1. Section 2: Weight and Drag Force
Newton’s second law can be used to explain the motion of falling objects.

2. 4.2 Weight and Drag Force Essential Questions
How are the weight and the mass of an object related?
How do actual weight and apparent weight differ?
What effect does air have on falling objects?
Weight and Drag Force
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3. 4.2 Weight and Drag Force
A. How are the weight and the mass of an object related? 1. Weight is the gravitational force experienced by an object. 2. Because weight is a force, the proper unit used to measure weight is the newton.

4. 4.2 Weight and Drag Force
3. Fg = mg, where m is the mass of the object and g is the gravitational field. a. Near Earth’s surface, g is 9.8 N/kg toward Earth’s center. b. 9.8 m/s2 = 9.8 N/kg since 1 N = 1 kg  m/s2

5. 4.2 Weight and Drag Force
4. The gravitational field is a vector quantity. a. Relates the mass of an object to the gravitational force it experiences at a given location. b. A field force whose magnitude is directly proportional to the mass of the object experiencing the force.

6. 4.2 Weight and Drag Force
Practice Problems 16. You place a watermelon on a spring scale calibrated to measure in newtons. If the watermelon’s mass is 4.0 kg, what is the scale’s reading?

7. 4.2 Weight and Drag Force
Practice Problems 17. You place a kg television on a spring scale. If the scale reads N, what is the gravitational field at that location?

8. 4.2 Weight and Drag Force
Practice Problems 18. A 0.50-kg guinea pig is lifted up from the ground. What is the smallest force needed to lift it? Describe the particular motion resulting from this minimum force.

9. 4.2 Weight and Drag Force
Practice Problems 19. A grocery sack can withstand a maximum of 230 N before it rips. Will a bag holding 15 kg of groceries that is lifted from the checkout counter at an acceleration of 7.0 m/s2 hold?

10. 4.2 Weight and Drag Force
B. How do actual weight and apparent weight differ? 1. Apparent weight is the support force exerted on an object during a vertical acceleration.

11. 4.2 Weight and Drag Force
2. Depends on how you are accelerating. a. Not accelerating: apparent weight = actual weight b. Accelerating upwards: apparent weight  actual weight c. Accelerating downwards: apparent weight  actual weight 3. Examples

12. 4.2 Weight and Drag Force
4. Weightlessness a. the condition where there are no contact forces acting to support the object b. the object’s apparent weight is zero.
Weight and Drag Force
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13. 4.2 Weight and Drag Force
Practice Problem You are in an elevator, standing on a bathroom scale. You notice that the scale reads a weight that is less than your actual weight. (Assume that the scale is correctly calibrated.) a. Is the elevator moving at constant velocity, or is it accelerating? b. If it is accelerating, what is the direction of the acceleration?
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Weight and Drag Force

14. 4.2 Weight and Drag Force
Practice Problems 19. On Earth, a scale shows that you weigh 585 N. a. What is your mass? b. What would the scale read on the Moon (g = 1.60 N/kg)?

15. 4.2 Weight and Drag Force
Practice Problems 20. Use the results from Example Problem 3 to answer questions about a scale in an elevator on Earth. What force would be exerted by the scale on a person in the following situations? a. The elevator moves upward at constant speed. b. It slows at 2.0 m/s2 while moving downward.

16. 4.2 Weight and Drag Force
Practice Problem 20 c. It speeds up at 2.0 m/s2 while moving downward. d. It moves downward at constant speed. e. In what direction is the net force as the elevator slows to a stop as it is moving down?

17. 4.2 Weight and Drag Force
C. What effect does air have on falling objects? 1. Drag force is the force exerted by a fluid on an object opposing motion through the fluid.

18. 4.2 Weight and Drag Force
2. Drag force is dependent on: a. the motion of the object b. the properties of the object c. the properties of the fluid (viscosity and temperature) that the object is moving through.

19. 4.2 Weight and Drag Force
3. The constant velocity that is reached when the drag force equals the force of gravity is called the terminal velocity. a. Drop an object, has a small drag force. b. As it falls, drag force increases as object’s velocity increases. c. As it continues to fall, drag force will equal force of gravity. d. At this point, Fnet = 0, so no acceleration.
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Weight and Drag Force

20. 4.2 Weight and Drag Force
4.2 Assignment: p. 105 #22-27 p #45-54 Due Tuesday, 11/10