3.  The force applied divided by the surface area  Pressure = Force/Area  Force (or weight) of an object depends on its mass and the pull of gravity  Area – the total surface of an object  Length (m) x width (m)  Unit used is meter squared (m 2 )

4.  As force increases, pressure also increases – directly proportional  As area increases, pressure decreases – indirectly proportional

5.  Why???

6.  Just like pressure is exerted on your foot- gas molecules exert pressure on things too.  Pressure can be measured in many different units: Inches of mercury Pounds per square inch (psi) Millimeters of mercury (mmHg) Atmospheres (atm)  That’s a lot of different units for one thing!!

7.  Certain units are used in scientific measurements  International System of Units (SI) – lets scientists all over the world to communicate with each other.  Chart

9.  Certain units are used in scientific measurements  International System of Units (SI) – lets scientists all over the world to communicate with each other.  Chart There are 2 different “kinds” of units: - Base Units - Derived Units

10.  Base Units – There are 7!! basic physical measurements (units in the metric system) Ex: mass and length  Derived units – Must be figured out mathematically Ex: area, force, volume and pressure

11. Force is measured in Newtons (N)  A bar of soap pushes down with about 1N of force. Pressure is measured in Pascals (Pa)  Pascal (Pa) = N/m 2  To find pressure need 2 things: 1. Force (units = N) 2. Area (units = m 2 )

12. Photo shows two bricks of the same mass lying on the ground.  1. Is each brick exerting the same total force on the ground? Explain.  2. Is each brick exerting the same pressure on the ground? Explain.

13.  Determine the area of the brick on the right side of the photo in Figure 2.7.  The dimensions of the brick surface touching the ground are 9.3 cm × 5.5 cm. ** Remember we want to have the units in meters(m)! So we will need to convert cm  m.