A2L Item 060
- Description: Recognize the direction of the net force on a ball on curved vertical track.
- Goal: Hone the vector nature of force and identify all the forces.
- Source: UMPERG
- Keywords: Vectors, Mechanics, Friction, Forces
The question for students:
A small ball is released from rest at position A and rolls down a vertical circular track under the influence of gravity as depicted below.
When the ball reaches position B, which of the indicated directions most nearly corresponds to the direction of the net force on the ball?
Enter (9) if the direction cannot be determined.
Commentary for teachers:
Answer
(9) The net force is the sum of the forces acting on the ball. If the ball rolls along the track there is a normal force, a friction force and a gravitational force being exerted on the ball. Although a best answer can be determined it would require a good understanding of dynamics, energy, and circular motion to achieve and we assume the student is addressing this question before all these elements are in place. [The actual answer is (2) but few students are able to appreciate that without much thought.]
Background
To become adept at identifying forces, students should consider a wide array of situations, even if the situations are too complex for them to fully analyze. To determine the direction of the net force students need to be able to judge the relative sizes of forces.
Questions to Reveal Student Reasoning
What forces are being exerted on the ball? What are the directions of these forces? What are the relative sizes of the different forces?
Suggestions
Consider a block sliding down an incline at 450? How does the block on an incline compare to the ball on the curved track? What are some similarities and differences?