# Homework Help: Block and spring on ramp; find compression

1. Oct 16, 2015

### physics8763

• Member warned to use the formatting template for homework posts.
An inclined plane of angle
θ = 20.0°
has a spring of force constant
k = 525 N/m
fastened securely at the bottom so that the spring is parallel to the surface as shown in the figure below. A block of mass
m = 2.31 kg
is placed on the plane at a distance
d = 0.312 m
from the spring. From this position, the block is projected downward toward the spring with speed
v = 0.750 m/s.
By what distance is the spring compressed when the block momentarily comes to rest?

I used equation 1/2mv^2+mgh=1/2kx^2 and got x=.084, which was incorrect.

2. Oct 16, 2015

### physics8763

Picture of problem

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3. Oct 16, 2015

### SteamKing

Staff Emeritus
Please show your detailed calculations. And don't omit the HW template. That is the rule at PF.

4. Oct 16, 2015

### physics8763

OK, like I said I got x=.084. Basically I have that KE (initial energy) + Gravitational PE is equal to the Work the spring does and solved for x. What did I do wrong?

5. Oct 16, 2015

### physics8763

Most likely there is a bug in the homework grading system. .084 is correct.

6. Oct 16, 2015

### throneoo

what did you use for 'h' ? You wouldn't know how much potential energy is lost unless you've also solved for the distance by which the spring is compressed

7. Oct 16, 2015

### SteamKing

Staff Emeritus
Well, did you include the units in your answer?

8. Oct 16, 2015

### physics8763

Units are already included and I substituted dsin(theta) for h

9. Oct 16, 2015

### physics8763

Maybe there isn't a bug. Hmm I am very confused

10. Oct 16, 2015

### SammyS

Staff Emeritus
Hello physics8763. Welcome to PF.

The equation you show could be used to solve the problem, provided that v, h, and, x are properly defined. (True also for m, g, and k)

Physics is much more than an exercise in "hunting for the magic equation".

That equation suggests that you are trying to solve this problem from a "conservation of energy" point of view.

As they say "The devil is in the details.", and you have given us very few details regarding how your equation translated into an answer of x=.084 . (Whatever x is.)

11. Oct 16, 2015

### throneoo

I don't think h is simply d*sin(theta)

12. Oct 16, 2015

### physics8763

Does anyone know how to do the problem?

13. Oct 16, 2015

### physics8763

I need help

14. Oct 16, 2015

### physics8763

Since all of you don't know how to do it I am going to just look up the answer.

15. Oct 16, 2015

### throneoo

Hint: h is a function of x

16. Oct 16, 2015

### Staff: Mentor

[Moderator's note: Thread title changed to conform to forum rules. Thread titles must be descriptive of the thread subject matter and identify the type of physics that is involved. Vague titles and pleas for help are not allowed]

Remember that as the spring compresses, the block continues to lose height. What does that do to the energy balance?

17. Oct 16, 2015

### SammyS

Staff Emeritus
I'm sure Steamking, gneill and I all know how to do the problem.

Check what the rules are regarding this Forum. We as HomeWork Helpers must also follow those rules. Putting it bluntly, we do not provide solutions. We are here to help you solve the problem.

18. Oct 16, 2015

### Mister T

Which of these is your desire?
2. A lesson in how to find the answer.
3, An understanding of the physics involved in reaching the answer.

We can't help with 1. We will help with 2 but only because the goal is 3.

19. Oct 17, 2015

### physics8763

Needed quadratic formula: (gsintheta+sqrt(((gsintheta)^2)+(k/m)(v^2+2sintheta*d))/(k/m) gives .1238, the correct answer