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Table of Contents

Table of Contents. The Nature of Force Friction and Gravity Newton’s First and Second Laws Newton’s Third Law Rockets and Satellites. Force. A force is simply a push or a pull. Isaac Newton accurately described the force of all moving objects in 1687 when he published his thesis.

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Table of Contents

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  1. Table of Contents • The Nature of Force • Friction and Gravity • Newton’s First and Second Laws • Newton’s Third Law • Rockets and Satellites

  2. Force • Aforceis simply a push or a pull. • Isaac Newton accurately described the force of all moving objects in 1687 when he published his thesis. • We still use these laws as the basis of all physics to this day.

  3. Units of Force • When describing the strength of a force we use the SI unit of Newtons (N) • The newton is the SI unit for force; it is equal to the amount of net force required to accelerate a mass of one kilogram at a rate of one meter per second squared. • F = ma: or multiplying mass (kg) by acceleration (m/s2) • Therefore: 1 N = 1 kg·m/s2

  4. Real World Examples of the Newton • 1 N is the force of Earth's gravity on a mass of about 102 g or roughly the mass of a small apple • On Earth's surface, a mass of 1 kg exerts a force of approximately 9.8 N [down] (or 1.0 kilogram-force; 1 kgf=9.80665 N by definition) • The force of Earth's gravity on a human being with a mass of 70 kg is approximately 686 N • It is common to see forces expressed in kilonewtons or kN (1 kN = 1,000 N)

  5. - The Nature of Force Combining Forces The combination of all forces acting on an object is called the net force. The net force determines whether an object moves and also the direction it will move!

  6. - The Nature of Force Unbalanced Forces • Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

  7. - The Nature of Force Balanced Forces • Balanced forces acting on an object do not change the object’s motion.

  8. - The Nature of Force Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. Question Answer What is a force? A force is a push or pull. What happens when forces combine? Forces combine to produce a net force.

  9. End of Section:The Nature of Force

  10. Friction • Friction is the force that is exerted on two surfaces as they rub against one another. • The strength of the friction force depends on two things: • How hard the surfaces are pushed together • What material the surface is made of

  11. Types of Friction • Static friction acts on objects that are at rest • Sliding friction occurs when two solid surfaces slide past one another • Rolling friction occurs when an object rolls across a surface • Fluid friction occurs when a solid object moves through a fluid

  12. What type of friction is occuring?

  13. What type of friction is occurring?

  14. Compare and Contrast

  15. What type of friction is occurring?

  16. What type of friction is occurring?

  17. Gravity • Gravity is a force that pulls objects toward one another • Gravity keeps objects in place unless they are acted on by an outside force! • Sir Isaac Newton accurately described this in his law of universal gravitation • Gravity acts on all objects regardless of their size and or location in the universe • This means that all objects are attracted to one another but there are some rules involved in how much attraction occurs • This is mostly due to differences in mass • What is the difference between mass and weight? • Mass is the amount of matter in an object. • Weight is the measure of gravitational force on an object.

  18. - Friction and Gravity Gravity • Two factors affect the gravitational attraction between objects: mass and distance

  19. - Friction and Gravity Gravity • The force of gravity on a person or object at the surface of a planet is known as weight. Can you find anything wrong with this picture?

  20. Gravity and Motion • An object is said to be in free fall when gravity is the only force acting on it. • This results in an unbalanced force and causes the object to accelerate in a downward motion at 9.8m/s • This acceleration is not dependent on mass! • Air resistance is fluid friction that acts in the opposite direction to motion • Air resistance is not the same for all objects! • Terminal velocity occurs when the force of air resistance equals the weight of the object. Therefore, the object will continue to fall with a constant velocity with no acceleration

  21. - Friction and Gravity Free Fall • Use the graph to answer the following questions.

  22. Time is on the horizontal axis, and speed is on the vertical axis. Interpreting Graphs: What variable is on the horizontal axis? The vertical axis? - Friction and Gravity Free Fall

  23. The slope is 9.8. The speed increases by 9.8 m/s each second. Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? - Friction and Gravity Free Fall

  24. 58.8 m/s Predicting: What will the speed of the object be at 6 seconds? - Friction and Gravity Free Fall

  25. The speed values would not change. Drawing Conclusions: Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? - Friction and Gravity Free Fall

  26. - Friction and Gravity Air Resistance • Falling objects with a greater surface area experience more air resistance.

  27. Projectile Motion • A projectile is any object projected into space by the exertion of a force. • Horizontal velocity does not affect how fast an object will fall! • Therefore, both the red and yellow ball will hit the ground at the same time • On our planet, all objects are accelerated by the force of gravity (9.8m/s) towards the center of the earth.

  28. What is going on in this diagram? http://onlinephys.com/kinematics1D.html

  29. - Friction and Gravity Comparing and Contrasting • As you read, compare and contrast friction and gravity by completing a table like the one below. Friction Gravity Pulls objects toward one another Opposes motion Effect on motion Types of surfaces involved, how hard the surfaces push together Depends on Mass and distance Measured in Newtons Newtons

  30. - Friction and Gravity Links on Friction • Click the SciLinks button for links on friction.

  31. - Friction and Gravity Free Fall • Click the Video button to watch a movie about free fall.

  32. End of Section:Friction and Gravity

  33. A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s2 - Newton’s First and Second Laws Calculating Force

  34. A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (Fnet) = __ What formula contains the given quantities and the unknown quantity? a = Fnet/m or Fnet = m X a Perform the calculation. Fnet = mXa= 55 kg X 2.0 m/s2 F = 110 kg • m/s2 F = 110 N - Newton’s First and Second Laws Calculating Force

  35. A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required to accelerate the water-skier. This may not seem like enough force, but it does not include the force of the speedboat's pull that overcomes friction. - Newton’s First and Second Laws Calculating Force

  36. Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s2? 3,000 N (1,000 kg X 3 m/s2) - Newton’s First and Second Laws Calculating Force

  37. Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s2? 350 N (25 kg X 14 m/s2) - Newton’s First and Second Laws Calculating Force

  38. - Newton’s First and Second Laws Outlining • As you read, make an outline about Newton’s first and second laws. Use the red headings for the main topics and the blue headings for the subtopics. Newton’s First and Second Laws • Newton’s First Law of Motion • Inertia • Inertia Depends on Mass • The Second Law of Motion • Changes in Force and Mass

  39. - Newton’s First and Second Laws More on Newton’s Laws • Click the PHSchool.com button for an activity • about Newton’s laws.

  40. End of Section:Newton’s First and Second Laws

  41. Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Read and Understand What information have you been given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s - Newton’s Third Law Calculating Momentum

  42. Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Plan and Solve What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass X Velocity Perform the calculation. Smaller sledgehammer = 3.0 km X 1.5 m/s = 4.5 kg•m/s Smaller sledgehammer = 4.0 km X 0.9 m/s = 3.6 kg•m/s - Newton’s Third Law Calculating Momentum

  43. Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Look Back and Check Does your answer make sense? The 3.0-kg hammer has more momentum than the 4.0-kg one. This answer makes sense because the 3.0-kg hammer is swung at a greater velocity. - Newton’s Third Law Calculating Momentum

  44. Practice Problem A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has the greater momentum? Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s The baseball has greater momentum. - Newton’s Third Law Calculating Momentum

  45. Practice Problem What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? 0.27 kg•m/s (0.018 kg X 15 m/s = 0.27 kg•m/s) - Newton’s Third Law Calculating Momentum

  46. - - Newton’s Third Law Conservation of Momentum • In the absence of friction, momentum is conserved when two train cars collide.

  47. - Newton’s Third Law Momentum Activity • Click the Active Art button to open a browser window and access Active Art about momentum.

  48. - Newton’s Third Law Previewing Visuals • Before you read, preview Figure 18. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions. Conservation of Momentum Q. What happens when two moving objects collide? A. In the absence of friction, the total momentum is the same before and after the collision. Q. What is the momentum of an object? A. Its mass multiplied by its velocity

  49. End of Section:Newton’s Third Law

  50. - Rockets and Satellites What Is a Satellite? • A projectile follows a curved path because the horizontal and vertical motions combine.

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