Physics 105

February 10, 2003

Today we will conclude our exploration of velocity by measuring the average velocity of an object using the photo gates The photo gates calculate the velocity of an object by measuring the time required to move the object a fixed distance (the fixed distance is equal to the distance between the leading edges of the of the black bands on the plastic plate if we use one photo gate or the distance between two photo gates if we use one to start the time measurement and another one to stop the time measurement).

When we determine the velocity in this manner we need to consider the impact of the size of the fixed distance used to determine the velocity on the calculated velocity. Under certain circumstances, which we will explore in today's experiment, the calculated velocity depends on the distance used to determine it.

Average and instantaneous velocity:

• Carry out experiment P03 to study the average velocity of your cart as function of the distance between the photo gates Do you understand the variations in average velocity ? Do not change the angle of inclination of the track!
• Put the first photo gate in the center of the track and use the "Velocity - Intro" setup to measure the "instantaneous velocity" at this point using the 12-bands on the top of the five-pattern picket fence. Compare this velocity with the average velocities determine in experiment P03.
• Change the angle of inclination of the track and repeat the previous two measurements.
• Change the angle of inclination of the track again and repeat the first two measurements once more.

The lab report that is due on Monday 2/17 should focus on velocity. The report should contain the following items:

• General discussion on velocity: when it it positive, when is it negative, how do you measure it? Describe the different methods we have used to measure velocity.
• Comparison of two techniques used to measure the velocity of a cart (motion sensor and photo gate). What are the advantages and disadvantages of each technique? Do both techniques give consistent results (note: make sure that you address the questions listed in the "Notes" window).
• Describe the measurements of the average velocity and the instantaneous velocity. What is the difference between these two velocities? Can you use the average velocity to estimate the instantaneous velocity?
• Describe the video analysis technique, and summarize your conclusions of the video analysis of the launch of the space shuttle and the launch of the lunar module.

© Frank L. H. Wolfs, University of Rochester, Rochester, NY 14627, USA

Last updated on Monday, February 10, 2003 10:36