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Demetrio Nila - Atwood Machine SE
General Physics (PHYS 2A)
California State University Fresno
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Student Exploration: Atwood Machine
Directions: Follow the instructions to go through the simulation. Respond to the questions and prompts in the orange boxes. 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?
Gizmo Warm-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 masses connected by a rope that passes over a pulley. As one mass moves down, the other will be pulled up. To begin, check that Mass A is 2 kg and Mass B is 3 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?
Activity A:
Up and down
Get the Gizmo ready: ●Click Reset ( ). ●Check that the Pulley is Frictional and has a Mass of 2 kg and a Radius of 0 m. ●Set Mass A to 1 kg and Mass B to 2 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 object descends depends on the difference between 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.
Mass A (kg) Mass B (kg) Time (s) 1 kg 2 kg 1 kg 3 kg 1 kg 4 kg 1 kg 5 kg
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?
Gather data : Record the descent time for each combination of masses.
Mass A (kg) Mass B (kg) Time (s) 1 kg 2 kg 2 kg 3 kg 3 kg 4 kg 4 kg 5 kg
D. Click Play. What happens?
- Calculate : The acceleration of an object is equal to the rate at which its velocity changes. If the initial velocity is zero, the acceleration is equal to the final velocity divided by time ( a = vfinal / tfinal ). Select the TABLE tab and scroll to the bottom.
A. What is the final velocity of mass A?
B. How long did it take for object A to reach the top?
C. What is the acceleration of mass A?
- Calculate : The Atwood machine was designed to demonstrate Newton’s second law , which states that force ( F ) is equal to the product of mass ( m ) and acceleration ( a ): F = ma. Because they are connected, you can treat both masses as part of the same system. A. Select the DESCRIPTION tab. What is the total mass of masses A and B?
B. What is the difference between the gravitational force on B and on A?
C. Based on Newton’s second law, what is the acceleration of each mass?
- Practice : Each mass is pulling the rope in the opposite direction with a force equal to its weight. Therefore, the net force is equal to the weight difference between mass A and mass B. For each combination of masses, calculate the total mass, net force, and acceleration.
Next, measure the final velocity, time, and acceleration for each combination using the Gizmo. Include all units.
Mass A Mass B
Total mass
Net force
Calculated acceleration
Final velocity Time
Measured acceleration 1 kg 3 kg 2 kg 3 kg 4 kg 5 kg
1 kg 5 kg
- Analyze : How well did your calculated values for acceleration match up with the measured values? What
are the possible sources of error?
- Explore : Click Reset. Select the BAR CHART tab and turn on Show numerical values. Set Mass A and Mass B to 2 kg. Click Play. On the SIMULATION pane you will notice that, while the objects do not move, a small Pull button appears below mass A. Click this button to apply an instantaneous force on mass A. A. What are the velocities of masses A and B?
B. Are masses A and B accelerating? Explain how you know.
- Think and discuss : Suppose you were designing an elevator. How could you use the concept of an Atwood machine to minimize the force required to move the elevator up and down? If possible, discuss your answer with your classmates and teacher.
Demetrio Nila - Atwood Machine SE
Course: General Physics (PHYS 2A)
University: California State University Fresno
