PHYSICS CHAPTER 2 ASSESS PG. 24-27 #1-48

1. What is the difference between force and net force on an object?

1. Force is a push or a pull; net force is the combination of all acting forces.

10. When you do pull-ups and you hang at rest, how much of your weight is supported by each arm?

10. Each arm supports half your weight.

11. What is the angle between a support force and the surface on object rests upon?

11. 90°; support force is perpendicular (normal) to the surface.

12. What two forces compress a spring inside a weighing scale when you weigh yourself?

12. Your downward push due to gravity and the upward force of the floor

13. When you are at rest and supported by a pair of weighing scales, how does the sum of the scale readings compare with your weight?

13. The sum of the readings will equal your weight when you are at rest

14. Can an object be moving and still be in equilibrium? Defend your answer.

14. Yes—if it moves at constant speed in a straight line. Then EF = 0.

15. If you push a crate across a factory floor at constant speed in a constant direction, what is the magnitude of the force of friction on the crate compared with your push?

15. Both forces are equal in magnitude, but in opposite directions. Thus, the net force is zero.

16. Distinguish between static equilibrium and dynamic equilibrium.

16. Objects at rest are in static equilibrium; objects moving at constant speed in a straightline path are in dynamic equilibrium.

17. According to the parallelogram rule for two vectors, what does the diagonal of a constructed parallelogram represent?

17. The diagonal is the resultant, or the sum of the two vectors.

18. Consider the suspension of Nellie in Figure 2.11. Name the three forces that act on her. What is your evidence that they cancel to zero?

18. Downward force is weight. Two upward forces are tensions in ropes. Being at rest (in equilibrium) is evidence that EF = 0.

19. Consider Nellie in Figure 2.12. What changes in rope tension occur when the ropes make a greater angle with the vertical?

19. Rope tensions increase.

2. What is the net force on a box that is being pulled to the right with a force of 40 N and pulled to the left with a force of 30 N?

2. Net force is 10 N to the right

20. When Nellie hangs from ropes at different angles, as shown in Figure 2.13, how does the vector resultant of the two rope tensions compare with her weight?

20. Resultant of both rope tensions is equal in magnitude and opposite in direction to the vector representing her weight.

21. Blocks A and B are supported by the table. Block C is partly supported by the table and partly by the rope. Rank the support forces provided by the table from greatest to least.

21. A = B = C

22. In the diagram below, identical blocks are suspended by ropes, each rope having a scale to measure the tension (stretching force) in the rope. Rank the scale readings from greatest to least.

22. B = D, C, A

23. Burl and Paul stand on their sign-painting scaffold. Tension in the left rope is measured by a scale. Rank the tensions in that rope from greatest to least.

23. C, D, A = B

24. Percy does gymnastics, suspended by one rope in A and by two ropes in positions B, C, and D. Rank the tensions in the ropes from greatest to least.

24. D, A, C, B

25. A cat lies on the floor. Can you say that no force acts on the cat? Or is it correct to say that no net force acts on the cat? Explain.

25. Correct to say no net force, as both gravity and support of the floor act on cat.

26. Consider two forces, one having a magnitude of 20 N and the other a magnitude of 12 N. What is the maximum net force possible for these two forces? The minimum ?

26. Maximum resultant occurs when forces are parallel in same direction: 32 N. The minimum occurs when they oppose each other: 8 N.

27. When a box of chocolate bars is in mechanical equilibrium, what can be correctly said about all the forces that act on it? Must the net force necessarily be zero?

27. The sum of all forces (i.e., the net force) must equal zero. Yes; in mechanical equilibrium, EF = 0.

28. Faina says that an object cannot be in mechanical equilibrium when only a single force acts on it. Do you agree or disagree?

28. Agree; if only a single nonzero force acts on an object, it will not be in mechanical equilibrium. There must be one or more additional forces to produce zero net force for equilibrium.

29. Phyllis Physics hangs at rest from the ends of the rope, as shown at right. How does the reading on the scale compare to her weight?

29. Scale reads half her weight. So, EF = upward pull of left rope + upward pull of right rope - weight = 0.

3. What name is given to the stretching force that occurs in a spring or rope being pulled?

3. Tension

30. Harry the painter swings year after year from his bosun's chair. His weight is 500 N and the rope, unknown to him, has a breaking point of 300 N. Why doesn't the rope break when he is supported as shown at the left? One day Harry is painting near a flagpole, and, for a change, he ties the free end of the rope to the flagpole instead of to his chair as shown at the right. Why did Harry end up taking his vacation early?

30. At left, Harry is supported by two strands of rope that share his weight (like Phyllis in Question 29). So each strand supports 250 N, below the breaking point. At right, Harry is supported by just one strand, which requires tension of 500 N. This is above the breaking point of the rope, which breaks and changes his vacation plans.

31. How many significant forces act on a your physics book when it is at rest on a table? Identify the forces.

31. Two forces—weight and support force

32. Why doesn't the support force that acts on a book resting on a table cause the book to rise from the table?

32. The book doesn't rise because the net force on it is zero: weight - support force = 0.

33. Nicole stands on a bathroom scale and reads her weight. Does the reading change if she stands on one foot instead of both feet? Defend your answer.

33. No; the reading is the same. Pressure against the scale is less on one foot, but not the weight.

34. Justin sets a hockey puck sliding across the ice at a constant speed. Is the puck in equilibrium? Why or why not?

34. Yes, it is in dynamic equilibrium; it is not undergoing a change in its motion.

35. Alyssa pulls horizontally on a crate with a force of 200 N, and it slides across the floor at a constant speed in a straight line. How much friction is acting on the crate?

35. 200 N; constant speed in a straight line, so EF = 0 = force of pulling - friction.

36. Consider a heavy refrigerator at rest on a kitchen floor. When Anthony and Daniel start to lift it, does the support force on the refrigerator provided by the floor increase, decrease, or remain unchanged? What happens to the support force on Anthony's and Daniel's feet?

36. Support force on the refrigerator decreases as it's lifted. When entirely lifted from the floor, the support force provided by the floor is zero, and the support force on the men's feet increases as the load transfers from the floor to them

37. Sneezlee is supported by two thin wires. Is the tension in each wire less than, equal to, or more than half his weight? Use the parallelogram rule to defend your answer.

37. If perfectly vertical, then tension in each wire is half of Sneezlee's weight. But the wires are only nearly vertical, so tension in each is greater than half the weight

38. Sneezlee's wire supports are repositioned as shown. How does the tension in each wire compare with the tension of the previous question?

38. Greater tension, as a parallelogram would show. (Interestingly, a 60° angle results in tension equal to the weight. If angle exceeds 60°, tension in the wire exceeds the weight.)

39. If a picture frame were supported by a pair of vertical wires, tension in each wire would be half the weight of the frame. When the frame is supported by wires at an angle, as shown below, how does the tension in each wire compare with that of vertical wires?

39. Tension in each wire is greater than half the weight of the picture.

4. What two quantities are necessary to determine a vector quantity?

4. Magnitude and direction

40. A monkey hangs by a strand of rope and holds onto the zoo cage as shown. Since her arm holding the cage is horizontal, only the rope supports her weight. How does the tension in the rope compare with her weight?

40. Tension in the rope is greater than her weight.

41. Why can't the strong man pull hard enough to make the chain perfectly straight?

41. Chain tensions on both sides of the book must form a parallelogram with a resultant that equals the weight of the book. This can only occur if each side of the chain makes an angle to the horizontal.

42. Two vertical chains are used to hold up a 1000-N log. One chain has a tension of 400 N. Find the tension in the other chain.

42. From EF = 0, total upward tensions = weight of log. 400 N + tension in other chain = 1000 N. Tension in other chain = 1000 N - 400 N = 600 N.

43. Lucy Lightweight stands with one foot on one bathroom scale and her other foot on a second bathroom scale. Each scale reads 300 N. What is Lucy's weight?

43. If each scale reads 300 N, Lucy's total weight = 600 N

44. Harry Heavyweight, who weighs 1200 N, stands on a pair of bathroom scales so that one scale reads twice as much as the other. What are the scale readings?

44. 800 N on one, 400 N on the other

45. The sketch shows a painter's staging in mechanical equilibrium. The person in the middle weighs 250 N, and the tensions in both ropes are 200 N. What is the weight of the staging?

45. EF = 0, upward forces are 400 N, and downward forces are 250 N + weight of staging. So staging weighs 150 N.

46. A staging that weighs 300 N supports two painters, one 250 N and the other 300 N. The reading in the left scale is 400 N. What is the reading in the right scale?

46. EF = 0, upward forces are 400 N + tension in right scale, and downward forces are 250 N + 300 N + 300 N = 850 N. Reading on the right scale is 450 N.

47. Two children push on a heavy crate that rests on a basement floor. One pushes horizontally with a force of 150 N and the other pushes in the same direction with a force of 180 N. The crate remains stationary. Show that the force of friction between the crate and the floor is 330 N.

47. From EF = 0, Eforces in one direction = Eforces in opposite direction. So, 150 N + 180 N = force of friction = 330 N in opposite direction to the children's pushes.

48. Two children push on a crate. They find that when they push together horizontally with forces of 155 N and 187 N, respectively, the crate slides across the floor at a constant speed. Show that the force of friction between the crate and the floor is 342 N.

48. Crate moves at constant speed in a straight line, so EF = 0. Eforces in one direction = Eforces in opposite direction. So, 155 N + 187 N = force of friction = 342 N in opposite direction to the children's pushes.

5. How does a vector quantity differ from a scalar quantity?

5. Vector quantity needs both magnitude and direction for its description. Scalar quantity is described by magnitude only, a number

6. Give an example of a vector quantity. Give an example of a scalar quantity

6. Force is a vector quantity; time, area, and volume are scalar quantities

7. How much tension is in a rope that holds up a 20-N bag of apples at rest?

7. 20 N

8. What does EF = 0 mean?

8. It means that the vector sum of all the forces that act on an object in equilibrium equal zero.

9. What is the net force on an object at rest?

9. Zero, as the rule EF = 0 states