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Forces

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  • 1. FORCES Unit 3 Chapter 6
  • 2. FORCE
    • How does a team win a game of tug-of-war?
  • 3. FORCE
    • A force is a push or a pull
    • Forces are vectors
      • Magnitude
        • The number value indicates the strength of the force
      • Direction
        • Which direction the force is applied (right, left, up, down)
  • 4. FOUR TYPES OF FORCES
    • Gravitational
      • Weakest of the four forces
      • An attractive force between objects
      • Based on the size and mass of the objects in question
        • Larger planets / stars have a lot of gravitational force
      • Can be broken – we do it everyday!
  • 5. FOUR TYPES OF FORCES
    • Electromagnetic
      • Force of electron bonds
      • Stonger than gravitational force
  • 6. The New Planets in Our Solar System
  • 7. FOUR TYPES OF FORCES
    • Weak Nuclear
      • Force that comes from Radioactive decay
      • Force provides power harnessed by Nuclear Power Plants and supplied to the public as electricity
      • But… there have been problems…
  • 8. FOUR TYPES OF FORCES
    • Strong Nuclear
      • The force that hold protons and neutrons together in the nucleus
      • When this force is broken…
  • 9. NEWTON’S LAWS
    • Video
    • First Law – An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force.
    • The unseen force that stops objects on earth is Friction
      • Friction occurs any time two surfaces touch.
      • The smoother the surfaces, the less the force of friction impeeds motion
      • The rougher the surfaces, the more the force of friction impeeds motion
  • 10. INERTIA
    • CQ: If an 18-wheeler and a car are going the same speed, which one is harder to stop? Which one is harder to start moving from rest?
    • Inertia – The tendency of an object to continue to move, or resist starting to move, is based on its mass
      • The heavier an object is, the more force it takes to stop the object once it is in motion, or start the object once it has been stopped
  • 11. MASS VS. WEIGHT ►
  • 12. MASS VS. WEIGHT
    • To calculate weight…
    • F w = mg
    • Where: F w = Force of Weight (N)
    • m = mass (kg)
    • g = Acceleration due to gravity (m/s 2 )
    • So, 1 N = 1 kg· m
    • s 2
    • (Newtons are a derived unit)
  • 13. MASS VS. WEIGHT
    • Stacey has a mass of 55kg. What is her weight on earth?
    • W = -539 N
    • Ashley has a weight of -97N on the moon where the acceleration due to gravity is -1.62 m/s 2 .
      • What is her mass?
      • What is her weight on earth ?
      • m = 59.88kg
      • W = -586.82N
  • 14. MASS VS. WEIGHT
    • What is your mass?
      • Your weight (in pounds)
      • 2.2
    • What is your weight (in Newtons) on the earth?
    • F w = mg
    • What is your weight (in Newtons) on the moon?
    • F w = mg
    • Moon’s Accel. Due to Gravity = -1.62 m/s 2
    • How many pounds would that be (on the moon)? Weight in Newtons Pounds
        • 4.45
  • 15. MASS VS. WEIGHT
    • So, how many Newtons® are in a Newton?
    • Find the mass of the Newtons® in grams
    • Convert the mass to kilograms
    • Find the weight (in Newtons) of the Newtons®
    • Set up a proportion…
    • # Given Newtons® ? # Newtons® Weight in Newtons 1 N
  • 16. NEWTON’S SECOND LAW
    • Video
    • Have you ever picked something up that you thought was heavy… but it wasn’t and it seemed to fly out of your hand?
    • The force required to change an object’s motion (1 st Law) is directly proportional to its mass – Think Inertia!
    • How fast the object moves (accelerates) is dependant on its mass and the force applied.
  • 17. NEWTON’S SECOND LAW
    • Formula
    • F NET = ma
    • Where F NET = Net (Total) Force (N)
    • m = mass (g)
    • a = acceleration (m/s 2 )
    • Compare our two formulas…
    • F w = mg F NET = ma
  • 18. NEWTON’S SECOND LAW
    • What net force is needed to accelerate a 4kg box at a rate of 2 m/s 2 ?
    • A) F NET = 8N
    • A net force of 56N is used to accelerate a book at a rate of 1.2 m/s 2 . What is the book’s mass?
      • A) m = 46.67 kg
  • 19. NEWTON’S SECOND LAW
    • A 72N net force is used to lift a 24kg box. What is the box’s acceleration?
    • A) a = 3m/s 2
    • A car starts from rest and accelerates to 25m/s in 3 seconds. How much force is used to accelerate the 1500kg car?
    • A) a = 8.33 m/s 2 ; F = 12495 N
  • 20. NEWTON’S THIRD LAW
    • Video
    • For every action, there is an equal and opposite rection.
    • Forces occur in pairs!
    • An object sitting on a table has 2 forces acting on it:
      • Weight (F W ) pushes down
      • The Normal Force (F N ) pushes up
    F W F N
  • 21. WARM UPS (3/19/08)
    • 11. What is the weight of a 60kg person:
    • a) On earth?
    • b) On the sun (g = -274 m/s 2 )
    • 12. It takes 5000N of net force to make a 1500kg car accelerate. What is the rate of acceleration?
    • 13. How much net force is needed to accelerate a 100kg piano at 4.5 m/s 2 ?
  • 22. QUICK REVIEW
    • 1 st Law
      • An object in motion stays in motion, an object at rest stays at rest until acted upon by an outside force
      • Inertia is the tendency of an object to stay still or keep moving due to its mass
      • Mass and Weight are different!
        • Mass is in kilograms – a scalar
        • Weight is in Newtons – and negative – a vector!
  • 23. QUICK REVIEW
    • 2 nd Law
      • The force required to move an object is directly proportional to the object’s mass
      • The amount of acceleration given to an object depends on the force given and the mass of the object
    • 3 rd Law
      • For every action there is an equal and opposite reaction
  • 24. DIFFERENT FORCES
    • CQ: A book sits on a table.
      • Is it moving?
      • Are there any forces acting on it?
  • 25. DIFFERENT FORCES
    • Net Force
      • The sum of the forces acting on an object
        • This is where vecotrs come in… Direction matters!
      • Don’t forget the Normal Force (F N )!
      • If the Net Force on an object is 0N…
        • The object is not moving
        • Think F NET = ma…
          • if a = 0m/s 2, then F NET = 0N
      • If the Net Force on an object is not 0N…
        • Then a ≠ 0 m/s 2 … the object must be moving!
  • 26. PRESSURE
    • Pressure
      • Not really a force, but the way a force is delivered
      • Pressure is the application of a force over an area
    • Formula
    • Where P = Pressure (Pa)
    • F = Force (N)
    • A = Area (m 2 )
  • 27. PRESSURE
    • Find the pressure exerted by a -50N rectangular box that has a surface area of 2m 2 .
    • Find the pressure exerted by a -80N rectangular box that is 40cm by 30cm in dimension.
    • Find the pressure exerted by a 10kg square brick that has a side length of 25cm.
  • 28. WARM UPS (3/20/08)
    • 15. You are traveling at 35 m/s when you slam on your brakes and come to a stop in 3 seconds. What force is needed to bring your 1250kg car to a stop?
    • 16. Mrs. Peak’s cat, Sylvester, has a mass of 9.5kg. Find the pressure of all four of his paws, if his paws have a total area of 0.008 m 2 .
    • 17. Find the pressure given by a book with a weight of -10N acting on an square area with a
    • side length of 0.2m.
    • 18. Find the pressure of a 10kg weight sitting on a rectangular area with side lengths of 45cm and 26cm.
  • 29. FINAL PROJECT
    • Physics in a Photo
  • 30. DIFFERENT FORCES…
    • Tension
      • The force in a rope, cable or wire is called tension
      • Tension units are the same as force – Newtons!
      • In order for a rope to hold an object – the object has to be in equilibrium
  • 31. TENSION
  • 32. TENSION
    • Find the tension in each of the ropes if the traffic light has a weight of -1000N.
  • 33. FORCE OF FRICTION
    • Acts opposite of motion
    • The smoother a surface, the less friction it has
    • The rougher a surface the more friction it has
    • Two types of Friction
      • Static Friction – the friction that on an object at rest to keep it from moving
      • Kinetic Friction – the friction that acts on an object while it is in motion to try to stop it
    • In order to move, you have to overcome the force of friction and have an unbalanced net force in your favor!
  • 34. FORCE OF FRICTION
    • The coefficient of friction is a measure of how much friction exists between two objects
    • The coeffificent of friction, μ:
      • Unitless
      • Less than 1
      • The closer μ is to 1, the rougher the surfaces
      • The closer μ is to 0, the smoother the surfaces
  • 35. FORCE OF FRICTION
    • Formula
    • F f = μ F N
    • Where F f = Force of Friction (N)
    • μ = coefficient of friction
    • F N = Normal Force (N)
    • Recall that the Normal Force (F N ) is
    • equal and opposite
    • of the Force of Weight
    • (Third Law!)
  • 36. FORCE OF FRICTION
    • It takes100N of force to keep a 50kg box in motion. If the coefficient of riction is 0.20 between the box and the floor,
      • A) What is the box’s weight?
      • B) What is the normal force on the box?
      • C) What is the force of friction on the box?
      • D) What is the Net Force on the box?
      • E) What is the box’s acceleration?
  • 37. WARM UPS (4/1/08)
    • 20. The coeficient of static friction between your tire and the road is 0.9. If your car has a mass of 1250kg, what is the force of friction on your car?
    • 21. You use 150N of force to push a 90kg block across a floor that has a coefficient of friction 0.15.
    • a) What is the force of friction that acts on the box?
    • b) What is the box’s acceleration?
    • (we’ll do this together)
  • 38. FORCE OF FRICTION
    • It takes 350N of force to keep a 58kg box in motion. If the coefficient of friction is 0.60 between the box and the floor,
      • A) What is the box’s weight?
      • B) What is the normal force on the box?
      • C) What is the force of friction on the box?
      • D) What is the Net Force on the box?
      • E) What is the box’s acceleration?
  • 39. FORCE OF FRICTION
    • A 200N force is used to push a 60kg rollerblader across the ground where the coefficient of friction is 0.03.
      • A) What is the rollerblader’s weight?
      • B) What is the normal force on the ‘blader?
      • C) What is the force of friction on the ‘blader?
      • D) What is the Net Force on the ‘blader?
      • E) What is the rollerblader’s acceleration?
  • 40. FORCE OF FRICTION
    • It takes 600N of force to pull a 40kg child in his 10kg wagon on the sidewalk where the coefficient of friction is 0.8.
    • You pull your 20kg backpack down the hall by the straps with a force of 50N. If the coefficient of friction between your backpack and the floor is 0.23,
      • A) What is the force of friction that acts on your backpack?
      • B) What is the backpack’s acceleration?

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