2019 #1 Free Response Question - AP Physics 1 - Exam Solution
TLDRIn this engaging physics lesson, the focus is on solving a free-response question from the 2019 AP Physics 1 exam. The scenario involves two blocks on a frictionless surface, where one block is pushed by a plunger and eventually collides with the other. The challenge is to sketch the speed of the center of mass of the two-block system over time. The discussion highlights the importance of understanding Newton's laws of motion and the conservation of momentum and angular momentum. The lesson emphasizes the need for careful graph interpretation and the application of physical principles to solve problems accurately.
Takeaways
- ๐ The task involves solving a free response question from the 2019 AP Physics 1 exam, requiring an understanding of Newton's laws of motion and the concept of center of mass velocity.
- ๐ The problem describes two identical blocks on a frictionless surface, with one block being pushed by a plunger and the other remaining stationary until they collide at point E.
- ๐ Block 1 experiences constant positive acceleration from A to B due to the plunger's force, resulting in a graph with a constant positive slope during this interval.
- ๐๏ธ Upon reaching B, Block 1 moves at a constant velocity from B to C as there is no net external force acting on it, represented by a horizontal line on the graph.
- ๐ From C to D, Block 1 experiences constant negative acceleration due to friction, which is depicted by a line with a constant negative slope.
- ๐ฅ At the collision point E, Block 1 sticks to Block 2, and their combined mass doubles, halving the velocity of the system, which should be reflected on the graph before E.
- ๐ The center of mass velocity of the two-block system remains constant from D to F as there is no net external force or friction acting on the system post-collision.
- ๐ฏ The question emphasizes the importance of careful graph drawing and understanding the underlying physics principles for accurate problem-solving.
- ๐ข The grading for Part A of the problem places significant weight on the correct depiction of the graph, with 5 out of 7 points awarded for accurate graphing.
- ๐ค Part B of the problem introduces a solid sphere and asks when the sphere's angular momentum changes, which occurs only when there is a net torque acting on it.
- ๐ ๏ธ The friction force from C to D is the only force that causes a net torque on the sphere, leading to a change in angular momentum during this interval.
Q & A
What is the scenario described in the transcript?
-The transcript describes a physics problem involving two identical blocks on a horizontal surface, where one block is pushed by a plunger causing various types of motion, including acceleration, constant velocity, and deceleration due to friction. The problem also involves a sphere experiencing similar conditions but with a focus on angular momentum in Part B.
How does the frictionless surface affect the motion of the blocks?
-On the frictionless surface, except for the region between points C and D, the blocks experience zero net external force which results in uniform motion. This means that once the plunger stops pushing block 1, it continues moving at a constant velocity from point B to C.
What happens when block 1 collides with block 2?
-Upon collision, block 1 sticks to block 2, and they move together as a single system. Due to the conservation of linear momentum and identical masses of the blocks, the combined velocity of the two-block system is half the velocity of block 1 just before the collision.
Why does the speed of the center of mass of the two-block system remain constant from point D to F?
-The speed of the center of mass remains constant from point D to F because the combined system experiences no net external force or net torque after the collision. The internal forces between the blocks do not affect the overall center of mass velocity.
What is the significance of the time axis in the problem?
-The time axis is crucial for sketching the graph of the speed of the center of mass of the two-block system as a function of time. It helps in identifying the intervals of acceleration, constant velocity, and deceleration as the blocks move across the surface.
How does the shape of the graph represent the motion of the blocks?
-The shape of the graph, with its lines of varying slopes, represents the different phases of motion: constant positive acceleration (incline), constant velocity (horizontal line), and constant negative acceleration (decline). The graph visually encapsulates the physics of the blocks' motion over time.
What is the key to solving Part A of the problem?
-The key to solving Part A is understanding the physics of the blocks' motion, including the principles of constant acceleration, constant velocity, and the conservation of linear momentum during the collision. Accurately representing these phases on the graph is crucial.
What does the problem in Part B focus on?
-Part B focuses on the conservation of angular momentum. It examines whether the sphere's angular momentum changes when it is subjected to different forces, including the plunger's force, normal force, gravity, and friction.
Which force causes a change in the sphere's angular momentum in Part B?
-The force of friction acting on the sphere from point C to D causes a change in the sphere's angular momentum because it is the only force not acting through the center of mass, thus producing a net torque.
How does the grading of Part A emphasize the importance of graphing?
-Grading Part A highlights that drawing the graph correctly, with accurate representation of the motion phases, is worth the majority of the points. This underscores the importance of being able to visualize and graphically represent physical concepts accurately.
What advice is given for students solving similar problems?
-Students are advised to read problems carefully and slowly, to understand the physics concepts thoroughly, and to draw graphs meticulously. They should focus on the shape of the graph rather than the exact numerical values and remember that internal forces within a system do not affect its overall motion or angular momentum.
Outlines
๐ Physics Problem Solving: Two-Block System
This paragraph introduces a physics problem-solving scenario involving a two-block system. The problem is from the 2019 AP Physics 1 exam, and it describes a situation where two identical blocks are placed on a frictionless surface, except for a rough patch between points C and D. Block 1 is pushed by a mechanical plunger from point A to B, after which it collides with and sticks to block 2 at point E. The combined system continues to move across the surface. The task is to sketch the speed of the center of mass of the two-block system as a function of time from the initial push until the blocks pass point F. The discussion includes the analysis of forces and accelerations experienced by block 1 and the system as a whole, highlighting the importance of understanding Newton's laws of motion and the conservation of momentum in solving the problem.
๐จ Correcting the Graph for the Two-Block System
This paragraph focuses on correcting the initial attempt to sketch the speed of the two-block system's center of mass. It emphasizes the need to consider the shape of the graph rather than specific numerical values. The discussion clarifies that the net force on the system is zero from D to F, even during the collision, leading to a constant velocity for the center of mass in that interval. The paragraph also includes grading insights, noting that drawing lines correctly on the graph is worth a significant portion of the points for this problem. It concludes with a reminder to read the problem statement carefully and to understand that the internal forces between the blocks do not affect the system's overall acceleration or momentum.
๐ Angular Momentum in a Sphere's Motion
The final paragraph shifts focus to a different physics problem involving a solid sphere pushed by a plunger on a horizontal surface. The task is to identify intervals where the sphere's angular momentum changes. The explanation revolves around the concept of net torque being zero for conservation of angular momentum. It is determined that only the friction force acting on the sphere from C to D causes a net torque, as all other forces act through the center of mass and thus do not create a torque. The paragraph concludes with a brief note on the importance of understanding the underlying physics principles rather than just memorizing problem-solving steps.
Mindmap
Keywords
๐กFree Response Question
๐กConstant Horizontal Force
๐กFriction
๐กCenter of Mass
๐กCollision
๐กLinear Momentum
๐กNewton's Laws of Motion
๐กAngular Momentum
๐กNet Torque
๐กGraph
๐กAcceleration
Highlights
The task involves solving a free response question from the 2019 AP Physics 1 exam.
The problem describes two identical blocks on a frictionless surface, except between points C and D.
Block 1 is pushed by a mechanical plunger from point A to B, then continues to move towards block 2.
After colliding, blocks 1 and 2 stick together and move as a single system.
The speed of the center of mass of the two block system is to be sketched as a function of time.
Block 1 experiences constant positive acceleration from A to B due to the plunger's force.
From B to C, block 1 moves at a constant velocity as there is no net external force.
Block 1 experiences constant negative acceleration from C to D due to friction.
The speed of block 1 at time D cannot be zero as it continues moving to the right.
The collision at E results in the speed of block 1 being halved due to conservation of linear momentum.
The graph should show a horizontal line from D to E, indicating no change in the center of mass velocity due to no external force.
The question asks for the speed of the center of mass, not the speed of block 1 alone.
The net force on the two-block system is zero from D to F, resulting in constant center of mass velocity.
The importance of drawing graphs carefully and the impact of incorrect graphing on grading is discussed.
Part B of the problem involves a uniform solid sphere pushed by a plunger and the conservation of angular momentum.
Angular momentum is conserved when the net torque acting on an object equals zero.
The force of friction from C to D is the only force causing a net torque on the sphere.
The explanation for Part B emphasizes understanding the physics concept rather than just answering the question.
The grading notes highlight that a concise explanation is sufficient for Part B, focusing on net torque and its causes.
The session concludes with a reflection on the learning process and an encouragement for careful reading and understanding of problems.
Transcripts
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