# Compression of a spring with friction

Hi again

I try to solve this by my own with little help from the net, but still I'm not sure if it's the correct!

Here is my work, and please someone tell me if I'm wrong, again. (x = 20,8 m ???)

Thanks

P.S. Here is the full problem http://i.minus.com/j8Fbhtd9CJtCQ.jpg [Broken]

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Some parts of the problem text are washed out, and the solution is out of focus. I don't think you want any guesswork on our part, so you should give us something more readable if you need help.

Some parts of the problem text are washed out, and the solution is out of focus. I don't think you want any guesswork on our part, so you should give us something more readable if you need help.

A sledge of mass m=2 kg starts with initial velocity Va=7 m/s at ha=40m. Compute, neglecting friction.
a) the speed Vb at hb=20m

The sledge then travels from C (hc=30m) to D horizontally on a rough surface with uk=0.6.
From D the surface becomes frictionless. In E a spring in equilibrium condition (i.e. neither streched nor compressed) is positioned.
b) Determine the maximum compression of the spring, knowing that CD=20m and the spring constant k=0.2 N/cm.

PE+KE=Wf+PEs

mgh + 1/2 mv2= mgμd + 1/2 kx2
Check for zero PE reference level.
Gravitational force is conservative, only levels are relevant not the path between the levels.

PE+KE=Wf+PEs

mgh + 1/2 mv2= mgμd + 1/2 kx2
Check for zero PE reference level.
Gravitational force is conservative, only levels are relevant not the path between the levels.
So, I take Vc and d as CD?

PE+KE=Wf+PEs

mgh + 1/2 mv2= mgμd + 1/2 kx2
Check for zero PE reference level.
Gravitational force is conservative, only levels are relevant not the path between the levels.
So, for v I take Vc and for d = CD?

CAF123
Gold Member
So, for v I take Vc and for d = CD?
Yes.

Yes.
And then, the final result is x=7.6m if I'm not wrong again?

Check the unit of spring constant k.

CAF123
Gold Member
And then, the final result is x=7.6m if I'm not wrong again?
I get a very similar answer(≈7.7m).

mgh + 0.5mv2=mgμd+0.5ky2
2x9.8x10 +0.5x2x7x7=2x9.8x0.6x20 + 0.5x0.2/0.01 x y2
196+49 =235.2 +10xy2
9.8=10xy2

y2= 0.98
y=1m

CAF123
Gold Member
mgh + 0.5mv2=mgμd+0.5ky2
2x9.8x10 +0.5x2x7x7=2x9.8x0.6x20 + 0.5x0.2/0.01 x y2
196+49 =235.2 +10xy2
9.8=10xy2

y2= 0.98
y=1m
Why is h = 10 m and v = 7m/s? I computed the velocity at C to be 15.7m/s and the motion is at a height of 30m. (I took the reference of zero potential at ground level).

Why is h = 10 m and v = 7m/s? I computed the velocity at C to be 15.7m/s and the motion is at a height of 30m. (I took the reference of zero potential at ground level).
As i mentioned on previous reply, it is a conservative force. It is not he path but the different of height that determines the ΔPE. So the ΔPE is height between A and C.
Conservation of energy. The kinetic energy remain the same. As in projectile, the horizontal velocity remains constant.

From point A to C no energy dissipated.

add: If level A equal to level C, do you need to take zero reference at ground level?

CAF123
Gold Member
As i mentioned on previous reply, it is a conservative force. It is not he path but the different of height that determines the ΔPE. So the ΔPE is height between A and C.
Conservation of energy. The kinetic energy remain the same. As in projectile, the horizontal velocity remains constant.

From point A to C no energy dissipated.

add: If level A equal to level C, do you need to take zero reference at ground level?
Ah of course! But I still don't see why the velocity at C is 7m/s. Can you show this mathematically? I have tried but get values greater than this.

PhanthomJay
Homework Helper
Gold Member
There are too many steps here that cause errors. Eliminate all the intermediate steps. Start at A and end at E. $(KE + PE)_{initial- at- A} + W_f = (KE + PE)_{final- at -E}$
Work done by friction is numerically a negative value.

So, guys, what is the final and also the right result?

PhanthomJay
Homework Helper
Gold Member
Oh heck you have a good understanding of work energy concepts, so it's just math now for the 2nd part and using g = 10 , then

1/2(2)(7)^2 + (2)(10)(40) + 0 - .6(2)(10)(20) = 0 + 2(10)(30) + 1/2(20)(x^2)

From which x = 1 m.

Which looks like what azizlwl got using a slightly different approach between a different set of points. You should confirm this on your own, as there are lots of ways to skin a cat, as they say.

Oh sorry I didn't pay attention to part A I guess that's solved already.

Okay, the part a) is just fine, no problem at all with it...
Many thanks to everybody, I hope the answer of 1m is correct :P

CAF123
Gold Member
Can anyone enlighten me as to why the velocity at C is 7m/s? Many thanks

PhanthomJay
Homework Helper
Gold Member
You should confirm it is correct on your own.....I didn't take the time pouring through your pages of calcs to see why you arrived at a different answer...maybe too many steps and a number got transposed somewhere... or maybe our answer is wrong you know..... Can anyone enlighten me as to why the velocity at C is 7m/s? Many thanks
At point A only energy for the system is kinetic energy with velocity of 7m/s.
From point A to point C, gravity has done work on the sledge equal to mgh.
No energy is taken out from the sledge ONLY added work by gravity.

So at Point C we have original energy PLUS energy added by gravity.

Last edited:
CAF123
Gold Member
You should confirm it is correct on your own.....I didn't take the time pouring through your pages of calcs to see why you arrived at a different answer...maybe too many steps and a number got transposed somewhere... or maybe our answer is wrong you know..... I think you think I am the OP :) I just came in and did the question and azizlwl pointed out my error in the way in which I was forgetting to use potential energy. However, I am still unsure of why the velocity at C is 7m/s. Computing it gives a much larger value of about 25m/s. I computed it like: $$\frac{1}{2}mv_b^2 + mg(h_A - h_B) = \frac{1}{2}mv_c^2 + mg(h_C-h_B),$$ which when using $v_b$= 21m/s yields $v_c$ as ≈25m/s. (reference of potential, 0 at ground)
See any errors?
Thanks

PhanthomJay
Homework Helper
Gold Member
Can anyone enlighten me as to why the velocity at C is 7m/s? Many thanks
The velocity at C is what you got...15 something m/s...velocity at A is 7 m/s......

CAF123
Gold Member
The velocity at C is what you got...15 something m/s...velocity at A is 7 m/s......
$v_c =$ 15.7m/s was what I got when I did my potential energy contributions incorrect. I now get $v_c =$ 25m/s if my equation is correct that I posted in my previous post.

PhanthomJay
I think you think I am the OP :) I just came in and did the question and azizlwl pointed out my error in the way in which I was forgetting to use potential energy. However, I am still unsure of why the velocity at C is 7m/s. Computing it gives a much larger value of about 25m/s. I computed it like: $$\frac{1}{2}mv_b^2 + mg(h_A - h_B) = \frac{1}{2}mv_c^2 + mg(h_C-h_B),$$ which when using $v_b$= 21m/s yields $v_c$ as ≈25m/s. (reference of potential, 0 at ground)