Student Exploration: Atwood Machine
Vocabulary: acceleration, Atwood machine, Newton’s second law, pulley, tension, weight
Prior Knowledge Question (Do this BEFORE using the Gizmo.)
Tarzan is standing on a tree branch, high above the forest floor, and he wants to get down to the ground. Jane is standing on the ground and wants to get onto Tarzan’s branch. Tarzan holds a vine that reaches to the ground.
How could Jane get to the branch at the same time that Tarzan travels to the ground?
Tarzan would have to jump down first, so his impact on the ground and his force will cause Jane to go up.
Tarzan could tie the vine to the branch and then slide down after Jane climbed up. But there may be an even easier way—what if Tarzan jumped over the branch while holding the vine, pulling Jane up as he came down?
A similar scenario is shown in the Atwood Machine Gizmo™. An Atwood machine has two weights connected by a rope that passes over a pulley. As one weight moves down, the other will be pulled up. To begin, check that Mass A is 2.0 kg and Mass B is 3.0 kg.
Which mass do you think will move down?
Which mass do you think will move up?
Click Play (). What happens?.
What is the force that pulls mass B downward?.
Up and down
Get the Gizmo ready:
Click Reset ().
Check that the Pulley is Frictional and has a Mass of 2.0 kg and a Radius of 0.20 m.
Set Mass A to 1.0 kg and Mass B to 2.0 kg.
Question: What controls how quickly the two weights on an Atwood machine move?
Predict: How do you think the speed at which the heavier mass descends depends on the weight difference of the two masses?.
Gather data: Click Play. The time it takes for mass B to hit the bottom is shown at bottom right. Record this time, and then repeat for each combination of masses.
Analyze: How does the difference in masses affect the speed at which mass B descends?
Think and discuss: What do you notice about the effect of adding more and more mass? (In other words, does each 1-kg addition of mass have the same effect?)
Predict: Next, you will investigate different mass combinations in which the mass difference is always the same. If the difference in mass is 1 kg, how do you think the total mass will affect how quickly the two objects move?
(Activity A continued on next page)
Activity A (continued from previous page)
Gather data: Record the descent time for each combination of masses.
Analyze: Given the same mass difference, how does the total mass affect how quickly the weights move?.
Think and discuss: Given the same mass difference, why do you think the masses move most quickly when the total mass is smallest?.
Challenge: Given what you have learned so far, what combination of unequal masses will result in the longest time for mass B to reach the bottom?.
Try this with the Gizmo. How long did it take for mass B to reach the ground?