Block Sliding on a Level Plane

Names_____________________________________________  Date______________

This lab activity allows you to better understand the relationships between forces acting on bodies at rest and in motion. You will basically investigate the nature of these forces and the resulting acceleration. It is one of a classic set of situations to which we apply Newton's Laws of Motion.

Here is the setup:

Let M1 be the mass of the block on the table. Actually, you'll be using an air track to do this. Let M2 be the mass of the object that falls vertically. Assuming no friction in the system, you can calculate the acceleration of the blocks. remember, strings pull. They don't compress or stretch (in theory anyway) and in a system with a massless, frictionless pulley, the force is transmitted undiminished throughout the string. Using the air track the end acts like a massless, frictionless pulley. You can perform the calculation in a straight forward manner:

        For Block 1:    

S F =  M1 a  = T

For Block 2:M2g

S F =  -M2 a  = T - M2g

Solving for acceleration we get:         a =  M2 g / (M 1 +  M2 )

What you will basically do is set up the air track and make it level. Recall how to do this from your previous lab. Check it at several places along the track. That way you know the acceleration is due to the falling mass and not to any incline of the air track. Instead of a pulley with a wheel we use an air cushioned "pulley." It works fine if you use a piece of recording tape from a cassette that has gone bad. Your instructor will supply some for you.

You will use a new timing device that has several marks on it. Your instructor will show you how it works. It can readily measure time, speed and acceleration electronically. This will be better than the flags we used in the last lab. What kind of uncertainty will you use? Why? You need to make this clear to the reader of your report.

Release the glider from rest at a marked point and allow it to move towards the timer. You can set the timer to measure speed and also use it to measure acceleration. It is recommended to use it to directly measure acceleration (during the short period while the glider passes the timer.) The acceleration should be uniform all along the track. Be sure to estimate uncertainties for this quantity.

Your lab report should include a thorough discussion of what you did all along the way and show the reader (mom or whoever) what you did and explain why you did that. The important this is to compare measurement with theory. i.e., you need to compare the calculated value of acceleration with the measured value. Your discussion should address the differences and why you might expect them to be that way or not. Cool!