- #1
Bashyboy
- 1,421
- 5
The puzzle is:
"The figure below represents a small, flat puck with mass m = 2.24 kg sliding on a frictionless, horizontal surface. It is held in a circular orbit about a fixed axis by a rod with negligible mass and length R = 1.03 m, pivoted at one end. Initially, the puck has a speed of v = 4.63 m/s. A 1.40-kg ball of putty is dropped vertically onto the the puck from a small distance above it and immediately sticks to the puck.
(a) What is the new period of rotation?
(b) Is the angular momentum of the puck–putty system about the axis of rotation constant in this process?
(c) Is the momentum of the system constant in the process of the putty sticking to the puck?
(d) Is the mechanical energy of the system constant in the process?"
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I was able to solve part (a).
For part (b), even though there is a collision, causing the putty and the puck to exert a force on each other, the forces are balanced, meaning the net force is zero, and the angular momentum is conserved. Is that right? So momentum would be conserved?
For part (c), didn't we sort of answer this question in part (b)?
For part (d), the answer is that mechanical energy is not conserved (constant) in the process. Why isn't it, though? Yes, I admit that the force of gravity pulls on the piece of putty, having it descend into the puck; there is force, but it is not being applied over a distance (work) to cause its state of energy to change--the force isn't even in the same direction as the motion, it is perpendicular to it.
"The figure below represents a small, flat puck with mass m = 2.24 kg sliding on a frictionless, horizontal surface. It is held in a circular orbit about a fixed axis by a rod with negligible mass and length R = 1.03 m, pivoted at one end. Initially, the puck has a speed of v = 4.63 m/s. A 1.40-kg ball of putty is dropped vertically onto the the puck from a small distance above it and immediately sticks to the puck.
(a) What is the new period of rotation?
(b) Is the angular momentum of the puck–putty system about the axis of rotation constant in this process?
(c) Is the momentum of the system constant in the process of the putty sticking to the puck?
(d) Is the mechanical energy of the system constant in the process?"
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I was able to solve part (a).
For part (b), even though there is a collision, causing the putty and the puck to exert a force on each other, the forces are balanced, meaning the net force is zero, and the angular momentum is conserved. Is that right? So momentum would be conserved?
For part (c), didn't we sort of answer this question in part (b)?
For part (d), the answer is that mechanical energy is not conserved (constant) in the process. Why isn't it, though? Yes, I admit that the force of gravity pulls on the piece of putty, having it descend into the puck; there is force, but it is not being applied over a distance (work) to cause its state of energy to change--the force isn't even in the same direction as the motion, it is perpendicular to it.