Physics 123 - Minilab 5

NEWTON'S LAWS - I

Purpose

Study the effect of velocity and acceleration on the tension in a pulley string holding a weight.

Introduction

According to Newton's Second Law, the net force on a mass must change if its acceleration changes in either magnitude or direction. No net force means the body moves at constant velocity (which need not be zero).

In this lab you will record the force on a body connected by a string over a pulley to a force probe. The probe directly measures the attached force, such as the tension in the string. Handle the probe very carefully.

Procedure

In the Course files folder, open the file for Lab 5, which will start up the LoggerPro program. You should see a blank plot of force (in Newtons) versus time.

Attach the force probe to the bar that holds the pulley by putting the bar through the ~1cm hole in the force probe. Tighten the screw. With no weight hanging from the probe, click “Collect” and plot the data. If the plot does not read zero, you must set it to zero using the “zero” in the “Experiment” icon menu.

1. Attach the 550 g mass to the hook of the force probe. Record the force reading in item 1 on the next page.

Now remove the force probe from the bar and thread the string through the pulley. Attach the force probe to the string with its hook upwards--it’s now upside down, so you have to check its zero again while you hold it. Then attach the 550g mass to the string on the side near the metal post, while you provide tension in the string by holding tightly the force probe attached to the other side. Be careful that the force probe does not go crashing upward into the pulley. Record (item 2) the force vs. time while the mass is not moving. Answer the question after item 2.

Next, record (item 3) force vs. time for accelerated motion. Start from rest, pull the force probe quickly down about 50 cm and then quickly stop the motion.

Repeat this, in the opposite (upward) direction. Record in item 4.

Now try this for constant velocity. Move the probe downward at a slower, constant speed. Record in item 5.

Repeat, but move the probe upward at a constant speed. Record in item 6.

Names: ______

Record your observations (don’t forget units!) in the spaces here, and answer the questions.

1. Mass at rest connected directly to force probe ______

2. Mass at rest connected directly to string/pulley: ______

Do these observed forces agree with your expectations? Explain. Are the readings equal to the weight of the mass?

3. Sketch the force vs. time plot you obtained for procedure 3. For the speeding up and slowing down sections, draw a force diagram for the 550 g mass with the length of the force vectors (Tension and Gravity) indicating their relative sizes. Also indicate the direction of the motion (v) and the acceleration (a).

4. Sketches from procedure 4, as in item 3.

5. Sketches from procedure 5, as in item 3, but draw the force diagram for the section at constant speed.

6. Sketches from procedure 6, as in item 5.

7. In parts 3 and 4, when the 550 g mass is accelerating, how is the tension related to its weight?

How is the tension related to the weight when the mass is moving at constant speed in parts 5 and 6? Does the magnitude or direction of the velocity make any difference?