When two masses are suspended over a pulley, the larger mass will accelerate towards the ground while the smaller accelerates towards the pulley at the same rate. How does this behavior change as we change the masses? What happens if we increase the total mass, but keep the difference between the large mass and small mass the same? Let's construct an Atwood's machine and find out!

Set up your equipment as shown in the image. Attach your pulley to a ring stand so that it hangs over the edge of the table. We'll need to suspend two groups of masses over the pulley from a string. Each side should be able to move at least 30 cm vertically before it strikes the floor or the other mass strikes the pulley. Always stop the masses before this happens.

To get data from our Atwood's machine we'll need to use a sensor. Please plug in your Arduino if you haven't already and attach an encoder pulley to digital pin 2.

In this section we'll start with 200 grams of mass hanging from the left side of the pulley and 180 grams on the right. Then we'll move mass from one side to the other. This way, the total mass in the system won't change, just the distribution. Here's the procedure we'll follow:

First record a run of data:

1. Following the table below, setup the masses for the left and right side.
2. Pull the smaller mass to the floor.
3. Click Start Recording and release the smaller mass.
4. Catch the larger mass before it strikes the floor, and click Stop Recording.

Then we'll analyze the data:

1. On the plot, find a section where the velocity is smoothly sloping upwards and click on the left side of it.
2. Then, click on the right side of the same section. Part of the plot will be marked with L and R icons, and a fit line will be drawn.
3. The dark blue line is the best fit of a line to the segment you've highlighted. Adjust your selection by clicking on either side until the line generally follows the upward shape of the plot.
4. Notice that the Acceleration block changes value as you move your selection. That's because the slope of the velocity plot is the acceleration. (It's a mesaurement of how fast the velocity changes.)
5. Click Save in the table for the row you're gathering data for.
6. Click Clear Data to clear your plot.

Repeat for each row in the table.

In this section we'll start with 200 grams of mass hanging from the left side of the pulley and 180 grams on the right again. But this time, instead of moving mass from one side to the other, we'll add mass to both sides, 20 grams at a time. This way the sides are always 20 grams apart, but the total mass keeps increasing. Here's the procedure we'll follow:

First record a run of data:

1. Following the table below, setup the masses for the left and right side.
2. Pull the smaller mass to the floor.
3. Click Start Recording and release the smaller mass.
4. Catch the larger mass before it strikes the floor, and click Stop Recording.

Then we'll analyze the data:

1. On the plot, find a section where the velocity is smoothly sloping upwards and click on the left side of it.
2. Then, click on the right side of the same section. Part of the plot will be marked with L and R icons, and a fit line will be drawn.
3. The dark blue line is the best fit of a line to the segment you've highlighted. Adjust your selection by clicking on either side until the line generally follows the upward shape of the plot.
4. Notice that the Acceleration block changes value as you move your selection. That's because the slope of the velocity plot is the acceleration. (It's a mesaurement of how fast the velocity changes.)
5. Click Save in the table for the row you're gathering data for.
6. Click Clear Data to clear your plot.

Repeat for each row in the table.