Materials

Pasco Dynamics Track

Pasco cart with Dual Range Force Sensor Attached

SuperPulley

LabPro

Laptop

Pasco ME- 8967 Mass set

Introduction

In this activity, we will explore Newton’s second law. We will exert a force on a cart by connecting a string to a hanging mass. We will simultaneously measure the velocity and acceleration of the cart as well as the force exerted on the cart. This is a somewhat challenging procedure. The more care you take in performing the experiment, the better your results will be.

Procedure

1. Set-up

Position the Pasco Dynamics track so that the pulley attached to one end of the track will be suspended over the end of the table. Level the track by adjusting the feet on the bottom. The cart should remain stationary when placed on the track. Connect the LabPro to the computer and connect the Force sensor to CH 1 on the front of the LabPro and connect the Motion detector to DIG/Sonic 1. Make sure that the force sensor is set to the 10 N position. Start LoggerPro and open the experiment file called “L04IN2(DFS)” by following the path Tools for Scientific Thinking=>Mechanics=> L04IN2(DFS)

2. Set up the cart

Attach the force sensor to the top of the cart and then use the electronic balance to measure the mass of the cart (plus sensor) and record the value.

mass of cart = ___________ kg

Obtain a piece of string approximately 1.0 m in length and tie one end to the hook on the force sensor and tie the other end around the mass hanger. Suspend a 10 g mass on the hanger for a total of 15 g.

Place the motion detector so that it faces the cart on the side opposite the pulley. The position of the motion detector is key in this experiment. You need to position it .5 m from the release point of the cart and angled slightly up so that it detects the cart over the maximum range possible as opposed to detecting the track. Experiment with the angle the detector points by pointing it and then letting the cart move away form the detector while collecting data. Adjust until the detector “sees” the cart over as much of its motion as possible.

3. Zero the Force Sensor

We want the force sensor to read 0 when the cart is sitting horizontally on the track with no tension in the string. Place the cart horizontally on the track and make sure that the string is slack. Click on the Zero button next to the collect button. A window will appear. Click on the Zero Force button. After a few seconds, the force sensor will be ready to go. You should zero the force sensor before every data run.

Data Analysis and Questions

Position the cart so that the bottom of the mass hanger is just below the pulley. Click on the collect button and once the motion detector starts clicking release the cart.

1. Once the cart starts moving, sketch the appearance of the velocity vs. time graph. Pay attention to the main trend while the cart is moving, and not the details of the starting of the cart or when the cart is no longer detected by the motion detector.

2. What does the corresponding part of the acceleration vs. time graph look like in this case?

3. Did a constant acceleration produce a linearly increasing velocity?

4. What does the corresponding part of the graph of force vs. time look like?

5. Did a constant force produce a constant acceleration?

6. Click on the acceleration graph. Click and drag over the data in the region where the cart was accelerating. Click on the button which says STAT. This will give you among other information, the average (mean) acceleration. Record in the data table below.

7. Click on the force graph. Click and drag over the data in the region where the cart was accelerating. Click on the button which says STAT. This will give you among other information, the average (mean) force. Record in the data table on the next page.

Mass on Hanger (g)	Acceleration (m/s/s)	Force (N)
15
25
35
45
55

Add 10 g to the hanger so that the total mass on the hanger is 25 g, and repeat the procedure. Be sure to zero the force probe before taking data. Record the measured average value of force and acceleration. Repeat three more times adding 10 g to the hanger each time.

8. Each time you added an additional 10 g to the mass hanger, what happened to the force that you measured?

9. Each time that you added an additional 10 g to the hanger, what happened to the acceleration that you measured?

10. Does there seem to be a relationship between the force and the acceleration? If so what type?

11. What should a graph of Force vs. acceleration look like, if your answer to question 10 is correct?

13. Click on the New icon to open a window to enter a graph. Construct a graph of force vs. acceleration (remember it is y vs. x). Be sure to label the axes correctly and give the graph a title.

14. Do your data appear to lie along a line?

15. Add a best-fit line to the graph by clicking on the following icon

Print a copy of your graph and attach it to your report.

16. Record the slope of the line including units.

Slope = ________________

According to Newton’s second law the relationship between force and acceleration is

F_net = ma.

17. Compare your graph to Newton’s second law. The slope should be the mass of something. What object’s mass should your slope be?

18. Compare your slope to the appropriate mass. Are they close?

19. Newton’s second law relates the net force to the acceleration. The force probe measures the tension due to the string. Do any other forces affect the motion of the cart?

20. If you do a careful analysis of the problem it turns out that friction would show up on your graph as a y-intercept. Examine your graph, is the y-intercept different than 0?

21. Record the value of the y-intercept. Include units. This is the force of friction acting on the cart.