Rotation with String Slipping & Not Slipping

In summary, the conversation discusses a problem involving two blocks on a horizontal axle with a pulley of negligible friction. Block 2 falls a distance of 75.0 cm in 5.00 s without the cord slipping on the pulley. The summary includes calculations for the magnitude of acceleration of the blocks, the tension of T2, the tension of T1, the magnitude of the pulley's angular acceleration, and the rotational inertia of the pulley. The conversation also mentions difficulties in setting up the same problem if the string slips and the disk does not move.
  • #1
Morenzio
1
0

Homework Statement


Block's 1 (460g) & 2 (500g) are mounted on a horizontal axle of negligible friction (R = 5.00cm). When release from rest, block 2 falls 75.0 cm in 5.00s without the cord slipping on the pulley.

a) What is magnitude of acceleration of blocks?
b) Tension of T2
c) Tension of T1
d) What is magnitude of pulley's angular acceleration?
e) What is its rotational inertia?

Block 1 - 460g
Block 2 - 500g
Radius Axle - 5.00cm
Distance = 75.0 cm
Time = 5.00s

Homework Equations



x = x_0 + v_0 t + (1/2) a t^2

\vec{F}_{net} = \Sigma \vec{F} = m \vec{a}

Can't seem to find other equation I was looking for on site involving (1/2)Mv2 + (1/2)Iω2

The Attempt at a Solution



a) The initial velocity is 0 m/s so plugging in givens into the first relevant equation results:
.75m = (1/2)a(5s)2
a=.06m/s2

b) After drawing a FBD, a is downwards thus:
F = ma
m2g - T2 = m2a
T2 = m2(g - a)
T2 = 4.87N

c) T1 - m1g = m1a
T1 = m1(g + a)
T1 = 4.54N

d) α = a/R = .06m/s2/.05m = 1.2 rad/s2

e) There is a loss of PE as block 2 falls and gain of PE as block 1 rises. The difference can be found:

m2gd = (.5kg)(9.8m/s2)(.75m) = 3.675 J
m1gd = (.46kg)(9.8m/s2)(.75m) = 3.381
Total PE Lost = .294 J

This can be equated to the final relevant equation (well I just assumed I probably am wrong) giving:

.294 J = .5Mv2 + .5Iω2
.294 J = .5(m1+m2)v2 + .5Iω2

However, we need v and we need ω:

v = at = (.06m/s2)(5s) = .3 m/s
ω = v/r = (.3 m/s)/(.05m) = 6 rad/s

Plugging into where I left off:

.294 J = .0432 kgm2/s2 + I(36 rad/s)
.2508 = 18I

Rotational inertia, I = .0139 kgm2

Problems I'm Having
If everything I did above is correct, I'm having trouble setting up this exact same problem if the string slips and the disk does not move. I would need to calculate the distance block 2 falls in 5 seconds. I was just assuming I was over thinking the problem and just think of it as free falling?
 
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  • #2
If the inertia of the pulley is to be ignored, there is a net force (difference in the weights) and a total mass (all undergoing the same acceleration). Or, more pedantically, set the two tensions equal.
 

1. What is the difference between rotation with string slipping and not slipping?

Rotation with string slipping refers to a scenario where the string is able to move freely along the rotating object, causing the object to spin faster than it would if the string was fixed. Rotation with string not slipping, on the other hand, occurs when the string is fixed to the object, causing the object to rotate at a slower rate.

2. How does friction affect rotation with string slipping and not slipping?

Friction plays a crucial role in determining whether an object will rotate with string slipping or not slipping. In the case of string slipping, friction between the string and the object is low, allowing the string to slide and increase the object's rotation speed. In contrast, high friction between the string and the object in string not slipping prevents the string from sliding, resulting in a slower rotation.

3. Can rotation with string slipping and not slipping occur simultaneously?

Yes, it is possible for rotation with string slipping and not slipping to occur simultaneously. This can happen when there is a combination of low and high friction areas along the string's surface, allowing some parts to slip while others remain fixed. In such cases, the object will rotate at an intermediate speed.

4. How does the length of the string affect rotation with string slipping and not slipping?

The length of the string can impact rotation with string slipping and not slipping. A shorter string will have a shorter distance for the object to rotate, resulting in a higher rotation speed. In contrast, a longer string will have a longer distance, and the object will rotate at a slower rate. However, the length of the string does not affect whether the rotation occurs with string slipping or not slipping.

5. What are some real-life examples of rotation with string slipping and not slipping?

One example of rotation with string slipping is a spinning top, where the string is able to slide along the top's surface, causing it to spin faster. An example of rotation with string not slipping is a yo-yo, where the string is fixed to the yo-yo's axle, resulting in a controlled and slower rotation. Other examples could include a merry-go-round (slipping) and a bicycle wheel (not slipping).

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