Archived Rotation with String Slipping & Not Slipping

Click For Summary
The discussion revolves around a physics problem involving two blocks and a pulley system, focusing on the calculations of acceleration, tension, angular acceleration, and rotational inertia. The calculated acceleration of the blocks is 0.06 m/s², with tensions T2 and T1 found to be 4.87 N and 4.54 N, respectively. The angular acceleration of the pulley is determined to be 1.2 rad/s², and the rotational inertia is calculated as 0.0139 kgm². The user expresses difficulty in adapting the problem to a scenario where the string slips and the disk does not move, questioning how to calculate the distance block 2 would fall under those conditions. The conversation highlights the complexities of analyzing motion in a pulley system with varying conditions.
Morenzio
Messages
1
Reaction score
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?
 
Physics news on Phys.org
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.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

Similar threads

  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 49 ·
2
Replies
49
Views
7K
Replies
5
Views
20K
  • · Replies 10 ·
Replies
10
Views
5K