- #1
danielbaker453
- 26
- 2
Homework Statement
A block of mass 200g is suspended through a vertical spring. The spring is stretched by 1.0 cm when the block is in equilibrium. A particle of mass 120g is dropped on the block from a height of 45 cm. The particle sticks to the block after the impact. Find the maximum extension of the spring. Take g=10 m/s^2.
Homework Equations
Initial Momentum of system = Final Momentum of system
Ui+Ki=Uf+Kf
The Attempt at a Solution
Since the spring is initially in equilibrium,
kxi = Mg
using this we can find the spring constant.
Also, at the point of maximum elongation,
kxf=(M+m)g
using this we can find the maximum elongation.
The answer I get is 1.6 cm which is wrong (the correct answer is 6.1 cm). When I looked at the solution, I saw that conservation of momentum was applied first and then conservation of energy. But, I don't see why the momentum can be conserved. There is a net external force on the system because of gravity.
Could someone explain to me what is wrong with my approach and also why momentum conservation can be applied in this case?
Here is a diagram of this problem I drew