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### Forces

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