Pressure Mat

Introduction

How much force does the atmosphere actually exert? Use atmospheric pressure to pick up a lab stool and make a deceptively ordinary rubber mat nearly impossible to lift.

Concepts

• Force
• Surface area
• Pressure
• Atmospheric pressure

Background

Pressure is not the same as force. A force is a push or pull that one body exerts on another. When two objects are in contact with each other, an equal and opposite force is exerted between the two objects at the points of contact. The contact points can have large or small surface area, but the force will be the same no matter how large or small the surface. A force on a given surface area results in a pressure on that surface. Pressure is a force per unit area. Therefore, the larger the surface area is for a given

P = Pressure
F = Force
A = Area

Earth’s atmosphere exerts a pressure on everything within it (due to the force from the rapidly moving air molecules colliding with objects within the atmosphere). At sea level, atmospheric pressure is 101,325 Pascal (the SI unit of pressure, 1 Pa = 1 newton per square meter). This is equal to 1 atmosphere, 760 mm of mercury (Hg), or 14.7 pounds per square inch (lb/in²). For every square inch of surface at sea level, the atmospheric molecules exert a force of 14.7 pounds.

The dimensions of the Pressure Mat are 12" x 12", or 144 in². Therefore, the amount of atmospheric force that is exerted on this mat (at sea level) is approximately 2100 pounds! Suppose the mat is laid flat on a tabletop such that the mat makes an air-tight seal and prevents air from entering this space. When the mat is pulled up by the handle, a partial vacuum is created between the mat and the tabletop. A partial vacuum exerts less force on the underside of the mat compared to the force of the atmosphere on the outer surface. The atmospheric pressure pushes down on the Pressure Mat as it is pulled up due to this difference in pressure (and force). This makes it difficult for the Pressure Mat to be lifted off the table. By lifting one corner of the Pressure Mat, the air-tight seal is easily broken allowing air to rush in between the mat and the tabletop, equalizing the pressure and allowing the Pressure Mat to be removed from the tabletop.

Materials

Safety Precautions

The materials in this demonstration are considered safe. Do not attempt to pick up objects that are more than 10 pounds. Do not pick objects up too far off the floor or tabletop. The object could suddenly fall to the ground if the mat’s seal is broken.

Disposal

The materials should be saved and stored for future use. Store the rubber mat in a cool, dry location.

Procedure

• Demonstration 1
  1. Before class, place the pressure mat on a smooth tabletop. Smooth out the rubber mat, if necessary, to remove any trapped air between the mat and the tabletop.
  2. During class, select a volunteer to attempt to pick the pressure mat up by pulling the handle. (The student should experience difficulty and may not be able to lift it at all.)
  3. Once the student returns to his or her seat, pick the mat up by one corner and show how easy it “should” be to pick up the mat.
  4. Place the mat down again and select another volunteer to repeat step 2.
  5. Select as many volunteers as necessary to illustrate the principles of force, pressure, surface area and atmospheric pressure.
• Demonstration 2
  1. Position a lab stool with a flat seat in the center of the demonstration area.
  2. Place the pressure mat in the center of the flat seat.
  3. Press down on the mat slightly and then in a continuous motion, pull up on the pressure mat. (The pressure mat should be “stuck” to the lab stool and the lab stool will rise up with the pressure mat.) Caution: Do not pick the lab stool up more than 30–50 cm. A sudden seal breakdown will “release” the lab stool and it will come crashing to the floor.
  4. Repeat this demonstration as often as necessary to illustrate the principles of pressure, surface area and atmospheric pressure.

Student Worksheet PDF

13298_Student.pdf

Teacher Tips

• The pressure mat can be used indefinitely. Do not attempt to pick up objects weighing more than 10 pounds. This could result in a tear of the rubber mat.
• The first demonstration works best on a smooth, clean tabletop. Test the surface of the tabletop with the pressure mat to determine its sealing ability with the rubber mat. Wash the tabletop with soap and water to remove any dirt or dust, if necessary.
• A lab stool with a flat seat and without a cushion works best for the second demonstration. Make sure the seat is clean and large enough to fit thepressure mat. The Wooden Laboratory Stool  (Catalog no. AP6208) works well with this demonstration and is available from Flinn Scientific.
• Experiment with other objects that can be lifted. Try large, flat and smooth sheets of wood or plastic. Make sure the materials do not weigh over 10 pounds.
• As with any rubber product, the pressure mat will decompose and degrade over time. The rubber is very high grade and should provide years of use. When cracks begin to appear, the pressure mat may no longer resist strenuous pulling. To prolong the life of the rubber mat, store in a large zipper-lock bag.
• Additional pressure demonstrations that are available from Flinn Scientific include: the Surface Tension Jar (AP6648), the Bottomless Bottle—A Demonstration of Pascal’s Law (AP6643), Bed of Nails (AP6769), Pressure Paradox (AP6301), Breaking Board Paradox (AP4713) and the Collapsing Can Demonstration (AP4695).

Answers to Questions

1. Define the terms force and pressure.

   A force is the actual push or pull that is caused by interactions of different objects. Pressure is produced by force acting on a given surface area—it is the distribution of the force over the surface area. Pressure is the ratio of the force to the given surface area on which the force acts.

2. Explain the science involved in the Pressure Mat demonstration using terms such as force, pressure and atmospheric pressure.

   See Background information.

3. Circle the correct answer.
   1. Increasing the surface area for a given force increases/decreases the pressure.
   2. Increasing the force on a given surface area increases/decreases the pressure.
   3. When worn by the same person, the heel of a high-heeled shoe will produce greater/smaller pressure compared to the heel of a running shoe.
   4. A 10-lb, rectangular brick standing vertically on its end produces greater/smaller pressure compared to the same brick lying horizontally on its long side.
   5. Which force produces the greatest pressure: 10 pounds on a 100 in² surface, or 1 pound on a 1 in² surface?

      10 pounds/1 pound.