Physics question needs to be checked out?

I dont know how to get the answer to this.



Suppose it requires 100 dynes to change the length of a certain spring 5 cm. What force is required to change the length of the spring 20 cm?Physics question needs to be checked out?
i suppose you need to assume that the spring obeys Hooke's law: F=kx

where x is the length of the spring minus its unstretched length.

http://en.wikipedia.org/wiki/Hooke's_law

there's a minus sign there, which simply means that if the displacement is positive (spring stretched) then the force is negative (pulling) and if the displacement is negative (spring compressed) then the force is positive (pushing).

this will be consistent with your everyday experience with springs....



the dyne is ';old school';, so convert to Newtons first.

1 dyne = 10^(-5) N

-%26gt; 100 dyne = 10^(-3) N

you don't *need* to convert to dynes for this problem, but it is always good practice to rationalize all quantities to SI units first, so the equations work out nicely.



anyway, from F=k*x, the force is linear in the displacement. and you quadrupled the displacement. so the force quadrupled too.



-%26gt; F = 400 dynes = 4x10^(-3) N



cheersPhysics question needs to be checked out?
You don't need to worry about whether you're dealing in dynes or Newtons beyond keeping track. Remember to use your units in your answer!