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# i need help ive done this problem 4 times and i don't know what im doing wrong ?

A.)A massless spring with force constant 250 N/m is fastened at its left end to a vertical

wall, as shown below. The acceleration of gravity is 9.8 m/s

2. 7 kg 4 kg 250 N/m Initially, the 7 kg block and 4 kg block rest on a horizontal surface with the 7 kg block in contact with the spring (but not compressing it) and with the 4 kg block in contact with the 7 kg block. The 7 kg block is then moved to the left, compressing the spring a distance of 0.3 m, and held in place while the 4 kg block remains at rest as shown below. 7 kg 4 kg k 0.3 m Determine the elastic energy U stored in the compressed spring. Answer in units of J

B.)The 7 kg block is then released and accelerates

to the right, toward the 4 kg block. The

surface is rough and the coefficient of friction

between each block and the surface is 0.5 . The

two blocks collide, stick together, and move

to the right. Remember that the spring is not

attached to the 7 kg block.

Find the speed of the 7 kg block just before

it collides with the 4 kg block.

Answer in units of m/s.

c.)Find the final speed of both blocks (stuck

together) just after they collide.

Answer in units of m/s.

d.)Find the horizontal distance the blocks move

before coming to rest.

Answer in units of m.

### 1 Answer

- NCSLv 71 month ago
A) This part is easy:

U = ½kx² = ½ * 250N/m * (0.3m)² = 11 J

B) converted into KE:

11J = ½mv² = ½ * 7kg * v²

v = 1.79 m/s

C) conserve momentum:

7kg * 1.7m/s + 0 = 11kg * V

V = 1.14 m/s

D) KE = ½ * 11kg * (1.14m/s)² = 7.15 J

becomes friction work:

7.15 J = 0.5 * 11kg * 9.8m/s² * d

d = 0.13 m

or 13 cm

Hope this helps!

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