# Homework Help: How to calculate work done on spring from object on slope?

1. Oct 9, 2016

### Eclair_de_XII

1. The problem statement, all variables and given/known data
"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?"

2. Relevant equations
$U_{spring}=\frac{1}{2}kx^2$
$W=\frac{1}{2}m(v^2-v_0^2)$

3. 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.

$U_{spring}=\frac{1}{2}ks^2=K_g=F⋅d=[(mg)][(cos(\frac{\pi}{2}-\theta))(d)]$
$d=\frac{sec(\frac{\pi}{2}-\theta)}{2mg}(ks^2)=\frac{10s^2}{(12kg)(98m)}(13,500\frac{N}{m})(\frac{121}{40000}m^2)=(\frac{1s^2}{294m⋅kg})(\frac{27}{16})(121J)=\frac{3267}{4704}m$

$d+x=\frac{606,656}{940,800}m$

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.

2. Oct 9, 2016

### haruspex

How far has the block moved in losing its PE when it comes momentarily to rest?

3. Oct 9, 2016

### Eclair_de_XII

It's moved $(d+x)(sin\theta)$ in terms of vertical distance, I'm guessing?

4. Oct 9, 2016

### haruspex

Right. But you used $mg(d sin\theta)$ for lost PE in post #1, yes?

5. Oct 9, 2016

### Eclair_de_XII

Right. I assumed that gravity stopped doing work when the block touched the spring. So I'm thinking it's actually:

$U_{spring}=(mg)(sin\theta)(d+x)$
$(d+x)=\frac{3267}{4704}m$

And the distance I'm looking for is:

$(sin\theta)(d+x)=\frac{3267}{9408}m$

6. Oct 9, 2016

### Eclair_de_XII

So I'm just going to attempt part (b) now.

$W=(mg)(sin\theta)(d)=\frac{1}{2}mv^2$
$v=\sqrt{2(mg)(sin\theta)(d)}=\sqrt{2(12kg)(\frac{98}{10}\frac{m}{s^2})(sin\frac{\pi}{6})(\frac{606,656}{940,800}m)}=1.77\frac{m}{s}$

Doing this with $v_y^2=v_0^2+2as$ gives me the same equation. I don't know why I'm off by a factor of -0.1.

7. Oct 9, 2016

### haruspex

Your expression for v is dimensionally wrong. You forgot to cancel an m.

8. Oct 9, 2016

### Eclair_de_XII

Right, it's: $v=\sqrt{2(g)(sin\theta)(d)}$. But I didn't really take the mass calculation into account when I evaluated the expression. So I'm still off by 0.1, according to the book.

9. Oct 9, 2016

### haruspex

I suspect you have somewhere used an incorrect value from your original (d instead of d+x) calculation.

10. Oct 9, 2016

### Eclair_de_XII

Okay.

$U_{spring}=\frac{1}{2}kx^2$
$W=F⋅(d+x)=(mg)(sin\theta)(d+x)$
$(mg)(sin\theta)(d+x)=\frac{1}{2}kx^2$
$d+x=\frac{csc\theta}{2mg}kx^2=\frac{2(10s^2)}{2(12kg)(98m)}(13500\frac{N}{m})(\frac{121}{40000}m^2)=\frac{5s^2}{(6kg)(49m)}(\frac{27}{80})(121J)=\frac{5s^2}{294m⋅kg}(\frac{3267}{80}J)=(\frac{3267}{(16)(294)}m)=\frac{3267}{4704}m$
$d=(d+x)-x=(\frac{3267}{4704}m)-(\frac{11}{200}m)=\frac{1}{940,800}(3267⋅200-11⋅4704)=\frac{1}{940,800}(653,400-51,744)m=\frac{601,656}{940,800}m$

So apparently I made a mistake in the numerator; it's $601,656$, not $606,656$.

$\frac{1}{2}mv^2=(mg)(sin\theta)(d)$
$v^2=2(g)(sin\theta)(d)$
$v=\sqrt{2(g)(sin\theta)(d)}=\sqrt{2(\frac{98}{10}\frac{m}{s^2})(sin\frac{\pi}{6})(\frac{601,656}{940,800}m)}$

I also have to point out that in my previous calculation, I neglected to cancel out $sin\theta$ and $2$; so I actually took the square root of half the value of $(g)(d)$.

Last edited: Oct 9, 2016
11. Oct 9, 2016

### haruspex

Your d+x seems to about double what it should be. The expression with the 13500 looks right, but not the next one.
Instead of all that manual cancellation, it is safer just to plug all the numbers into a calculator or spreadsheet.

12. Oct 9, 2016

### Eclair_de_XII

Oh, I see what the problem is. I took out 2 from the denominator twice in that expression. That's why.

$d+x=\frac{3267}{9408}m$
$d=(d+x)-x=\frac{3267}{9408}m-\frac{11}{200}m=\frac{1}{1881600}(653400-103488)m=\frac{549,912}{1,881,600}m$
$v=\sqrt{2g(sin\theta)(d)}=\sqrt{2(\frac{98}{10}\frac{m}{s^2})(sin\frac{\pi}{6})(\frac{549,912}{1,881,600}m)}=1.69\frac{m}{s}$

Thank you for your help, again.