Newton's Second Law

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Purpose

To study the relationship between ________________, __________________ , and _________________________.

Apparatus

• dynamics cart
• weight hanger
• string
• masses
• smart pulley
• track
• ULI and computer

Introduction

1. Set up the apparatus as shown below.

   

2. As masses are added to the weight hanger, the cart will accelerate, rotate the smart pulley, and the result will be recorded on the computer as a v vs. t graph.
3. As you perform the lab, you may change the maximum values on the axes on any graph to fit your particular needs.

Logging On

1. Have one student in the group log on to the network to give you access to the T-drive (template).
2. Once you are logged on, double-click on the Logger Pro icon, then follow the following steps:
   • File-Open
   • Experiment arrow down [ ]
   • Template
   • All Physics
   • Physics I
   • Forces
   • Pulley
   • F=ma
3. You should see a v vs t graph. Remember, you can change the values on the axes to fit your data collection needs. Follow the directions in the text window.

PART I

The hanging weight is an indication of the net force F_net which will accelerate the cart. Increasing the hanging mass increases the net force on the cart. What effect will increasing hanging weight have on the acceleration of the cart?

1. Place the empty weight hanger over the pulley, so that when you let it go, it falls and pulls the cart along the track.
2. When the cart is ready to be released, press ENTER to allow the pulley to start collecting data. To stop taking data, press ENTER again BEFORE the hanging weight hits the floor and the cart reaches the end of the track. One of the group members should catch the weight hanger before it hits the ground, And the cart should be stopped before it reaches the end of the track.
3. Highlight the area of the graph which appears to be the best constant acceleration data by clicking and dragging on the best part of the graph, and going to Analyze, Linear Fit. What does the slope of the graph represent? Record the best value for the slope of your graph.
4. Take the slotted mass off of the cart and place it on the hanger, so that you are changing the net force acting on the system without changing the total mass of the system.
5. Repeat steps 3 and 4 , recording your best estimate of the acceleration in each case.

PART II

In this part of the lab, you will allow an empty weight hanger (50 g) to accelerate the cart three times, with the cart being heavier each time, but the hanging mass (indicating the net force) staying the same.

1. Place the empty weight hanger on the string and allow it to accelerate the cart carrying only one slotted mass. Record your best estimate of the acceleration of the cart as you did in Part I, steps 3 and 4.
2. Repeat the previous step, adding a heavier mass to the cart, and recording your best estimate of the acceleration.
3. Discuss and answer the following lab questions with your lab group.

Questions

1. State the specific purpose of Part I.

2. State Newton's second law in words, and give the equation.

3. In Part I, you worked with two masses: the hanging mass and the cart mass. In this experiment, which mass does the m in Newton's second law refer to? Explain.

4. Which of the quantities in Newton's second law were you changing in Part I? Explain.

5. Which of the quantities remained constant throughout Part I?

6. Write a general statement describing the trend of the results you obtained in Part I, and relate your results to Newton's second law.

7. In Part II, what quantity were you changing and what quantity were you keeping constant?

8. Write a general statement describing the trend of the results you obtained in Part II, and relate your results to Newton's second law.

9. If the cart starts from rest at a distance of 0.8 m from the pulley and accelerates toward the pulley, using your data from the first trial of Part II, calculate

(a) the time it takes for the cart to reach the pulley.

(b) the velocity of the cart just before it reaches the pulley.

(c) the tension in the string.

(d) Is the tension in the string the net force which is accelerating the mass of the entire system (hanging mass + cart)? Explain.