How to Solve Vertical Spring and Pulley Homework Problems?

[Vxismyname]

I have two homework problems i need help with one is:
1) A 4kg load is hanging on the bottom of a vertically positioned spring that stretches 10 cm down. Find K of the sping.

2) There are three blocks connected by a string. m1 is 10 kg, m2 is 20kg.
m1, and m2 are connected at the top of a surface and m3=1kg, is hanging of the end of a pulley. looks kinda like this:

[ m1 ]----[ m2 ]-----O
xxxxxxxxxxxxxxxxxxxxx|
xxxxxxxxxxxxxxxxxxxxx|
xxxxxxxxxxxxxxxxxxx [m3]

a) find the acceleration of the system.
b) the tension in the two strings

 

[OlderDan]

I have two homework problems i need help with one is:
1) A 4kg load is hanging on the bottom of a vertically positioned spring that stretches 10 cm down. Find K of the sping.

2) There are three blocks connected by a string. m1 is 10 kg, m2 is 20kg.
m1, and m2 are connected at the top of a surface and m3=1kg, is hanging of the end of a pulley. looks kinda like this:

[ m1 ]----[ m2 ]-----O
xxxxxxxxxxxxxxxxxxxxx|
xxxxxxxxxxxxxxxxxxxxx|
xxxxxxxxxxxxxxxxxxx [m3]

a) find the acceleration of the system.
b) the tension in the two strings

The first of these is a direct application of Hooke's Law. The restoring force of a spring is proportional to the amount it is stretched.

For your second problem, call the tensions in the strings T12, and T23. Use Newton's second law to write an F = ma equation for each of the three masses in the problem. This will give you three equations. Note that all the masses must have the same magnitude of acceleration, and that the motion will be in a clockwise sense. You will have three equations for three unknowns: T12, T23 and a. See if you can set up and solve those three equations.

 

[Vxismyname]

im sorry i must be a complete idiot but i really cannot figure this out. I find that F= -kx. So For the first one do i multiply the 4kg weight by 9.8 to get Newtons and then divide by 10cm but convert that to meters so .1 meters. So 4kg (9.8) = 39.2 / .1 = 392 N/m is -k?

 

[supersix2]

Yes that's correct but use -9.8 for gravity so k becomes a positive number that way -k is actually a negative value. Understand?

 

[Vxismyname]

ok i think i got that last one thank you, as for my other problem i haven't a clue to figure it. For Tension do i just put T= (m1m2m3/m1+m2+m3)g? and what do i do to find acceleration?