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Preview. Section 1 Newton's Second Section 5 Extra questions. Net Force - the Sum of the Forces. This car is moving with a constant velocity. F forward = road pushing the tires F resistance = force caused by friction and air Forces are balanced

lionel-madden
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  1. Preview Section 1 Newton's Second Section 5Extra questions

  2. Net Force - the Sum of the Forces • This car is moving with a constant velocity. • Fforward = road pushing the tires • Fresistance = force caused by friction and air • Forces are balanced • Velocity is constant because the net force (Fnet) is zero.

  3. Equilibrium • The state in which the net force is zero. • All forces are balanced. • Object is at rest or travels with constant velocity. • In the diagram, the bob on the fishing line is in equilibrium. • The forces cancel each other. • If either force changes, acceleration will occur.

  4. Classroom Practice Problem • An agricultural student is designing a support system to keep a tree upright. Two wires have been attached to the tree and placed at right angles to each other (parallel to the ground). One wire exerts a force of 30.0 N and the other exerts a force of 40.0 N. Determine where to place a third wire and how much force it should exert so that the net force on the tree is zero. • Answer: 50.0 N at 143° from the 40.0 N force

  5. Newton’s Second Law • Increasing the force will increase the acceleration. • Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N? • Answer: the 7 N force • Increasing the mass will decrease the acceleration. • A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration? • Answer: the 3 kg airplane

  6. Newton’s Second Law (Equation Form) • F represents the vector sum of all forces acting on an object. • F = Fnet • Units for force: mass units (kg)  acceleration units (m/s2) • The units kg•m/s2 are also called newtons (N).

  7. Classroom Practice Problem • Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude? • Answer: 2600 kg•m/s2 or 2600 N

  8. What do you think? • How do the quantities weight and mass differ from each other? • Which of the following terms is most closely related to the term friction? • Heat, energy, force, velocity • Explain the relationship.

  9. Weight and Mass • Mass is the amount of matter in an object. • Kilograms, slugs • Weight is a measure of the gravitational force on an object. • Newtons, pounds • Depends on the acceleration of gravity • Weight = mass  acceleration of gravity • W = mag where ag = 9.81 m/s2 on Earth • Depends on location • ag varies slightly with location on Earth. • ag is different on other planets.

  10. Normal Force • Force on an object perpendicular to the surface (Fn) • It may equal the weight (Fg), as it does here. • It does not always equal the weight (Fg), as in the second example. • Fn = mg cos 

  11. Static Friction • Force that prevents motion • Abbreviated Fs • How does the applied force (F) compare to the frictional force (Fs)? • Would Fs change if F was reduced? If so, how? • If F is increased significantly, will Fs change? If so, how? • Are there any limits on the value for Fs?

  12. Kinetic Friction • Force between surfaces that opposes movement • Abbreviated Fk • Does not depend on the speed • Using the picture, describe the motion you would observe. • The jug will accelerate. • How could the person push the jug at a constant speed? • Reduce F so it equals Fk.

  13. Friction Click below to watch the Visual Concept. Visual Concept

  14. Calculating the Force of Friction (Ff) • Ff is directly proportional to Fn(normal force). • Coefficient of friction (): • Determined by the nature of the two surfaces • s is for static friction. • k is for kinetic friction. • s > k

  15. Typical Coefficients of Friction • Values for  have no units and are approximate.

  16. Everyday Forces Click below to watch the Visual Concept. Visual Concept

  17. Classroom Practice Problem • A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. • Draw a free-body diagram and use it to find: • the weight • the normal force (Fn) • the force of friction (Ff) • Find the coefficient of friction. • Answer: s = 0.32

  18. Classroom Practice Problem • A student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0˚ with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.500. Find the magnitude of the acceleration of the box. • Start with a free-body diagram. • Determine the net force. • Find the acceleration. • Answer: a = 0.12 m/s2

  19. The Four Fundamental Forces • Electromagnetic • Caused by interactions between protons and electrons • Produces friction • Gravitational • The weakest force • Strong nuclear force • The strongest force • Short range • Weak nuclear force • Short range

  20. Preview • Multiple Choice • Short Response • Extended Response

  21. Multiple Choice • Use the passage below to answer questions 1–2. • Two blocks of masses m1 and m2 are placed in contact with each other on a smooth, horizontal surface. Block m1 is on the left of block m2. A constant horizontal force F to the right is applied to m1. • 1. What is the acceleration of the two blocks? • A. C. • B. D.

  22. Multiple Choice • Use the passage below to answer questions 1–2. • Two blocks of masses m1 and m2 are placed in contact with each other on a smooth, horizontal surface. Block m1 is on the left of block m2. A constant horizontal force F to the right is applied to m1. • 1. What is the acceleration of the two blocks? • A. C. • B. D.

  23. Multiple Choice, continued Use the passage below to answer questions 1–2. Two blocks of masses m1 and m2 are placed in contact with each other on a smooth, horizontal surface. Block m1 is on the left of block m2. A constant horizontal force F to the right is applied to m1. 2. What is the horizontal force acting on m2? F.m1a G.m2a H. (m1 + m2)a J.m1m2a

  24. Multiple Choice, continued Use the passage below to answer questions 1–2. Two blocks of masses m1 and m2 are placed in contact with each other on a smooth, horizontal surface. Block m1 is on the left of block m2. A constant horizontal force F to the right is applied to m1. 2. What is the horizontal force acting on m2? F.m1a G.m2a H. (m1 + m2)a J.m1m2a

  25. Multiple Choice, continued 3. A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N, and downward with a force of 236 N. Find the magnitude and direction of the net force on the crate. A.3.30 N at 96° counterclockwise from the positive x-axis B. 3.30 N at 6° counterclockwise from the positive x-axis C. 3.30 x 102 at 96° counterclockwise from the positive x-axis D. 3.30 x 102 at 6° counterclockwise from the positive x-axis

  26. Multiple Choice, continued 3. A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N, and downward with a force of 236 N. Find the magnitude and direction of the net force on the crate. A.3.30 N at 96° counterclockwise from the positive x-axis B. 3.30 N at 6° counterclockwise from the positive x-axis C. 3.30 x 102 at 96° counterclockwise from the positive x-axis D. 3.30 x 102 at 6° counterclockwise from the positive x-axis

  27. Multiple Choice, continued 5. A freight train has a mass of 1.5 x 107 kg. If the locomotive can exert a constant pull of 7.5 x 105 N, how long would it take to increase the speed of the train from rest to 85 km/h? (Disregard friction.) A.4.7 x 102s B. 4.7s C. 5.0 x 10-2s D. 5.0 x 104s

  28. Multiple Choice, continued 5. A freight train has a mass of 1.5 x 107 kg. If the locomotive can exert a constant pull of 7.5 x 105 N, how long would it take to increase the speed of the train from rest to 85 km/h? (Disregard friction.) A.4.7 x 102s B. 4.7s C. 5.0 x 10-2s D. 5.0 x 104s

  29. Multiple Choice, continued Use the passage below to answer questions 6–7. A truck driver slams on the brakes and skids to a stop through a displacement Dx. • 6. • A.Dx/4 • B.Dx • C.2Dx • D.4Dx If the truck’s mass doubles, find the truck’s skidding distance in terms of Dx. (Hint: Increasing the mass increases the normal force.)

  30. Short Response Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 10. How long does the ball take to hit the ground?

  31. Short Response Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 10. How long does the ball take to hit the ground? Answer: 6.00 s

  32. Short Response, continued Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 11. How far from the building does the ball hit the ground?

  33. Short Response, continued Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 11. How far from the building does the ball hit the ground? Answer: 72.0 m

  34. Short Response, continued Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 12. When the ball hits the ground, what is its speed?

  35. Short Response, continued Base your answers to questions 10–12 on the information below. A 3.00 kg ball is dropped from rest from the roof of a building 176.4 m high.While the ball is falling, a horizontal wind exerts a constant force of 12.0 N on the ball. 12. When the ball hits the ground, what is its speed? Answer: 63.6 m/s

  36. Extended Response 16. A student pulls a rope attached to a 10.0 kg wooden sled and moves the sled across dry snow. The student pulls with a force of 15.0 N at an angle of 45.0º. If mk between the sled and the snow is 0.040, what is the sled’s acceleration? Show your work.

  37. Extended Response 16. A student pulls a rope attached to a 10.0 kg wooden sled and moves the sled across dry snow. The student pulls with a force of 15.0 N at an angle of 45.0º. If mk between the sled and the snow is 0.040, what is the sled’s acceleration? Show your work. Answer: 0.71 m/s2

  38. Extended Response, continued 17. You can keep a 3 kg book from dropping by pushing it horizontally against a wall. Draw force diagrams, and identify all the forces involved. How do they combine to result in a zero net force? Will the force you must supply to hold the book up be different for different types of walls? Design a series of experiments to test your answer. Identify exactly which measurements will be necessary and what equipment you will need.

  39. Extended Response, continued 17. You can keep a 3 kg book from dropping by pushing it horizontally against a wall. Draw force diagrams, and identify all the forces involved. How do they combine to result in a zero net force? Will the force you must supply to hold the book up be different for different types of walls? Design a series of experiments to test your answer. Identify exactly which measurements will be necessary and what equipment you will need. Answer: Plans should involve measuring forces such as weight, applied force, normal force, and friction.

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