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July 7, 2015

July 7, 2015

Posted by **~christina~** on Tuesday, February 26, 2008 at 10:26pm.

Block 1 of mass 0.200 kg is sliding to the right over a frictionless elevated surface at a speed of 8.00 m / s. The block undergoes an elastic collision with stationary block 2, which is attached to a spring of spring constant 1208.5 N / m. (Assume that the spring does not affect the collision.) After the collision, block 2 oscillates in SHM with a period of 0.140 s, and block 1 slides off the opposite end of the elevated surface landing a distance d from the base of that surface after falling height h = 4.90 m.

I came up with

x(t)= Acos(omega*t + pi/2)

not sure about +/- for the angle though and how you know which sign to have.(need help on this determination)

I found omega and m2 through: T= 2pi/omega= 2pi sqrt(m/k)

m1v1i + m2v2i = m1v1f + m2v2f

and found v2f and v1f

Then I was thinking of using the v1f and v2f in energy equation to find the distance that the spring compresses (Amplitude) so I can plug it into the equation for cos

1/2mv1f + 1/2m2vf = 1/2kx^2

Is this alright?

once again I'm not sure if phi's angle or even if phi is correct.

x(t)= A cos (omega*t + pi/2)

v(t)= -omega A sin( omega*t + pi/2)

a(t)= -omega^2 cos (omega*t + pi/2)

I think I'd just plug into the equation after I find the values from a

I know this is projectile motion problem with I think v in x direction...but if it is then would an angle be included? I think yes but I haven't worked with many problems with a object falling after sliding off a level surface.

how would I approach this?

Thank you drwls =)

- Physics..=> drwls - I need your help -
**drwls**, Wednesday, February 27, 2008 at 12:36ama) The displacement of block 2 will be 0 at t=0, and it will vibrate about this position. They already tell you the period is P = 0.140 s. Figure out tne mass m2 from the relation

P = 2 pi sqrt (m2/k) = 0.140 s

m2/k = [P/(2 pi)]^2 = 4.965*10^-4.

I get m2 = 0.600 kg, 3 times the mass of m1.

You need to compute the amplitude of vibration to complete this part. You can get that by using energy and momentum conservation to compute the velocity of m2 right after collision. I believe you will find this to be 4.00 m/s. Vmax of mass2 equals omega*A, where A is the amplitude. In your case,

omega = 2 pi/P = 44.88 rad/s

Therefore A = (4.00 m/s)/44.88 rad/s) = 8.91*10^-2 m

Another way to get the amplitude is to set (1/2) k A^2 equal to the kinetic energy of m2 right after the collision, which you suggested. It should give the same answer. Try it and see.

The displacement equation for mass 2 is, if I'm right,

X = 8.91*10^-2 m * sin(2 pi t/P)

= 8.91*10^-2 sin (44.88 t)

2) This step is straightforward since you know know X(t) -- assuming I did the calculations correctly. This you need to verify.

3) Yes, just plug in the numbers. Since that is exactly 3 periods later, you should get the same values you had at t=0

4) In doing the elastic collision problem, you should find that mass 1 bpunces back with a velocity of 4.0 m/s. The time it takes to fall a veritcal height of 4.90 m is

t = sqrt (2H/g) = sqrt 1 = 1.00 s

The distance d will therefore be 4.0 m from the base

- Physics..=> drwls - I need your help -
**~christina~**, Wednesday, February 27, 2008 at 1:05amX = 8.91*10^-2 m * sin(2 pi t/P)

= 8.91*10^-2 sin (44.88 t)

**I don't get this...why did you use sin? and how did you get 2pi t/P ??**

I used these formulas below but are they incorrect?

x(t)= A cos (omega*t + pi/2)

v(t)= -omega A sin( omega*t + pi/2)

a(t)= -omega^2 cos (omega*t + pi/2)

P.S.- I was also wondering if there is a good site on the web that can explain the relation of the position of a spring to the sin/cos function since I have problems visualizing the two and which one I should use for which situation.

- Physics..=> drwls - I need your help -
**drwls**, Wednesday, February 27, 2008 at 3:15amI used sin because the displacement at time =0, and then it becomes positive at first.

cos (wt + pi/2) is the same thing as -sin wt, anyway.

Your formula is OK if you define positive motion to be opposite to the direction of m1 before impact.

However, in the first part I think they want you to provide the actual value of A.

2 pi t/P is the same thing as w t, since 1/P is the frequency f and

2 pi f = w

- Physics..=> drwls - I need your help -
**~christina~**, Wednesday, February 27, 2008 at 12:15pmThanks drwls

I was wondering though if I used sin then wouldn't the

v(t)= omega A cos (omega* t)?

and

a(t)= -omega^2 A sin(omega* t)?