PHYS 2425 – Engineering Physics I

Newton’s Second Law

 

Leader: _____________________________  Recorder: ___________________________

Skeptic: _____________________________ Encourager: _________________________

 

Introduction

      In this activity, we will explore Newton’s second law.  We will exert a force on a cart by connecting the cart by a string to a hanging mass.  We will simultaneously measure the velocity, acceleration of the cart and the tension force acting on the cart.

 

Materials

Pasco Dynamics Track                                     LabPro

Cart with Dual-range force probe attached        Laptop

Pasco ME- 8967 mass set                                Super Pulley

String                                                               Motion Detector

 

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 LabPro and connect the Motion detector to DIG/SONIC 1 on the LabPro.  Make sure that the force sensor is set to the 10 N position.  Start LoggerPro and open the experiment file called L04IN2.  This file can be found by clicking on the open icon and then double clicking on the “Additional Physics Folder” then double clicking on the "Tools for Scientific Thinking"  folder and then the "Mechanics" folder.

 

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.  Place the cart upside down on the scale while weighing so that it does not role off.

 

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.  Place the motion detector so that it faces the cart on the side opposite the pulley.

      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 so that the rear of the cart is .5 m from the detector.  Also the detector needs to be 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 the detector it “sees” the cart over as much of its motion as possible.

      Attach the string to the cart and the other end to the 5 g plastic mass hanger.  Place a 10 g mass on the hanger.

 

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 which is next to the collect button.  (Alternatively, you can click on the Experiment menu and then choose Zero…) A window will appear.  Click OK in the window and after a few seconds, the force sensor will be ready to go.  Before every data run, you should zero the force probe.

 

Data Analysis and Questions

      Position the cart so that the rear of the cart is 50 cm in front of the motion detector.  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.  Focus on the appearance of the graph while the cart is moving, not the extraneous details about the cart starting, stopping, bouncing at the end, and so on.

 

 

 

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 below.


 

Total 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 and repeat the procedure.  Record the measured value of force and acceleration.  Repeat three more times adding 10 g to the hanger each time.  Note that you need to close the statistics boxes and choose a new area each time you take data.

 

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

 

 

9.  Each time that you added 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?

 

12.  Gently disconnect the USB cable from the back of the laptop.  Click on the New icon  and LoggerPro will give you a window where you can enter the data from your data table.  Construct a correctly labeled graph of Force vs. Acceleration.  Add a best fit line to your graph by click on the following icon

 

13. Record the slope of the line of the best fit line including units.

 

Slope = ________________

 

 

14.  Use that a newton is expressed in fundamental units as kg m/s/s and reduce the units of the slope to lowest terms.  What physical quantity has these units?  _________

 

15.  The slope of the line then represents the ____________ of the cart.  How does the value you determined from the slope compare to the value you measured directly?

 

 

16.  Discuss what sources of error might result in the values not agreeing.

 

 

17.  List the forces that act on the cart.

 

 

18.  Draw a free body diagram for the cart.

 

 

19.  Which of the forces that you listed did you measure with the force probe?  Explain.

 

 

20.  Does the force probe measure the net force on the cart?  Explain

 

 

21.  It turns out that the y-intercept of the graph gives the frictional force acting on the cart.  If you subtract the constant friction force from your data, what will your graph look like?

 

 

22.  We can use LoggerPro to carry out the subtraction.  Click on Data and then choose New Calculated Column… The following window will appear

Name the new column Net Force and give it appropriate units.  Click in the Equation box and then choose the name you entered for the Force column from the Variables (Columns): list.  Enter – and then the value of the y-intercept of your previous graph and click Done.  Click on the y-label of the graph and uncheck the current column and check the net force column.  You should now have a graph displayed of Net Force vs. Acceleration.

 

23.  What kind of relationship is shown between the net force on the cart and the acceleration of the cart?

 

24.  Add a best fit line to this graph.  Is the slope different than before?

 

 

25.  Based on your answers, complete the following

Fnet = _______ x a  This result is Newton’s 2nd law.