A2L Item 183
- Description: Identify the orientation for which a two block and spring system will jump highest.
- Goal: Reason with impulse and energy
- Source: CT151.2S02-46
- Keywords: Dynamics, Impulse, Mechanics, Reasoning, Springs
The question for students:
Two
blocks are connected to the ends of a spring as shown. Assume that the
mass is proportional to the size of the block. The spring is compressed
(same amount) and released suddenly. In which orientation will the
system achieve the largest height?
- A
- B
- both go to the same height
- cannot be determined
Commentary for teachers:
Answer
(2) This is a very rich problem for reasoning. It IS possible for students to reason to the correct solution if they consider appropriate concepts. To help them along suggest the following: Draw free body diagrams for each of the masses separately. Combine them to get a valid free body diagram for the system. Such a process reveals that the normal force is responsible for the impulse causing the system to jump. The spring force is internal to the system and does not appear on the system’s free body diagram.
Students can deduce the answer using analogy or experience. Pogo sticks or even the human body are analogous systems.