Pulleys
Summary: The basic consideration in making useful mechanical arrangements are two folds (i) improve the convenience of applying force and (ii) reduce the magnitude of force required.
A pulley is a part of a convenient arrangement that makes it possible to transfer force with change of direction. Unless otherwise stated, a pulley is considered to have negligible mass and friction. This is a relative approximation with respect to mass and friction involved with other elements. Pulleys are used in different combination with other elements – almost always with strings and blocks.
It is relatively difficult to fetch a bucket of water from a well with a “string” as compared to a “pulley and string” system. The basic consideration in making useful mechanical arrangements are two folds (i) improve the convenience of applying force and (ii) reduce the magnitude of force. The example of fetching of a bucket of water with “pulley and string” achieves the goal of improving convenience as we find it easier applying force in the level of arms horizontally rather than applying force vertically.
Had it been possible to reduce force for doing a mechanical activity, then that would have been wonder and of course against the well founded tenets of physical laws. What is meant here by reducing force is that we can fulfill a task (which comprises of force and motion) by reducing force at the expense of extending motion.
The important characterizing aspects of pulley are discussed in the sections named :
- Static or fixed pulley
- Moving pulley
- Combination or Multiple pulley system
Static pulley
The pulley is fixed to a frame. In this situation, we are only concerned with the accelerations of the bodies connected to the string that passes over the pulley. Since string is a single piece inextensible element, the accelerations of the bodies attached to it are same.
We are at liberty to choose the direction of acceleration of the blocks attached to the pulley. A wrong choice will be revealed by the sign of acceleration that we get after solving equations. However, it is a straight forward choice here as it is obvious that the bigger mass will pull the blocks - string system down.
Example 1
Free body diagram of body of mass 10 kg
Free body diagram of body of mass 10 kg
∑
F
y
=
T
-
10
g
=
10
a
y
⇒
T
-
10
x
10
=
10
a
⇒
T
=
100
+
10
a
∑
F
y
=
T
-
10
g
=
10
a
y
⇒
T
-
10
x
10
=
10
a
⇒
T
=
100
+
10
a
Free body diagram of body of mass 20 kg
Free body diagram of body of mass 20 kg
∑
F
y
=
20
g
-
T
=
20
a
y
⇒
20
x
10
-
T
=
20
a
⇒
T
=
200
-
20
a
∑
F
y
=
20
g
-
T
=
20
a
y
⇒
20
x
10
-
T
=
20
a
⇒
T
=
200
-
20
a
⇒
200
-
20
a
=
100
+
10
a
⇒
a
=
100
30
=
3.33
m
/
s
2
⇒
200
-
20
a
=
100
+
10
a
⇒
a
=
100
30
=
3.33
m
/
s
2
Problem : Two blocks of masses 10 kg and 20 kg are connected by a string that passes over a pulley as shown in the figure. Neglecting friction between surfaces, find acceleration of the blocks and tension in the string (consider
g
=
10
m
/
s
2
g = 10 m /
s
2
Static pulley system  Figure 1 |
Solution : The blocks are connected by a taut string. Hence, their accelerations are same. Let us assume directions of accelerations as shown in the figure. Also, let the magnitude of accelerations be “a”.
Static pulley system  Figure 2 |
The external forces are (i) weight of block, 10g, and (ii) tension, T, in the string.
Free body diagram  Figure 3 |
The external forces are (i) weight of block, 20g, and (ii) tension, T, in the string.
Free body diagram  Figure 4 |
From two equations, we have :
There is a very useful technique to simplify the solution involving "mass-less" string and pulley. As string has no mass, the motion of the block-string system can be considered to be the motion of a system comprising of two blocks, which are pulled down by a net force in the direction of acceleration.
Let us consider two blocks of mass "
m
1
m
1
m
2
m
2
m
2
>
m
1
m
2
>
m
1
m
2
m
2
m
1
m
1
Now the force pulling the system in the direction of acceleration is :
The total mass of the system is :
Applying law of motion, the acceleration of the system is :
Clearly, this method to find acceleration is valid when the block - string system can be combined i.e. accelerations of the constituents of the system are same.
We can check the efficacy of this technique, using the data of previous example. Here,
The acceleration of the block is :
Moving pulley
Moving pulley differs to static pulley in one important respect. The displacement of pulley and block, which is attached to the string passing over it may not be same. As such, we need to verify this aspect while applying force law. The point is brought out with clarity in the illustration explained here.
We consider a block attached to a string, which passes over a mass-less pulley. The string is fixed at one end and the Pulley is pulled by a force in horizontal direction as shown in the figure.
Moving pulley system  Figure 5 |
In order to understand the relation of displacements, we visualize that pulley has moved a distance “x” to its right. The new positions of pulley and block are as shown in the figure. To analyze the situation, we use the fact that the length of string remains same in two situations. Now,
Moving pulley system  Figure 6 |
Length of the string, L, in two situations are given as :
L = AB + BC = AB + BB’ + B’C’
Therefore,
B’C’ = BC – BB’
Now, displacement of the block is :
C’C = B’C – B’C’
⇒ C’C = B’C – (BC – BB’) = B’C –BC + BB’ = 2 BB’
If the displacement of pulley is x, the displacement of block is 2x,. Further, as acceleration is second derivative of displacement with respect to time, the relation between acceleration of the block (
a
B
a
B
a
P
a
P
This is an important result that needs some explanation. It had always been emphasized that the acceleration of a taut string is always same through out its body. Each point of a string is expected to have same velocity and acceleration! What happened here ? One end is fixed, while other end is moving with acceleration. There is, in fact, no anomaly. Simply, the acceleration of the pulley is also reflected in the motion of the loose end of the string as they are in contact and that the motion of the string is affected by the motion of the pulley.
But the point is made. The accelerations of two ends of a string need not be same, when in contact with a moving body.
Example 2
Free body diagram of pulley
Free body diagram of pulley
∑
F
x
=
F
-
2
T
=
m
p
a
p
∑
F
x
=
F
-
2
T
=
m
p
a
p
⇒
F
=
2
T
⇒
T
=
F
2
⇒
F
=
2
T
⇒
T
=
F
2
∑
F
x
=
T
=
m
a
x
=
2
m
a
∑
F
x
=
T
=
m
a
x
=
2
m
a
⇒
a
=
F
4
m
⇒
a
=
F
4
m
Problem : A block of mass, “m” is connected to a string, which passes over a smooth pulley as shown in the figure. If a force “F” acts in horizontal direction, find the accelerations of the pulley and block.
| Moving pulley system Figure 7 |
Solution : Let us consider that the acceleration of pulley is “a” in the direction of applied force. Now as analyzed before, acceleration of the block is “2a” and is in the same direction as that of pulley.
Moving pulley system  Figure 8 |
As motion is confined to x-direction, we draw free body diagram considering forces in x-direction only.
The external forces are (i) force, F, (ii) tension, T, in the string and (ii) tension, T, in the string.
Free body diagram  Figure 9 |
As mass of the pulley is zero,
This is an unexpected result. The pulley is actually accelerated, but the forces on it form a balanced force system. This apparent contradiction of force law is due to our approximation that pulley is “mass-less”. Look at the force analysis above. The equality of forces in two opposite directions results, because we evaluate force equation for the condition of m = 0. Had it not been so, then the forces will not form a balanced force system.
Free body diagram of block
Free body diagram of block
The external force is (i) tension, T, in the string.
Free body diagram  Figure 10 |
Combining two equations, we have :
Thus, acceleration of the pulley is F/4m and that of block is F/2m.
Combination or Multiple pulley system
Multiple pulleys may involve combination of both static and moving pulleys. This may involve combining characterizing aspects of two systems.
Let us consider one such system as shown in the figure.
Multiple pulley system  Figure 11 |
Let us consider that accelerations of the blocks are as shown in the figure. It is important to note that we have the freedom to designate direction of acceleration without referring to any other consideration. Here, we consider all accelerations in downward direction.
Multiple pulley system  Figure 12 |
We observe that for given masses, there are five unknowns
a
1
,
a
2
,
a
3
,
T
1
and
T
2
a
1
,
a
2
,
a
3
,
T
1
and
T
2
However, we can add two additional equations; one that relates three accelerations and the one that relates tensions in the two strings. Ultimately, we get five equations for five unknown quantities.
1: Accelerations
The pulley “A” is static. The accelerations of block 1 and pulley “B” are, therefore, same. The pulley “B”, however, is moving. Therefore, the accelerations of blocks 2 and 3 may not be same as discussed for the case of moving pulley.
We need to know the relation among accelerations of block 1, 2 and 3. In order to obtain this relation, we first establish the relation among the positions of moving blocks and pulley. Then we can obtain relation among accelerations by taking second differentiation of position with respect to time. Let the positions be determined from a horizontal datum drawn through the static pulley as shown in the figure.
Multiple pulley system  Figure 13 |
Now, the lengths of the two strings are constant. Let they be
L
1
and
L
2
L
1
and
L
2
Eliminating
x
0
x
0
This is the needed relation for the positions. We know that acceleration is second derivative of position with respect to time. Hence,
This gives the relation of accelerations involved in the pulley system.
2: Tensions in the strings
We can, now, find the relation between tensions in two strings by considering the free body diagram of pulley “B” as shown in the figure.
Free body diagram  Figure 14 |
Here,
Note:
This result appears to be simple and on expected line. But it is not so. Note that pulley "B" itself is accelerated. The result, on the other hand, is exactly same as for a balanced force system. In fact this equality of forces in opposite direction is possible, because we have considered that pulley has negligible mass. This aspect has been demonstrated in the force analysis of the example given earlier (you may go through the example again if you have missed the point).
3: Free body diagrams of the blocks
The free body diagrams of the blocks are as shown in the figure.
Free body diagram  Figure 15 |
Thus, we have altogether 5 equations for 5 unknowns.
There is one important aspect of the motions of blocks of mass "
m
2
m
2
m
2
m
2
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Last edited by Sunil Kumar Singh on Jul 10, 2007 5:35 am GMT-5.