P11 Constant Force

PASCO scientific Science Workshop

Physics Lab Manual: P11 - 1 Newton's Second Law – Constant Force

Experiment P11: Newton's Second Law – Constant Force (Force Sensor, Motion Sensor) Concept Newton’s Laws

Time SW Interface Macintosh® file 30 m 500 or 700 P11 Constant Force

EQUIPMENT NEEDED • Science Workshop™ Interface • ±50 Newton force sensor • motion sensor • adjustable feet, track (two) • balance (for measuring mass)

• • • • •

Windows® file P11_CONF.SWS

dynamics cart mass set with hanger pulley with table clamp string track, 1.2 meter

PURPOSE The purpose of this laboratory activity is to investigate the relationship between force, mass, and acceleration. THEORY Newton described the relationship between acceleration, force, and mass as follows: The acceleration of an object is directly proportional to and in the same direction as the net force, and inversely proportional to the mass of the object, or a=

Fnet m

where a is acceleration, F net is net force, and m is mass. PROCEDURE For this activity, a motion sensor measures the motion of a cart that is pulled by a string. The string is attached to a mass and suspended over a pulley. A force sensor mounted on the cart measures the force that accelerates the cart. The Science Workshop program calculates the velocity of the moving cart. The graph of velocity versus time reveals the cart’s acceleration, which is compared to the theoretical value. PART I: Computer Setup 1.

jh

Connect the Science Workshop interface to the computer, turn on the interface, and turn on the computer.

© 1996, PASCO scientific

P11 - 1


P11 - 2: Physics Lab Manual Newton's Second Law – Constant Force

2.

Connect the motion sensor’s stereo phone plugs to Digital Channels 1 and 2. Plug the yellow-banded (pulse) plug into Digital Channel 1 and the second plug (echo) into Digital Channel 2.

3.

Connect the DIN plug of the force sensor to Analog Channel A.

4.

Open the Science Workshop file titled as shown: Macintosh P11 Constant Force

Windows P11_CONF.SWS

•

The document will open with a Graph display of Velocity (m/sec) versus Time (sec), and a Table display of Force (Newtons).

•

Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click on its window or select the name of the display from the list at the end of the Display menu. Change the Experiment Setup window by clicking on the Zoom box or the Restore or Maximize button in the upper right hand corner of that window.

5.

The Sampling Options… are: Periodic Samples = Fast at 200 Hz and Digital Timing = 10000 Hz.

PART II: Sensor Calibration and Equipment Setup Sensor Calibration •

You do not need to calibrate the motion sensor.

P11 - 2


jh


1.


To calibrate the force sensor, change the Experiment Setup window to full size by clicking on the Zoom box or the Maximize button in the upper right hand corner of that window.

Full size

Zoom box

Close box

Macintosh

2.

In the Experiment Setup window, double-click on the force sensor’s icon to open the Force Sensor setup window.

•

The Force Sensor setup window shows the default calibration values (50 Newtons produces 8 Volts, -50 Newtons produces -8 Volts). The force sensor is set up so that a pull away from the sensor is a ‘negative’ force. For example, if a one kilogram object is hung vertically from the hook, the force sensor measures -9.8 Newtons (since the force is downward).

jh


Restore button Minimize button

Control-menu button

Reduced size

Maximize button

Windows

P11 - 3



3.

Mount the force sensor on a horizontal rod so the force sensor’s hook is down. Do NOT put an object on the force sensor’s hook yet.

4.

For the High Value calibration point, press the tare button on the side of the force sensor to zero the sensor. Click the Read button for High Value. Since there is no object on the sensor’s hook, type 0 as the High Value.

5.

For the Low Value calibration, hang an object of known mass on the sensor’s hook.

6.

Click the Read button for Low Value. Enter the object’s weight in Newtons (mass in kilogram x 9.8 N/kg).

•

Remember, enter the object’s weight as a negative value (a force pulling away from the sensor).

Force Sensor

Object of known mass

•

In this example, the object has a mass of 549.5 grams (0.5495 kg), or a weight of 5.385 Newtons (entered as -5.385).

7.

Click OK to return to the Experiment Setup window.

P11 - 4


jh



Equipment Setup 1.

Place the track on a horizontal surface. Level the track by placing the dynamics cart on the track. If the cart rolls one way or the other, use the adjustable feet at one end of the track to raise or lower that end until the track is level and the cart will not roll one way or the other.

2.

Attach a pulley to the right end of the track.

3

Position the motion sensor at the left end of the track. The cart will be pulled away from the sensor. The minimum distance from the sensor to its target is 0.40 m (40 cm). Put a mark on the track 40 cm from the motion sensor.

4.

Use a #0 Phillips head screwdriver (available on the Force Sensor Bracket) to mount the force sensor onto the accessory tray of the cart. Carefully measure and record the total mass of the cart plus force sensor.

5.

Place the cart on the track so the hook end of the force sensor points away from the motion sensor. Minimum distance = 40 cm Force Sensor

Pulley

Motion Sensor

Dynamics cart

To Interface

Mass

6.

Use a string that is 10 cm longer than the length needed to reach the floor when the cart is next to the pulley. Attach one end to the force sensor’s hook.

7.

Add 20 or 30 grams of mass to the mass hanger. Carefully measure and record the total mass.

8.

Attach the mass hanger to the other end of the string, and put the string in the pulley’s groove. Adjust the height of the pulley so the string is parallel to the track.

•

You can fasten the string to the mass hanger by wrapping the string four or five turns around the notched area of the mass hanger.

PART IIIA: Data Recording

jh


P11 - 5



1.

Pull the cart toward the left end of the track but keep the cart at least 40 centimeters from the motion sensor, and don’t let the mass hanger bump into the pulley.

2.

Click the REC button (

3.

Click STOP (

4.

Stop the cart before it collides with the pulley.

•

Run #1 will appear in the Data list in the Experiment Setup window.

) to begin data recording and then release the cart.

) to end recording just before the cart reaches the pulley.

ANALYZING THE DATA 1.

Click the Statistics button ) in the lower left ( corner of the Graph display to open the Statistics area. Click the Autoscale button ) to rescale the graph ( to fit the data.

2.

In the plot of velocity, use the cursor to click-and-draw a rectangle around the region of the plot that shows the movement of the cart.

3.

Click the Statistics Menu button ( ) in the Statistics area. Select Curve Fit, Linear Fit from the Statistics menu.

•

The Statistics area will show the y-intercept (a1) and the slope (a2) for the line of ‘best fit’ for your data. The slope is the average acceleration of the cart.

4.

Record the slope (acceleration) of the best fit line in the Data section.

5.

Calculate and record the theoretical acceleration of the system.

6.

Click the Table to make it active. Click the Statistics button (

•

The Statistics area at the bottom of the Table shows Min (minimum), Max (maximum), Mean, and Std. Dev (standard deviation).

P11 - 6


).

jh



7.

Record the Mean of the Force in the Data section.

8.

Calculate and record the force exerted on the cart/force sensor (i.e., the weight at the end of the string).

jh


P11 - 7



DATA TABLE Item

Value

mass of cart + sensor

kg

mass of hanger + masses

kg

acceleration (slope)

m/sec/sec

acceleration (calculated)

m/sec/sec

Force (Mean)

Newtons

Force (calculated)

Newtons

QUESTIONS 1.

What is the percentage difference between the measured and theoretical values of acceleration? Remember, % difference =

measured − theoretical × 100% theoretical

2.

What is the percentage difference between the measured and calculated values of force?

3.

What are some possible reasons for any differences between the measured and calculated or theoretical values?

P11 - 8


jh