During the Push What kind of motion does the puck have at this time? Is it speeding up, slowing down, not moving, or moving at a steady speed?

Transcription

1 Elaborate The teacher is giving a demonstration with a hover puck. Make the requested predictions before using your observations to check if you were correct or if you need to make changes.. Predict how the puck will behave according to the descriptions. Pay close attention to the time frame for your predictions. Before means that nobody is touching the puck, during means the whole time somebody is pushing the puck, and immediately after means right after the hand was taken off. A. A hover puck is pushed while it is turned off. During the (while touching the puck) What kind of motion do you predict the puck will have at this time? What kind of motion do you predict the puck will have at this time? at a steady Observations During the What kind of motion does the puck have at this time? What kind of motion does the puck have at this time? at a steady B. A hover puck is pushed while it is turned on. During the What kind of motion do you predict the puck will have at this time? What kind of motion do you predict the puck will have at this time? at a steady Observations During the What kind of motion does the puck have at this time? What kind of motion does the puck have at this time? at a steady 12b. How can you tell if an object is speeding up, slowing down, or moving with a uniform You may use graphs in your explanation. 1

2 13. What is the difference between set up A (puck is turned off) and B (puck is turned on)? 14. In each of these examples, what was acting to cause a change in velocity (the puck speeding up or slowing down)? 15. What applied a force on the puck after the person stopped pushing it in test A.? 16. In which set up was the friction force bigger? When the hover puck was turned on or off? 17. What evidence do you have that supports your idea about the size of the friction force? 18. Suppose you could completely remove the friction force acting on the puck. What kind of motion (stationary, speeding up, slowing down, steady speed) do you think it would have after you pushed it? Explain why you think that. 19. Read the following student statement. Explain whether you agree or disagree with it and support your idea with evidence. When there is a push or force on an object, it speeds up or slows down. Another way to say it is that motion changes when a extra force is exerted on an object. 20. In the first situation, the hover puck is turned off. In the second situation, the hover puck is turned on. I II Choose one of the possible solutions (a-d) listed below the questions, and explain why you selected that option. What should you do to make the puck with the hover puck turned off to move with a uniform velocity? What should you do to make the turned on puck move with a uniform velocity? a) Give the puck a push from behind, and immediately release the puck. b) Constantly push the puck from behind as it glides across the table. c) Increase the strength of the push on the puck as it moves across the table. d) You cannot achieve a uniform velocity by just pushing with your finger. You must do something else. Describe what you must do. a) Give the puck a push from behind, and immediately release the puck. b) Constantly push the puck from behind as it glides across the table. c) Increase the strength of the push on the puck as it moves across the table. d) You cannot achieve a uniform velocity by just pushing with your finger. You must do something else. Describe what you must do. 21. If you release an object and allow it to fall under the influence of gravity alone, what kind of motion will it experience? 22. If it is accelerating, what must be acting on the object? 23. Is this a contact or non-contact force? 24. Would you describe the force applied to the object as balanced or unbalanced? Why? Balanced means that the forces acting are equal on each side of the object; unbalanced means they are unequal. You drop an object. 25. Prepare a free-body diagram for the object describing the force acting on the object while it is falling. Indicate what applies this force on your diagram. (Assume that air resistance is not a factor in this example.) 2

3 25b. What forces are acting on the object? 26. How should the lengths of the vectors (arrows) in your diagram compare? Why? You hold the object in place by a thread. 28. Prepare a free-body diagram for the suspended object. Label all of the forces acting on the object. 27. Would you describe the force exerted on the object as balanced or unbalanced? Why? 29. What forces are applied to the book? 30. How should the lengths of the vectors in your diagram compare? Why? A book rests on a table top. 32. Prepare a free-body diagram for the book. Label all of the forces acting on the book. 31. Would you describe the force applied to the book as balanced or unbalanced? Why? You will now prepare some free-body diagrams for set-ups involving pulleys. Consider the following set up where equal-sized weights are attached by strings to a low-friction cart. Prepare accurate free-body diagrams for each cart. The friction forces are so small that you may ignore them in your free-body diagram. Draw a free body diagram for the low-friction cart shown in the illustration. (there should be more than two force arrows in each set-up) Indicate whether all of the forces acting on the object are balanced or unbalanced. Describe the specific evidence that supports your interpretation of whether or not the forces are balanced or unbalanced. 33. Forces Balanced or Unbalanced? Observation 35. Actual motion Stationary Cart 34. Predicted motion. 36. Were the forces balanced or unbalanced based on the actual motion? 37. Forces Balanced or Unbalanced? Observation 39. Actual motion Consider the forces acting on the cart after the push Predicted Motion. 40. Were the forces balanced or unbalanced based on the actual motion?

4 40b. When forces are balanced how should the length of the vectors compare? 41. Can an object have forces applied to it, yet experience no change in motion? Give an example. 42. Can an object be moving even though the forces acting on it are balanced? Given an example. Consider the following situation. A stationary cart is prepared as in your original lab set-ups. One of the strings is cut. 43. Prepare a free-body diagram for the cart. 44. Explain whether the forces are balanced or unbalanced. 45. Predict the type of motion the cart will experience. Explain why this type of motion will be observed. Review We have examined some of the basic properties of forces in the last investigations. The simplest interpretation of a force is that it is a push or a pull. We have observed that a force is the result of an interaction between two objects. We classified forces as either contact or non-contact forces. Friction is an example of a contact force, while gravity is an example of a non-contact force. We have also observed that a force can result in the change in motion of an object. Forces can change the speed of an object, or the direction that an object is moving, or both. When forces result in the change in motion of an object, we say that the forces are unbalanced. In some cases, the forces acting on an object are balanced. When forces are balanced, we observed no change in motion. We used a spring scale to measure forces in a system where the forces were balanced. We also practiced identifying the forces acting on an object by drawing free-body diagrams. Free-body diagrams help us predict the motion of an object, or explain why the object moves as it does. Evaluate II. Conrad rolls a steel ball bearing near a magnet. The ball bearing is deflected as the diagram below illustrates. The speed of the ball bearing does not change. We are looking at the set-up from above. The magnet lies flat on the table and the steel ball is rolled passed the magnet as shown. 46. Was an unbalanced force applied to the ball bearing? How do you know? 47. Do you have to observe a change in speed in order to know that a net force was applied? Why or why not? 48. Was the force applied a contact or non-contact force? How do you know? 4

5 49. What is the direction of the force applied by the magnet on the ball bearing at point p? III. Isabella slides the salt across the table to her sister. While she is pushing the salt shaker it is speeding up. As soon as she stops pushing the shaker it starts slowing to a stop. The shaker then comes to rest in front of her sister. Friction forces affect the motion of the shaker during A and B. Fill the requested information in the table below. Salt Shaker Motion Free Body Diagram Forces Balanced or Unbalanced? How do you know? Contact forces affecting the shaker Non-contact forces affecting the shaker 5

6 IV. Fill the requested information in the table below. Illustration Free-body diagram Are the forces balanced or unbalanced? x vs. t graph (positiontime) sketch describing the motion of the object v vs. t graph (velocitytime) sketch describing the motion of the object 6