Newton s Second Law: Quantitative I

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1 / Then change the settings: forces ) describe the acceleration? How would you describe the velocity? (Use Set the force to 100 N and click on go. unbalanced force is constant. the value of the velocity. Describe what happens to the velocity when the graph. Which one of these graphs is similar in shape to the force versus time graph? Keep the force at 1 00 N. This time when you click go, watch what happens to 72 Observe and Describe words like constant, steadily increasing, steadily decreasing, etc.) a) Examine the velocity versus time graph and the acceleration versus time 71 Observe and Find a Pattern Select Graph Force, Graph Acceleration, and Graph Velocity Select Show total force only in upper right (deselect Show horizontal Turn friction off _Dimension Dimension. This can be accessed at: b) For this scenario, how would you describe the force? How would you For activities 7.1 through 7.5, use the PHET Simulation called: Forces in One Newton s Second Law: Quantitative

2 Look at the force diagram in the upper right hand corner. The forces in the a) What does FG represent? Keep the force at 1 00 N but set it for only about 4 sec. Then set the force to c) Did the cabinet stop moving? Did it slow down? Explain. direction? velocity? pushing? c) There is a tiny arrow that points in the positive horizontal direction. What a) What happened when the man stopped pushing? b) Look at the force diagram. What is Fnet after the man stopped 0. f) How is the direction of the unbalanced force related to the change in the e) Does the motion of the object change in the horizontal direction? b) Does the motion of the object change in the vertical direction? d) s there any other arrow that balances this arrow in the horizontal the object. vertical direction balance each other out. FN is the force the surface exerts on does this arrow represent? 7.3 Observe and Find a Pattern 7.4 Observe and Find a Patterr

3 d) What can you do to stop it? 7.5 Observe and Find a Pattern a) Fill in the table below by changing the value of the force Fman on object and finding the value of the acceleration. Force (N) Acceleration (m/s/s) 0 Q ou 100 i. Describe the pattern from the table in words ii. c c Graph your data and draw a best fit line. iii. Can you invent a mathematical expression for the relationship between F and a?

4 i. Describe the pattern from the table in words. 10 kg book kg dog kg 100 (m/s/s) 400kg kg 100 Object Acceleration Force (N) ii. Graph your data and draw a cabinet \J ) crate 0. ) refrigerator C d)fill in the table below for the different objects. different than the motion for the filing cabinet? the refrigerator. Run the simulation. How is the motion for the refrigerator c) This time, set the applied force to 100 N. Choose your object at r best fit line.

5 . than a will decreases. Let s think about an equality a = b x c. n this situation, if we increase c and proportional. keep b constant, than a will increase. f we decrease c and keep b constant, The type of relationshi we see between a andm is called in versely on it. The mass of the box is 3.6 kg. b) Draw a force diagram for the box assuming that any other forces c) Are the two diagrams consistent? Explain. d) What is the acceleration of the box? increase. Think about how this is different than if we increase or decrease b. our object of interest 7:6 Represent and Reason system. The acceleration a of the system is exerted on the box are balanced. Draw a motion diagram. a) You are pushing on a box on a smooth floor exerting a N force The iinit of force is called the newton (N) where 1 N = (1 kg)(1 m/s 2). constant, than a will decrease. f we decrease c and keep b constant, than a will objects and inversely proportional to the mass m of the system object:. t,t directly proportional to the unbalanced force exerted on the system by other the Newton s Second Law of Motion: We choose a particular object (objects) as Now think about an equality C. n this situation, if we increase c and keep b b a= a ins

6 e) What if you put a book of 0.7 kg on top of the box? What would change in your answers to questions a-d? jcarfl7 o a / C Ci&L(tt4C SC m z9fr 7.7 Represent and Reason Consider the scenario of the box in the previous problem. You will need your equations for linear motion from the previous unit. You push on the box for 2 seconds a) What is the speed of the box at the end of these 2 seconds? i 7CA vi b) magine that after these 2 seconds you stop pushing; what will happen to the box? Explain. Draw a motion diagram and a force diagram to support your answer. A J rj, ii 4 t 1 ( t 1 7 1i/ f 1o : F ) c) What if you put a book of 0.7 kg on top of the box while you were pushing it? What would change in your answers to questions a and b? (- C, U L V

7 7.8 Represent and Reason Use the velocity versus time graph for an object to find its acceleration (hint: slope!) and determine the unbalanced force exerted on it. Assume the mass of the object is 2.0 xl O kg (i.e kg). U Time (s)