# 2015 AP Physics 1 free response 1c | Summary and Q&A

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April 5, 2016
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2015 AP Physics 1 free response 1c

## TL;DR

Adding mass to a system reduces the acceleration because the same net force is now acting on a larger total mass.

## Key Insights

• 💆 Adding mass to a system reduces the acceleration because the same net force is divided among a larger total mass.
• 💱 Internal forces, such as tensions in the strings, change when additional masses are introduced.
• 🪐 Friction between a block and a surface does not affect the net force and acceleration calculations.
• 🥶 Free-body diagrams provide a visual representation of the forces acting on each block in the system.
• 👮 Newton's second law can be applied to determine net forces and accelerations in different directions.
• 🏋️ The difference in weights between the blocks is the main contributor to the net force in the system.
• 💆 The magnitude of the acceleration is inversely proportional to the total mass of the system.

## Transcript

• [Voiceover] Let's now tackle part C. So they tell us block 3 of mass m sub 3, so that's right over here, is added to the system as shown below. There is no friction between block 3 and the table. Alright, indicate whether the magnitude of the acceleration of block 2 is now larger, smaller, or the same as in the original two-block system. Explain ... Read More

### Q: How does adding a block of mass affect the acceleration in a system without friction?

Adding mass to the system reduces the acceleration because the same net force is now acting on a larger total mass. According to Newton's second law, acceleration is equal to force divided by mass. When the net force remains constant but the total mass increases, the acceleration decreases.

### Q: Why do the internal forces change when a block of mass is added to the system?

The internal forces in the system change because there are now two different tensions in the strings. The tension in the second string connected to block 2 is larger than the tension in the first string, causing a difference in forces and resulting in acceleration in different directions.

### Q: Why does the presence of friction between block 3 and the table not affect the acceleration?

Friction between block 3 and the table does not affect the acceleration because it is not part of the net force calculation. Friction is an internal force and cancels out when calculating the net force. The only forces considered for acceleration are the difference in weights between the blocks and the tensions in the strings.

### Q: How can a free-body diagram be used to analyze the forces in the system?

A free-body diagram helps visualize the forces acting on each block in the system. It includes the force of gravity, tensions in the strings, and the weight of each block. By applying Newton's second law to each block and considering the directions of acceleration, the net forces and magnitudes of acceleration can be determined.

## Summary & Key Takeaways

• Adding a block of mass to a system without friction reduces the acceleration of block 2 compared to the original two-block system.

• The net forces on the system remain the same, but the internal forces change due to the addition of the extra block and the presence of two different tensions in the strings.

• The acceleration is smaller because the added mass increases the total mass that needs to be accelerated by the same net force.