How much easier is it to lift a heavy object using a pulley system? Demonstrate the effectiveness of a block and tackle pulley system using this simple human pulley system.
- Simple machines
- Mechanical advantage
PVC pipe handle, 1" dia., 20" long*
PVC pipe handle with center hole, 1" dia., 8" long*
PVC pipe handle with center hole, 1" dia., 20" long*
Rope, ¼" dia., 20 feet*
Student volunteers, 3
*Materials included in kit.
Please follow normal laboratory safety guidelines. Wear cloth gloves when holding the longer handles to avoid rope abrasions. Clear out the demonstration area to remove objects that could be tripped over. Do not jerk on the rope. Pull the rope gently with an even force.
- Insert one end of the rope through the hole in one of the 20"-long PVC pipe handles.
- Wrap the end of the rope around the PVC handle two times and then securely tie the rope to the PVC pipe with a double (“square”) knot.
- Insert the free end of the rope through the hole in the 8"-long PVC pipe handle.
- Wrap the end of the rope around the PVC handle two times and then securely ties the rope to the PVC pipe with a double (“square”) knot.
- Select three student volunteers.
- Assign two volunteers as the “block and tackle,” or “handle” holders, and the other as the rope puller. The handle holders will hold the two 20" PVC pipe handles parallel to each other to act as “block and tackle.” The rope pullers will pull on the 8"-long PVC pipe handle.
- Have the two handle holders wear gloves and stand about four feet apart and extend their arms to hold the handles parallel to the floor at shoulder level.
- Wrap the rope around the middle of the free handle (see Figure 1) and give the free end of the rope to the rope puller.
The rope puller should stand behind and slightly to the side of one of the handle holders so the rope will be pulled perpendicular to the length of the “block and tackle” handles. The free end of the rope should go under the arms of the handle holder so that the rope is pulled parallel to the ground as well (see Figure 2).
- Have the two handle holders try as hard as they can to prevent the handles from coming together as the rope puller pulls on the rope. Can the single rope puller draw the two handle holders together?
- Repeat steps 4 and 5 several times. For each new trial wrap the rope around the handles a different number of times (see Figure 1). How much more difficult is it for the holders with each new trial? How much easier is it for the puller? What is the mechanical advantage of the puller as a new loop is added to the pulley system? How close are the handle holders drawn together compared to the amount of rope pulled by the rope puller during each new trial?
- The purpose of this activity is to show how to increase the mechanical advantage of a pulley, not a contest of strength. Pulling too vigorously on the rope or jerking back on the handles may result in injury. The rope puller should take care to pull the rope evenly and straight, and avoid rubbing the rope against the hands of the other two volunteers. As a safety precaution, the volunteer handle holders may wear cloth gloves, available from Flinn Scientific (Catalog No. SE1030) to avoid abrasions from the rope.
- To enhance the effect of mechanical advantage use two “strong” volunteers to hold the handles and a “weak” volunteer to pull the rope.
- Use one large student and one small student as handle holders and have them stand on a slippery floor (or just in their socks on the classroom floor). When they are pulled together, the smaller student moves much more than the large student as both students are pulled toward the center of mass of the pulley system.
- The rope has been seared at the ends to prevent fraying. If the rope ends begin to fray, use a match to burn and melt the ends as necessary.
Answers to Questions
- What is the mechanical advantage of the first pulley system?
This pulley system has a mechanical advantage of two. Some students may say one because there is only one rope loop. (Since both “pulleys” move, it is actually closer to 1.5 so answers of one or two should be considered correct.)
- For the first pulley system, how difficult is it for the puller to bring the PVC-handle holders together?
The rope puller appears to have a difficult time pulling the “pulleys” together. The individuals holding the PVC handles do not move very much as the puller struggles to pull them together.
- As more rope loops are added to the pulley system, does the mechanical advantage for the rope puller increase or decrease? What observations help you form this conclusion?
The mechanical advantage of the system increases. As more loops are added, the roper puller has a much easier time pulling the handles together. The handle holders have a much more difficult time standing in place and are pulled together a short distance.
- What is major disadvantage for the roper puller as more rope loops are added to the pulley system?
As more rope loops are added to the system, the puller has the advantage of being able to pull the handles together now, whereas when only one rope loop was used it was a real struggle. But, the rope puller must pull the rope a much farther distance while the handles move together only a small distance.
Pulleys are used extensively when heavy objects need to be lifted, especially by cranes used in shipping and construction areas. Pulleys are one of six types of simple machines used to easily change the direction and/or the magnitude of an applied force. (The lever and fulcrum, inclined plane, wheel and axle, wedge, and screw are the five other types of simple machines.) How does a pulley decrease the amount of force necessary to lift an object? The advantage of a pulley is its ability to change the number of “ropes” lifting an object. This gives a lifter a greater mechanical advantage. Mechanical advantage is a ratio of the output force compared to the input force. The greater the mechanical advantage is for a system, the greater the output force is compared to the input force. The greater the mechanical advantage, the easier it is to do the work. For a block and tackle pulley system, the mechanical advantage is determined by the number of support ropes that are lifting the object (see Figure 3). Therefore, the more times the rope is wrapped around the handles, the greater the mechanical advantage is for the puller.
However, a pulley does not give something for nothing. A block and tackle pulley system gives a high mechanical advantage, but the sacrifice is that the applied force must be carried over a longer distance compared to the distance the lifted object actually moves. Ideally, due to the conservation of energy, the work in must be equal to the work out. Work is defined as a force times a distance. Therefore, even though a pulley (or any simple machine) makes it easier to lift a heavy object, the total amount of work necessary to lift the object will be equal. A smaller force will be used over a larger distance in order to lift a heavy object a short distance.
Bilash, B. A Demo A Day™—A Year of Physical Science Demonstrations; Flinn Scientific: Batavia, IL, 1997; p 262.