How do you find the amplitude?

1. Jan 22, 2006

nrc_8706

a block of mass 4kg is hung from a spring, causing it to stretch 8cm at equilibrium. the 4kg block is then replaced by a 5kg block. the accleration of gravity is 9.8m/s^2. how far will the 5kg block fall before its direction is reversed?

i dont know if this helps but i found K for the first block

F=-kx
4*9.8=.8k
k=49

i dont know anything else. HELP PLEASE

2. Jan 22, 2006

durt

8 cm is .08 m, not .8 m.
Just find the distance to the from the old equilibrium point to the new one (the amplitude of oscillation) and multiply by two.

3. Jan 22, 2006

nrc_8706

?

how do you find the amplitude?

4. Jan 23, 2006

pseudovector

Use the conservation of energy principal:
In the beggining of the motion, the spring is unstretched, so it has no energy whatsoever, but the mass has potential energy m*g*x, where x is the distance it will fall before coming to a halt.
In the end of the motion, the mass stops falling and is momentarily at rest, so it has neither potential nor kinetic energy. The spring, however, is now stretched by distance x from equilibrium, so the energy it has equals 0.5k*x^2. The energy at the beginning must be equal to the energy at the end, so you can write an equation and solve it to find x.