match the curvature of Earth, it would go into orbit • Satellite – a projectile moving fast enough to fall around Earth rather than into it (v = 8 km/s, or 18,000 mi/h) • Due to air resistance, we launch our satellites into higher orbits so they will not burn up

  41. Satellites

  42. ARISTOTLE ON MOTION • Aristotle attempted to understand motion by classification • Two Classes: • Natural and Violent

  43. Natural Motion • Natural motion depended on nature of the object. • Examples: • A rocks falls because it is heavy, a cloud floats because it’s light • The falling speed of an object was supposed to be proportional to its weight.

  44. Natural Motion • Natural motion could be circular (perfect objects in perfect motion with no end).

  45. Violent Motion • Pushing or pulling forces imposed motion. • Some motions were difficult to understand. • Example: the flight of an arrow • There was a normal state of rest except for celestial bodies.

  46. Aristotle • Aristotle was unquestioned for 2000 years. • Most thought that the Earth was the center of everything for it was in its normal state. • No one could imagine a force that could move it.

  47. GALILEO AND THE LEANING TOWER • 17th Century scientist who supported Copernicus. • He refuted many of Aristotle's ideas. • Worked on falling object problem - used experiment.

  48. GALILEO'S INCLINED PLANES • Knocked down Aristotle's push or pull ideas. • Rest was not a natural state. • The concept of inertia was introduced. • Galileo is sometimes referred to as the • “Father of Experimentation.”

  49. NEWTON’S FIRST LAW OF MOTION • Newton finished the overthrow of Aristotelian ideas. • Law 1 (Law of Inertia) • An object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an outside force.

  50. Newton’s First Law (law of inertia) • INERTIA is a property of an object that describes how hard it is to change the motion of the object • More mass = more inertia • F=MA