- #1
Eclair_de_XII
- 1,083
- 91
Homework Statement
"In Fig. 8-40, a block of mass ##m=12kg## is released from rest on a friction-less incline of angle ##\theta=\frac{\pi}{6}##. Below the block is a spring that can be compressed ##\frac{1}{50}m## by a force of ##270 N##. The block momentarily stops when it compresses the spring by ##\frac{11}{200}m##. (a) How far does the block move down the incline from its rest position to this stopping point? (b) What is the speed of the block just as it touches the spring?"
Homework Equations
##U_{spring}=\frac{1}{2}kx^2##
##W=\frac{1}{2}m(v^2-v_0^2)##
The Attempt at a Solution
##m=12kg##
##\theta=\frac{\pi}{6}##
##x=\frac{1}{50}m##
##F=270N##
##s=\frac{11}{200}m##
(a)
To find the distance that the block slides down, ##d+x##, I started out by finding the spring constant ##k##.
##F=-kx##
##k=-\frac{F}{x}=-(50m^{-1})(270N)=13,500\frac{N}{m}##
And then I state the potential energy in the spring and equate it to the gravitational work.
##U_{spring}=\frac{1}{2}ks^2=K_g=F⋅d=[(mg)][(cos(\frac{\pi}{2}-\theta))(d)]##
##d=\frac{sec(\frac{\pi}{2}-\theta)}{2mg}(ks^2)=\frac{10s^2}{(12kg)(98m)}(13,500\frac{N}{m})(\frac{121}{40000}m^2)=(\frac{1s^2}{294m⋅kg})(\frac{27}{16})(121J)=\frac{3267}{4704}m##
Then add x...
##d+x=\frac{606,656}{940,800}m##
I assumed it wanted vertical distance from its position at rest, so the distance I'm looking for is:
##(sin\theta)(d+x)=\frac{606,656}{1,881,600}m≈0.32m##
I feel like I'm misunderstanding this question, though; when I remove the x from the equation, I get the right answer... Please correct me if I'm wrong in assuming that they are looking for the distance the block has covered with or without the distance covered when compressing the spring. I don't feel like I will be ready to move onto part (b) until I have made sure that I have done everything in (a) correctly.
But if anyone was curious, I ended up using the distance part (a) was looking for, and using the SUVAT equation to solve for ##v_f##. I also used it by equating ##W_g=\frac{1}{2}mv^2##. Either way, I ended up with ##v=\sqrt{2g(cos(\frac{\pi}{2}-\theta))(d)}##, and I don't think it's correct.