POGIL: Newton s First Law of Motion and Statics. Part 1: Net Force Model: Read the following carefully and study the diagrams that follow.

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1 POGIL: Newton s First Law of Motion and Statics Name Purpose: To become familiar with the forces acting on an object at rest Part 1: Net Force Model: Read the following carefully and study the diagrams that follow. Recall that a force is simply a push or a pull on an object. A force has both magnitude and direction. It has magnitude because you can push or pull with different amounts of effort. It has direction because you can direct a push or pull in a particular direction. The magnitude of a force is commonly measured in pounds (lb.) or Newtons (N). The greater the number of Newtons of force, the greater the magnitude of the push or pull. So a push with 5 N of force is a harder push than one with only 1 N. The net force is the total force on the object, taking into account the magnitude and direction of each individual push or pull. The following diagrams show the magnitude and direction of several forces acting on some boxes and the resulting net force on each one. 10 N 10 N net force = 10 N to the right net force = 0 N net force = 5 N to the left Critical Thinking Questions, Part I 1. Determine the net force (magnitude and direction) on the following objects. a. 35 N b. 9 N c. 8 N 6 N 8 N d. 5 N 9 N Part II: Newton s First Law of Motion states the following: An object maintains its state of motion unless it is acted upon by net force. This means that an object at rest will stay at rest unless the net force acting upon is non-zero. Critical Thinking Part II 2. Suppose each of the boxes in Question 1 are initially at rest. Which ones will remain at rest? 1

2 3. Each of the following boxes is initially at rest. Show the magnitude and direction of the force needed for the box to remain at rest. a. 20 N b. 9 N c. 6 N 6 N 13 N d. Hint: You need two forces for this one! 7 N 13 N e. In each of the above examples with the missing force drawn in, what is the net force on each box? 4. Look at a book resting on a table. One of the forces acting on it is gravity, pulling it downward with a force equal to the object s weight. a. What is the net force on the book? b. Suppose the book weighs 10 N. That means gravity is pulling the book downward with a force of 10 N. What must be the magnitude and direction of the second force that pushes on the book? c. What object exerts the force you determined in part b? 2

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4 POGIL: Newton s First Law of Motion and Statics Name Part 1: Newton s Second Law and Net Force Newton s Second Law of Motion states that the acceleration of an object is directly proportional to the net force on an object and inversely proportional to the mass of the object. We can write this in the form of an equation: F net a =, m where a is the acceleration of the object, F net is the net force acting on it, and m is the object s mass. The following diagrams show the magnitude and direction of several forces acting on some boxes with masses of 5 kg. The resulting net force and acceleration of each box is calculated using Newton s Second Law. 10 N 10 N net force = 10 N to the right acceleration = 10 N/5 kg = 2 m/s 2 to the right net force = 5 N to the left acceleration = 5 N/5 kg = 1 m/s 2 to the left net force = 0 N acceleration = 0 m/s 2 Critical Thinking Questions, Part 1 1. Both net force and acceleration have magnitude and direction. How do the directions of the net force and acceleration compare? 4

5 2. The following diagrams show the magnitude and direction of several forces acting on some boxes with different masses. For each one, find the magnitude (with the correct units) and direction (up, down, left, or right) of both the net force and acceleration. a. 5 N 20 kg 5 N net force: acceleration: b. 8 N 10 kg 20 N net force: acceleration: c N 8 N 0.4 kg net force: 1 acceleration: d. 7 N 20 kg 2 net force: 5 N acceleration: 5

6 Part 2: A Kinematics Reminder Recall that the velocity of an object is how fast it is moving and the direction it moves in. Acceleration is the rate of change of velocity. Therefore, if speed or direction changes (either one is considered a change in velocity), the object is accelerating. Critical Thinking Questions, Part 2 3. Is it possible to say for certain that any of the boxes in question 3 are at rest? Justify your answer. 4. Which of the boxes in question 3 could be moving with a constant velocity? Justify your answer. 5. Is it possible for the box in question 3b to be moving to the right? If so, is the box s speed increasing, decreasing, or staying the same? If not, explain why not. 6. Is it possible for the box in question 3c to be moving to the right? If so, is the box s speed increasing, decreasing, or staying the same? If not, explain why not. 7. Suppose the box in question 3d is moving to the right. a. Will the speed of the box increase, decrease, or remain the same? b. Will the direction of motion of the box change? 6

7 Part 3: Force Diagrams Model: Recall the following types of forces: gravity: an attractive force between two masses; the weight of an object is the gravitational force exerted on it by the planet on which the object resides normal force: the perpendicular force a surface exerts on an object tension: the force with which a rope or other material pulls on an object friction: the force that resists sliding between surfaces; the frictional force is parallel to the surface Imagine a 10 kg block sitting on a table, as in the following diagram. block 10 kg table We will now draw the block by itself with some arrows that show the directions of the forces acting on the block. normal force gravity This type of diagram is called a force diagram or free-body diagram. Every force is represented by an arrow showing the direction of the force and each force is labeled. Critical Thinking Questions, Part 3 8. If the 10 kg box in the example above is at rest, find: a. the acceleration of the box b. the net force on the box c. the weight of the box d. the magnitude of the support force 9. If the 10 kg box in the example above is moving at a constant velocity of 2 m/s to the right, find: a. the acceleration of the box b. the net force on the box c. the magnitude of the support force 7

8 10. Now imagine dragging the 10 kg box with a rope to the right with 40 N of force. Suppose also that friction is acting with 20 N of force between the box and tabletop. a. Draw a force diagram of the box below. Don t forget to label the forces. b. Find the magnitude and direction of the net force on the box. c. Find the magnitude and direction of the acceleration of the box d. Is the box speeding up, slowing down, or moving at a constant speed? Justify your answer. e. Suppose you stop pulling the box with the rope. Answer the following questions for right after letting go of the rope but before the box stops moving. i. What is the new magnitude and direction of the net force? ii. What is the new magnitude and direction of the acceleration? iii. Is the box now speeding up, slowing down, or moving with constant speed? Justify your answer. 8

9 Conclusions Write a paragraph in clear, complete sentences that addresses the following questions: How are the magnitudes of net force and acceleration related to each other? How do the directions of the net force and acceleration compare? Can an object have zero net force acting on it and still be moving? Why or why not? If an object is changing its direction of motion but maintaining a constant speed, can it have a net force of zero acting on it? Is the net force on an object always in the same direction as its motion? If so, use Newton s Second Law to explain why. If not, describe the motion of an object whose net force is in the opposite direction of its motion. 9